diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index 639c5d4c9309f..711092ae8eb25 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -159,6 +159,7 @@
 #![feature(unicode_internals)]
 #![feature(unsize)]
 #![feature(unwrap_infallible)]
+#![feature(wtf8_internals)]
 // tidy-alphabetical-end
 //
 // Language features:
@@ -232,6 +233,8 @@ pub mod sync;
 #[cfg(all(not(no_global_oom_handling), not(no_rc), not(no_sync)))]
 pub mod task;
 pub mod vec;
+#[cfg(all(not(no_rc), not(no_sync), not(no_global_oom_handling)))]
+pub mod wtf8;
 
 #[doc(hidden)]
 #[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]
diff --git a/library/alloc/src/wtf8/mod.rs b/library/alloc/src/wtf8/mod.rs
new file mode 100644
index 0000000000000..047994adc4486
--- /dev/null
+++ b/library/alloc/src/wtf8/mod.rs
@@ -0,0 +1,562 @@
+//! Heap-allocated counterpart to core `wtf8` module.
+#![unstable(
+    feature = "wtf8_internals",
+    issue = "none",
+    reason = "this is internal code for representing OsStr on some platforms and not a public API"
+)]
+// rustdoc bug: doc(hidden) on the module won't stop types in the module from showing up in trait
+// implementations, so, we'll have to add more doc(hidden)s anyway
+#![doc(hidden)]
+
+// Note: This module is also included in the alloctests crate using #[path] to
+// run the tests. See the comment there for an explanation why this is the case.
+
+#[cfg(test)]
+mod tests;
+
+use core::char::{MAX_LEN_UTF8, encode_utf8_raw};
+use core::hash::{Hash, Hasher};
+pub use core::wtf8::{CodePoint, Wtf8};
+#[cfg(not(test))]
+pub use core::wtf8::{EncodeWide, Wtf8CodePoints};
+use core::{fmt, mem, ops, str};
+
+use crate::borrow::{Cow, ToOwned};
+use crate::boxed::Box;
+use crate::collections::TryReserveError;
+#[cfg(not(test))]
+use crate::rc::Rc;
+use crate::string::String;
+#[cfg(all(not(test), target_has_atomic = "ptr"))]
+use crate::sync::Arc;
+use crate::vec::Vec;
+
+/// An owned, growable string of well-formed WTF-8 data.
+///
+/// Similar to `String`, but can additionally contain surrogate code points
+/// if they’re not in a surrogate pair.
+#[derive(Eq, PartialEq, Ord, PartialOrd, Clone)]
+#[doc(hidden)]
+pub struct Wtf8Buf {
+    bytes: Vec<u8>,
+
+    /// Do we know that `bytes` holds a valid UTF-8 encoding? We can easily
+    /// know this if we're constructed from a `String` or `&str`.
+    ///
+    /// It is possible for `bytes` to have valid UTF-8 without this being
+    /// set, such as when we're concatenating `&Wtf8`'s and surrogates become
+    /// paired, as we don't bother to rescan the entire string.
+    is_known_utf8: bool,
+}
+
+impl ops::Deref for Wtf8Buf {
+    type Target = Wtf8;
+
+    fn deref(&self) -> &Wtf8 {
+        self.as_slice()
+    }
+}
+
+impl ops::DerefMut for Wtf8Buf {
+    fn deref_mut(&mut self) -> &mut Wtf8 {
+        self.as_mut_slice()
+    }
+}
+
+/// Formats the string in double quotes, with characters escaped according to
+/// [`char::escape_debug`] and unpaired surrogates represented as `\u{xxxx}`,
+/// where each `x` is a hexadecimal digit.
+///
+/// For example, the code units [U+0061, U+D800, U+000A] are formatted as
+/// `"a\u{D800}\n"`.
+impl fmt::Debug for Wtf8Buf {
+    #[inline]
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&**self, formatter)
+    }
+}
+
+/// Formats the string with unpaired surrogates substituted with the replacement
+/// character, U+FFFD.
+impl fmt::Display for Wtf8Buf {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Some(s) = self.as_known_utf8() {
+            fmt::Display::fmt(s, formatter)
+        } else {
+            fmt::Display::fmt(&**self, formatter)
+        }
+    }
+}
+
+#[cfg_attr(test, allow(dead_code))]
+impl Wtf8Buf {
+    /// Creates a new, empty WTF-8 string.
+    #[inline]
+    pub fn new() -> Wtf8Buf {
+        Wtf8Buf { bytes: Vec::new(), is_known_utf8: true }
+    }
+
+    /// Creates a new, empty WTF-8 string with pre-allocated capacity for `capacity` bytes.
+    #[inline]
+    pub fn with_capacity(capacity: usize) -> Wtf8Buf {
+        Wtf8Buf { bytes: Vec::with_capacity(capacity), is_known_utf8: true }
+    }
+
+    /// Creates a WTF-8 string from a WTF-8 byte vec.
+    ///
+    /// Since the byte vec is not checked for valid WTF-8, this function is
+    /// marked unsafe.
+    #[inline]
+    pub unsafe fn from_bytes_unchecked(value: Vec<u8>) -> Wtf8Buf {
+        Wtf8Buf { bytes: value, is_known_utf8: false }
+    }
+
+    /// Creates a WTF-8 string from a UTF-8 `String`.
+    ///
+    /// This takes ownership of the `String` and does not copy.
+    ///
+    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
+    #[inline]
+    pub const fn from_string(string: String) -> Wtf8Buf {
+        Wtf8Buf { bytes: string.into_bytes(), is_known_utf8: true }
+    }
+
+    /// Creates a WTF-8 string from a UTF-8 `&str` slice.
+    ///
+    /// This copies the content of the slice.
+    ///
+    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
+    #[inline]
+    pub fn from_str(s: &str) -> Wtf8Buf {
+        Wtf8Buf { bytes: s.as_bytes().to_vec(), is_known_utf8: true }
+    }
+
+    pub fn clear(&mut self) {
+        self.bytes.clear();
+        self.is_known_utf8 = true;
+    }
+
+    /// Creates a WTF-8 string from a potentially ill-formed UTF-16 slice of 16-bit code units.
+    ///
+    /// This is lossless: calling `.encode_wide()` on the resulting string
+    /// will always return the original code units.
+    pub fn from_wide(v: &[u16]) -> Wtf8Buf {
+        let mut string = Wtf8Buf::with_capacity(v.len());
+        for item in char::decode_utf16(v.iter().cloned()) {
+            match item {
+                Ok(ch) => string.push_char(ch),
+                Err(surrogate) => {
+                    let surrogate = surrogate.unpaired_surrogate();
+                    // Surrogates are known to be in the code point range.
+                    let code_point = unsafe { CodePoint::from_u32_unchecked(surrogate as u32) };
+                    // The string will now contain an unpaired surrogate.
+                    string.is_known_utf8 = false;
+                    // Skip the WTF-8 concatenation check,
+                    // surrogate pairs are already decoded by decode_utf16
+                    unsafe {
+                        string.push_code_point_unchecked(code_point);
+                    }
+                }
+            }
+        }
+        string
+    }
+
+    /// Appends the given `char` to the end of this string.
+    /// This does **not** include the WTF-8 concatenation check or `is_known_utf8` check.
+    /// Copied from String::push.
+    unsafe fn push_code_point_unchecked(&mut self, code_point: CodePoint) {
+        let mut bytes = [0; MAX_LEN_UTF8];
+        let bytes = encode_utf8_raw(code_point.to_u32(), &mut bytes);
+        self.bytes.extend_from_slice(bytes)
+    }
+
+    #[inline]
+    pub fn as_slice(&self) -> &Wtf8 {
+        unsafe { Wtf8::from_bytes_unchecked(&self.bytes) }
+    }
+
+    #[inline]
+    pub fn as_mut_slice(&mut self) -> &mut Wtf8 {
+        // Safety: `Wtf8` doesn't expose any way to mutate the bytes that would
+        // cause them to change from well-formed UTF-8 to ill-formed UTF-8,
+        // which would break the assumptions of the `is_known_utf8` field.
+        unsafe { Wtf8::from_mut_bytes_unchecked(&mut self.bytes) }
+    }
+
+    /// Converts the string to UTF-8 without validation, if it was created from
+    /// valid UTF-8.
+    #[inline]
+    fn as_known_utf8(&self) -> Option<&str> {
+        if self.is_known_utf8 {
+            // SAFETY: The buffer is known to be valid UTF-8.
+            Some(unsafe { str::from_utf8_unchecked(self.as_bytes()) })
+        } else {
+            None
+        }
+    }
+
+    /// Reserves capacity for at least `additional` more bytes to be inserted
+    /// in the given `Wtf8Buf`.
+    /// The collection may reserve more space to avoid frequent reallocations.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity exceeds `isize::MAX` bytes.
+    #[inline]
+    pub fn reserve(&mut self, additional: usize) {
+        self.bytes.reserve(additional)
+    }
+
+    /// Tries to reserve capacity for at least `additional` more bytes to be
+    /// inserted in the given `Wtf8Buf`. The `Wtf8Buf` may reserve more space to
+    /// avoid frequent reallocations. After calling `try_reserve`, capacity will
+    /// be greater than or equal to `self.len() + additional`. Does nothing if
+    /// capacity is already sufficient. This method preserves the contents even
+    /// if an error occurs.
+    ///
+    /// # Errors
+    ///
+    /// If the capacity overflows, or the allocator reports a failure, then an error
+    /// is returned.
+    #[inline]
+    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
+        self.bytes.try_reserve(additional)
+    }
+
+    #[inline]
+    pub fn reserve_exact(&mut self, additional: usize) {
+        self.bytes.reserve_exact(additional)
+    }
+
+    /// Tries to reserve the minimum capacity for exactly `additional` more
+    /// bytes to be inserted in the given `Wtf8Buf`. After calling
+    /// `try_reserve_exact`, capacity will be greater than or equal to
+    /// `self.len() + additional` if it returns `Ok(())`.
+    /// Does nothing if the capacity is already sufficient.
+    ///
+    /// Note that the allocator may give the `Wtf8Buf` more space than it
+    /// requests. Therefore, capacity can not be relied upon to be precisely
+    /// minimal. Prefer [`try_reserve`] if future insertions are expected.
+    ///
+    /// [`try_reserve`]: Wtf8Buf::try_reserve
+    ///
+    /// # Errors
+    ///
+    /// If the capacity overflows, or the allocator reports a failure, then an error
+    /// is returned.
+    #[inline]
+    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
+        self.bytes.try_reserve_exact(additional)
+    }
+
+    #[inline]
+    pub fn shrink_to_fit(&mut self) {
+        self.bytes.shrink_to_fit()
+    }
+
+    #[inline]
+    pub fn shrink_to(&mut self, min_capacity: usize) {
+        self.bytes.shrink_to(min_capacity)
+    }
+
+    #[inline]
+    pub fn leak<'a>(self) -> &'a mut Wtf8 {
+        unsafe { Wtf8::from_mut_bytes_unchecked(self.bytes.leak()) }
+    }
+
+    /// Returns the number of bytes that this string buffer can hold without reallocating.
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.bytes.capacity()
+    }
+
+    /// Append a UTF-8 slice at the end of the string.
+    #[inline]
+    pub fn push_str(&mut self, other: &str) {
+        self.bytes.extend_from_slice(other.as_bytes())
+    }
+
+    /// Append a WTF-8 slice at the end of the string.
+    ///
+    /// This replaces newly paired surrogates at the boundary
+    /// with a supplementary code point,
+    /// like concatenating ill-formed UTF-16 strings effectively would.
+    #[inline]
+    pub fn push_wtf8(&mut self, other: &Wtf8) {
+        match ((&*self).final_lead_surrogate(), other.initial_trail_surrogate()) {
+            // Replace newly paired surrogates by a supplementary code point.
+            (Some(lead), Some(trail)) => {
+                let len_without_lead_surrogate = self.len() - 3;
+                self.bytes.truncate(len_without_lead_surrogate);
+                let other_without_trail_surrogate = &other.as_bytes()[3..];
+                // 4 bytes for the supplementary code point
+                self.bytes.reserve(4 + other_without_trail_surrogate.len());
+                self.push_char(decode_surrogate_pair(lead, trail));
+                self.bytes.extend_from_slice(other_without_trail_surrogate);
+            }
+            _ => {
+                // If we'll be pushing a string containing a surrogate, we may
+                // no longer have UTF-8.
+                if self.is_known_utf8 && other.next_surrogate(0).is_some() {
+                    self.is_known_utf8 = false;
+                }
+
+                self.bytes.extend_from_slice(other.as_bytes());
+            }
+        }
+    }
+
+    /// Append a Unicode scalar value at the end of the string.
+    #[inline]
+    pub fn push_char(&mut self, c: char) {
+        // SAFETY: It's always safe to push a char.
+        unsafe { self.push_code_point_unchecked(CodePoint::from_char(c)) }
+    }
+
+    /// Append a code point at the end of the string.
+    ///
+    /// This replaces newly paired surrogates at the boundary
+    /// with a supplementary code point,
+    /// like concatenating ill-formed UTF-16 strings effectively would.
+    #[inline]
+    pub fn push(&mut self, code_point: CodePoint) {
+        if let Some(trail) = code_point.to_trail_surrogate() {
+            if let Some(lead) = (&*self).final_lead_surrogate() {
+                let len_without_lead_surrogate = self.len() - 3;
+                self.bytes.truncate(len_without_lead_surrogate);
+                self.push_char(decode_surrogate_pair(lead, trail));
+                return;
+            }
+
+            // We're pushing a trailing surrogate.
+            self.is_known_utf8 = false;
+        } else if code_point.to_lead_surrogate().is_some() {
+            // We're pushing a leading surrogate.
+            self.is_known_utf8 = false;
+        }
+
+        // No newly paired surrogates at the boundary.
+        unsafe { self.push_code_point_unchecked(code_point) }
+    }
+
+    /// Shortens a string to the specified length.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `new_len` > current length,
+    /// or if `new_len` is not a code point boundary.
+    #[inline]
+    pub fn truncate(&mut self, new_len: usize) {
+        assert!(self.is_code_point_boundary(new_len));
+        self.bytes.truncate(new_len)
+    }
+
+    /// Consumes the WTF-8 string and tries to convert it to a vec of bytes.
+    #[inline]
+    pub fn into_bytes(self) -> Vec<u8> {
+        self.bytes
+    }
+
+    /// Consumes the WTF-8 string and tries to convert it to UTF-8.
+    ///
+    /// This does not copy the data.
+    ///
+    /// If the contents are not well-formed UTF-8
+    /// (that is, if the string contains surrogates),
+    /// the original WTF-8 string is returned instead.
+    pub fn into_string(self) -> Result<String, Wtf8Buf> {
+        if self.is_known_utf8 || self.next_surrogate(0).is_none() {
+            Ok(unsafe { String::from_utf8_unchecked(self.bytes) })
+        } else {
+            Err(self)
+        }
+    }
+
+    /// Consumes the WTF-8 string and converts it lossily to UTF-8.
+    ///
+    /// This does not copy the data (but may overwrite parts of it in place).
+    ///
+    /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”)
+    pub fn into_string_lossy(mut self) -> String {
+        if !self.is_known_utf8 {
+            let mut pos = 0;
+            while let Some((surrogate_pos, _)) = self.next_surrogate(pos) {
+                pos = surrogate_pos + 3;
+                // Surrogates and the replacement character are all 3 bytes, so
+                // they can substituted in-place.
+                self.bytes[surrogate_pos..pos].copy_from_slice("\u{FFFD}".as_bytes());
+            }
+        }
+        unsafe { String::from_utf8_unchecked(self.bytes) }
+    }
+
+    /// Converts this `Wtf8Buf` into a boxed `Wtf8`.
+    #[inline]
+    pub fn into_box(self) -> Box<Wtf8> {
+        // SAFETY: relies on `Wtf8` being `repr(transparent)`.
+        unsafe { mem::transmute(self.bytes.into_boxed_slice()) }
+    }
+
+    /// Converts a `Box<Wtf8>` into a `Wtf8Buf`.
+    pub fn from_box(boxed: Box<Wtf8>) -> Wtf8Buf {
+        let bytes: Box<[u8]> = unsafe { mem::transmute(boxed) };
+        Wtf8Buf { bytes: bytes.into_vec(), is_known_utf8: false }
+    }
+
+    /// Provides plumbing to core `Vec::extend_from_slice`.
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
+    #[inline]
+    pub unsafe fn extend_from_slice_unchecked(&mut self, other: &[u8]) {
+        self.bytes.extend_from_slice(other);
+        self.is_known_utf8 = false;
+    }
+}
+
+/// Creates a new WTF-8 string from an iterator of code points.
+///
+/// This replaces surrogate code point pairs with supplementary code points,
+/// like concatenating ill-formed UTF-16 strings effectively would.
+impl FromIterator<CodePoint> for Wtf8Buf {
+    fn from_iter<T: IntoIterator<Item = CodePoint>>(iter: T) -> Wtf8Buf {
+        let mut string = Wtf8Buf::new();
+        string.extend(iter);
+        string
+    }
+}
+
+/// Append code points from an iterator to the string.
+///
+/// This replaces surrogate code point pairs with supplementary code points,
+/// like concatenating ill-formed UTF-16 strings effectively would.
+impl Extend<CodePoint> for Wtf8Buf {
+    fn extend<T: IntoIterator<Item = CodePoint>>(&mut self, iter: T) {
+        let iterator = iter.into_iter();
+        let (low, _high) = iterator.size_hint();
+        // Lower bound of one byte per code point (ASCII only)
+        self.bytes.reserve(low);
+        iterator.for_each(move |code_point| self.push(code_point));
+    }
+
+    #[inline]
+    fn extend_one(&mut self, code_point: CodePoint) {
+        self.push(code_point);
+    }
+
+    #[inline]
+    fn extend_reserve(&mut self, additional: usize) {
+        // Lower bound of one byte per code point (ASCII only)
+        self.bytes.reserve(additional);
+    }
+}
+
+/// Creates an owned `Wtf8Buf` from a borrowed `Wtf8`.
+pub(super) fn to_owned(slice: &Wtf8) -> Wtf8Buf {
+    Wtf8Buf { bytes: slice.as_bytes().to_vec(), is_known_utf8: false }
+}
+
+/// Lossily converts the string to UTF-8.
+/// Returns a UTF-8 `&str` slice if the contents are well-formed in UTF-8.
+///
+/// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”).
+///
+/// This only copies the data if necessary (if it contains any surrogate).
+pub(super) fn to_string_lossy(slice: &Wtf8) -> Cow<'_, str> {
+    let Some((surrogate_pos, _)) = slice.next_surrogate(0) else {
+        return Cow::Borrowed(unsafe { str::from_utf8_unchecked(slice.as_bytes()) });
+    };
+    let wtf8_bytes = slice.as_bytes();
+    let mut utf8_bytes = Vec::with_capacity(slice.len());
+    utf8_bytes.extend_from_slice(&wtf8_bytes[..surrogate_pos]);
+    utf8_bytes.extend_from_slice("\u{FFFD}".as_bytes());
+    let mut pos = surrogate_pos + 3;
+    loop {
+        match slice.next_surrogate(pos) {
+            Some((surrogate_pos, _)) => {
+                utf8_bytes.extend_from_slice(&wtf8_bytes[pos..surrogate_pos]);
+                utf8_bytes.extend_from_slice("\u{FFFD}".as_bytes());
+                pos = surrogate_pos + 3;
+            }
+            None => {
+                utf8_bytes.extend_from_slice(&wtf8_bytes[pos..]);
+                return Cow::Owned(unsafe { String::from_utf8_unchecked(utf8_bytes) });
+            }
+        }
+    }
+}
+
+#[inline]
+pub(super) fn clone_into(slice: &Wtf8, buf: &mut Wtf8Buf) {
+    buf.is_known_utf8 = false;
+    slice.as_bytes().clone_into(&mut buf.bytes);
+}
+
+#[cfg(not(test))]
+impl Wtf8 {
+    #[rustc_allow_incoherent_impl]
+    pub fn to_owned(&self) -> Wtf8Buf {
+        to_owned(self)
+    }
+
+    #[rustc_allow_incoherent_impl]
+    pub fn clone_into(&self, buf: &mut Wtf8Buf) {
+        clone_into(self, buf)
+    }
+
+    #[rustc_allow_incoherent_impl]
+    pub fn to_string_lossy(&self) -> Cow<'_, str> {
+        to_string_lossy(self)
+    }
+
+    #[rustc_allow_incoherent_impl]
+    pub fn into_box(&self) -> Box<Wtf8> {
+        let boxed: Box<[u8]> = self.as_bytes().into();
+        unsafe { mem::transmute(boxed) }
+    }
+
+    #[rustc_allow_incoherent_impl]
+    pub fn empty_box() -> Box<Wtf8> {
+        let boxed: Box<[u8]> = Default::default();
+        unsafe { mem::transmute(boxed) }
+    }
+
+    #[cfg(target_has_atomic = "ptr")]
+    #[rustc_allow_incoherent_impl]
+    pub fn into_arc(&self) -> Arc<Wtf8> {
+        let arc: Arc<[u8]> = Arc::from(self.as_bytes());
+        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const Wtf8) }
+    }
+
+    #[rustc_allow_incoherent_impl]
+    pub fn into_rc(&self) -> Rc<Wtf8> {
+        let rc: Rc<[u8]> = Rc::from(self.as_bytes());
+        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const Wtf8) }
+    }
+
+    #[inline]
+    #[rustc_allow_incoherent_impl]
+    pub fn to_ascii_lowercase(&self) -> Wtf8Buf {
+        Wtf8Buf { bytes: self.as_bytes().to_ascii_lowercase(), is_known_utf8: false }
+    }
+
+    #[inline]
+    #[rustc_allow_incoherent_impl]
+    pub fn to_ascii_uppercase(&self) -> Wtf8Buf {
+        Wtf8Buf { bytes: self.as_bytes().to_ascii_uppercase(), is_known_utf8: false }
+    }
+}
+
+#[inline]
+fn decode_surrogate_pair(lead: u16, trail: u16) -> char {
+    let code_point = 0x10000 + ((((lead - 0xD800) as u32) << 10) | (trail - 0xDC00) as u32);
+    unsafe { char::from_u32_unchecked(code_point) }
+}
+
+impl Hash for Wtf8Buf {
+    #[inline]
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        state.write(&self.bytes);
+        0xfeu8.hash(state)
+    }
+}
diff --git a/library/std/src/sys_common/wtf8/tests.rs b/library/alloc/src/wtf8/tests.rs
similarity index 75%
rename from library/std/src/sys_common/wtf8/tests.rs
rename to library/alloc/src/wtf8/tests.rs
index b57c99a8452a1..291f63f9f9e54 100644
--- a/library/std/src/sys_common/wtf8/tests.rs
+++ b/library/alloc/src/wtf8/tests.rs
@@ -1,3 +1,5 @@
+use realalloc::string::ToString;
+
 use super::*;
 
 #[test]
@@ -82,82 +84,85 @@ fn code_point_to_char_lossy() {
 
 #[test]
 fn wtf8buf_new() {
-    assert_eq!(Wtf8Buf::new().bytes, b"");
+    assert_eq!(Wtf8Buf::new().as_bytes(), b"");
 }
 
 #[test]
 fn wtf8buf_from_str() {
-    assert_eq!(Wtf8Buf::from_str("").bytes, b"");
-    assert_eq!(Wtf8Buf::from_str("aé 💩").bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(Wtf8Buf::from_str("").as_bytes(), b"");
+    assert_eq!(Wtf8Buf::from_str("aé 💩").as_bytes(), b"a\xC3\xA9 \xF0\x9F\x92\xA9");
 }
 
 #[test]
 fn wtf8buf_from_string() {
-    assert_eq!(Wtf8Buf::from_string(String::from("")).bytes, b"");
-    assert_eq!(Wtf8Buf::from_string(String::from("aé 💩")).bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(Wtf8Buf::from_string(String::from("")).as_bytes(), b"");
+    assert_eq!(
+        Wtf8Buf::from_string(String::from("aé 💩")).as_bytes(),
+        b"a\xC3\xA9 \xF0\x9F\x92\xA9"
+    );
 }
 
 #[test]
 fn wtf8buf_from_wide() {
     let buf = Wtf8Buf::from_wide(&[]);
-    assert_eq!(buf.bytes, b"");
+    assert_eq!(buf.as_bytes(), b"");
     assert!(buf.is_known_utf8);
 
     let buf = Wtf8Buf::from_wide(&[0x61, 0xE9, 0x20, 0xD83D, 0xDCA9]);
-    assert_eq!(buf.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(buf.as_bytes(), b"a\xC3\xA9 \xF0\x9F\x92\xA9");
     assert!(buf.is_known_utf8);
 
     let buf = Wtf8Buf::from_wide(&[0x61, 0xE9, 0x20, 0xD83D, 0xD83D, 0xDCA9]);
-    assert_eq!(buf.bytes, b"a\xC3\xA9 \xED\xA0\xBD\xF0\x9F\x92\xA9");
+    assert_eq!(buf.as_bytes(), b"a\xC3\xA9 \xED\xA0\xBD\xF0\x9F\x92\xA9");
     assert!(!buf.is_known_utf8);
 
     let buf = Wtf8Buf::from_wide(&[0xD800]);
-    assert_eq!(buf.bytes, b"\xED\xA0\x80");
+    assert_eq!(buf.as_bytes(), b"\xED\xA0\x80");
     assert!(!buf.is_known_utf8);
 
     let buf = Wtf8Buf::from_wide(&[0xDBFF]);
-    assert_eq!(buf.bytes, b"\xED\xAF\xBF");
+    assert_eq!(buf.as_bytes(), b"\xED\xAF\xBF");
     assert!(!buf.is_known_utf8);
 
     let buf = Wtf8Buf::from_wide(&[0xDC00]);
-    assert_eq!(buf.bytes, b"\xED\xB0\x80");
+    assert_eq!(buf.as_bytes(), b"\xED\xB0\x80");
     assert!(!buf.is_known_utf8);
 
     let buf = Wtf8Buf::from_wide(&[0xDFFF]);
-    assert_eq!(buf.bytes, b"\xED\xBF\xBF");
+    assert_eq!(buf.as_bytes(), b"\xED\xBF\xBF");
     assert!(!buf.is_known_utf8);
 }
 
 #[test]
 fn wtf8buf_push_str() {
     let mut string = Wtf8Buf::new();
-    assert_eq!(string.bytes, b"");
+    assert_eq!(string.as_bytes(), b"");
     assert!(string.is_known_utf8);
 
     string.push_str("aé 💩");
-    assert_eq!(string.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(string.as_bytes(), b"a\xC3\xA9 \xF0\x9F\x92\xA9");
     assert!(string.is_known_utf8);
 }
 
 #[test]
 fn wtf8buf_push_char() {
     let mut string = Wtf8Buf::from_str("aé ");
-    assert_eq!(string.bytes, b"a\xC3\xA9 ");
+    assert_eq!(string.as_bytes(), b"a\xC3\xA9 ");
     assert!(string.is_known_utf8);
 
     string.push_char('💩');
-    assert_eq!(string.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(string.as_bytes(), b"a\xC3\xA9 \xF0\x9F\x92\xA9");
     assert!(string.is_known_utf8);
 }
 
 #[test]
 fn wtf8buf_push() {
     let mut string = Wtf8Buf::from_str("aé ");
-    assert_eq!(string.bytes, b"a\xC3\xA9 ");
+    assert_eq!(string.as_bytes(), b"a\xC3\xA9 ");
     assert!(string.is_known_utf8);
 
     string.push(CodePoint::from_char('💩'));
-    assert_eq!(string.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(string.as_bytes(), b"a\xC3\xA9 \xF0\x9F\x92\xA9");
     assert!(string.is_known_utf8);
 
     fn c(value: u32) -> CodePoint {
@@ -168,53 +173,53 @@ fn wtf8buf_push() {
     string.push(c(0xD83D)); // lead
     assert!(!string.is_known_utf8);
     string.push(c(0xDCA9)); // trail
-    assert_eq!(string.bytes, b"\xF0\x9F\x92\xA9"); // Magic!
+    assert_eq!(string.as_bytes(), b"\xF0\x9F\x92\xA9"); // Magic!
 
     let mut string = Wtf8Buf::new();
     string.push(c(0xD83D)); // lead
     assert!(!string.is_known_utf8);
     string.push(c(0x20)); // not surrogate
     string.push(c(0xDCA9)); // trail
-    assert_eq!(string.bytes, b"\xED\xA0\xBD \xED\xB2\xA9");
+    assert_eq!(string.as_bytes(), b"\xED\xA0\xBD \xED\xB2\xA9");
 
     let mut string = Wtf8Buf::new();
     string.push(c(0xD800)); // lead
     assert!(!string.is_known_utf8);
     string.push(c(0xDBFF)); // lead
-    assert_eq!(string.bytes, b"\xED\xA0\x80\xED\xAF\xBF");
+    assert_eq!(string.as_bytes(), b"\xED\xA0\x80\xED\xAF\xBF");
 
     let mut string = Wtf8Buf::new();
     string.push(c(0xD800)); // lead
     assert!(!string.is_known_utf8);
     string.push(c(0xE000)); // not surrogate
-    assert_eq!(string.bytes, b"\xED\xA0\x80\xEE\x80\x80");
+    assert_eq!(string.as_bytes(), b"\xED\xA0\x80\xEE\x80\x80");
 
     let mut string = Wtf8Buf::new();
     string.push(c(0xD7FF)); // not surrogate
     assert!(string.is_known_utf8);
     string.push(c(0xDC00)); // trail
     assert!(!string.is_known_utf8);
-    assert_eq!(string.bytes, b"\xED\x9F\xBF\xED\xB0\x80");
+    assert_eq!(string.as_bytes(), b"\xED\x9F\xBF\xED\xB0\x80");
 
     let mut string = Wtf8Buf::new();
     string.push(c(0x61)); // not surrogate, < 3 bytes
     assert!(string.is_known_utf8);
     string.push(c(0xDC00)); // trail
     assert!(!string.is_known_utf8);
-    assert_eq!(string.bytes, b"\x61\xED\xB0\x80");
+    assert_eq!(string.as_bytes(), b"\x61\xED\xB0\x80");
 
     let mut string = Wtf8Buf::new();
     string.push(c(0xDC00)); // trail
     assert!(!string.is_known_utf8);
-    assert_eq!(string.bytes, b"\xED\xB0\x80");
+    assert_eq!(string.as_bytes(), b"\xED\xB0\x80");
 }
 
 #[test]
 fn wtf8buf_push_wtf8() {
     let mut string = Wtf8Buf::from_str("aé");
-    assert_eq!(string.bytes, b"a\xC3\xA9");
+    assert_eq!(string.as_bytes(), b"a\xC3\xA9");
     string.push_wtf8(Wtf8::from_str(" 💩"));
-    assert_eq!(string.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(string.as_bytes(), b"a\xC3\xA9 \xF0\x9F\x92\xA9");
     assert!(string.is_known_utf8);
 
     fn w(v: &[u8]) -> &Wtf8 {
@@ -224,42 +229,42 @@ fn wtf8buf_push_wtf8() {
     let mut string = Wtf8Buf::new();
     string.push_wtf8(w(b"\xED\xA0\xBD")); // lead
     string.push_wtf8(w(b"\xED\xB2\xA9")); // trail
-    assert_eq!(string.bytes, b"\xF0\x9F\x92\xA9"); // Magic!
+    assert_eq!(string.as_bytes(), b"\xF0\x9F\x92\xA9"); // Magic!
 
     let mut string = Wtf8Buf::new();
     string.push_wtf8(w(b"\xED\xA0\xBD")); // lead
     string.push_wtf8(w(b" ")); // not surrogate
     string.push_wtf8(w(b"\xED\xB2\xA9")); // trail
-    assert_eq!(string.bytes, b"\xED\xA0\xBD \xED\xB2\xA9");
+    assert_eq!(string.as_bytes(), b"\xED\xA0\xBD \xED\xB2\xA9");
     assert!(!string.is_known_utf8);
 
     let mut string = Wtf8Buf::new();
     string.push_wtf8(w(b"\xED\xA0\x80")); // lead
     string.push_wtf8(w(b"\xED\xAF\xBF")); // lead
-    assert_eq!(string.bytes, b"\xED\xA0\x80\xED\xAF\xBF");
+    assert_eq!(string.as_bytes(), b"\xED\xA0\x80\xED\xAF\xBF");
     assert!(!string.is_known_utf8);
 
     let mut string = Wtf8Buf::new();
     string.push_wtf8(w(b"\xED\xA0\x80")); // lead
     string.push_wtf8(w(b"\xEE\x80\x80")); // not surrogate
-    assert_eq!(string.bytes, b"\xED\xA0\x80\xEE\x80\x80");
+    assert_eq!(string.as_bytes(), b"\xED\xA0\x80\xEE\x80\x80");
     assert!(!string.is_known_utf8);
 
     let mut string = Wtf8Buf::new();
     string.push_wtf8(w(b"\xED\x9F\xBF")); // not surrogate
     string.push_wtf8(w(b"\xED\xB0\x80")); // trail
-    assert_eq!(string.bytes, b"\xED\x9F\xBF\xED\xB0\x80");
+    assert_eq!(string.as_bytes(), b"\xED\x9F\xBF\xED\xB0\x80");
     assert!(!string.is_known_utf8);
 
     let mut string = Wtf8Buf::new();
     string.push_wtf8(w(b"a")); // not surrogate, < 3 bytes
     string.push_wtf8(w(b"\xED\xB0\x80")); // trail
-    assert_eq!(string.bytes, b"\x61\xED\xB0\x80");
+    assert_eq!(string.as_bytes(), b"\x61\xED\xB0\x80");
     assert!(!string.is_known_utf8);
 
     let mut string = Wtf8Buf::new();
     string.push_wtf8(w(b"\xED\xB0\x80")); // trail
-    assert_eq!(string.bytes, b"\xED\xB0\x80");
+    assert_eq!(string.as_bytes(), b"\xED\xB0\x80");
     assert!(!string.is_known_utf8);
 }
 
@@ -269,15 +274,15 @@ fn wtf8buf_truncate() {
     assert!(string.is_known_utf8);
 
     string.truncate(3);
-    assert_eq!(string.bytes, b"a\xC3\xA9");
+    assert_eq!(string.as_bytes(), b"a\xC3\xA9");
     assert!(string.is_known_utf8);
 
     string.truncate(1);
-    assert_eq!(string.bytes, b"a");
+    assert_eq!(string.as_bytes(), b"a");
     assert!(string.is_known_utf8);
 
     string.truncate(0);
-    assert_eq!(string.bytes, b"");
+    assert_eq!(string.as_bytes(), b"");
     assert!(string.is_known_utf8);
 }
 
@@ -287,11 +292,11 @@ fn wtf8buf_truncate_around_non_bmp() {
     assert!(string.is_known_utf8);
 
     string.truncate(4);
-    assert_eq!(string.bytes, b"\xF0\x9F\x92\xA9");
+    assert_eq!(string.as_bytes(), b"\xF0\x9F\x92\xA9");
     assert!(string.is_known_utf8);
 
     string.truncate(0);
-    assert_eq!(string.bytes, b"");
+    assert_eq!(string.as_bytes(), b"");
     assert!(string.is_known_utf8);
 }
 
@@ -361,7 +366,7 @@ fn wtf8buf_from_iterator() {
         Wtf8Buf { bytes: b"a\xC3\xA9 \xF0\x9F\x92\xA9".to_vec(), is_known_utf8: true }
     );
 
-    assert_eq!(f(&[0xD83D, 0xDCA9]).bytes, b"\xF0\x9F\x92\xA9"); // Magic!
+    assert_eq!(f(&[0xD83D, 0xDCA9]).as_bytes(), b"\xF0\x9F\x92\xA9"); // Magic!
     assert_eq!(
         f(&[0xD83D, 0x20, 0xDCA9]),
         Wtf8Buf { bytes: b"\xED\xA0\xBD \xED\xB2\xA9".to_vec(), is_known_utf8: false }
@@ -401,7 +406,7 @@ fn wtf8buf_extend() {
         Wtf8Buf { bytes: b"a\xC3\xA9 \xF0\x9F\x92\xA9".to_vec(), is_known_utf8: true }
     );
 
-    assert_eq!(e(&[0xD83D], &[0xDCA9]).bytes, b"\xF0\x9F\x92\xA9"); // Magic!
+    assert_eq!(e(&[0xD83D], &[0xDCA9]).as_bytes(), b"\xF0\x9F\x92\xA9"); // Magic!
     assert_eq!(
         e(&[0xD83D, 0x20], &[0xDCA9]),
         Wtf8Buf { bytes: b"\xED\xA0\xBD \xED\xB2\xA9".to_vec(), is_known_utf8: false }
@@ -449,8 +454,8 @@ fn wtf8buf_show_str() {
 
 #[test]
 fn wtf8_from_str() {
-    assert_eq!(&Wtf8::from_str("").bytes, b"");
-    assert_eq!(&Wtf8::from_str("aé 💩").bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(&Wtf8::from_str("").as_bytes(), b"");
+    assert_eq!(&Wtf8::from_str("aé 💩").as_bytes(), b"a\xC3\xA9 \xF0\x9F\x92\xA9");
 }
 
 #[test]
@@ -461,7 +466,7 @@ fn wtf8_len() {
 
 #[test]
 fn wtf8_slice() {
-    assert_eq!(&Wtf8::from_str("aé 💩")[1..4].bytes, b"\xC3\xA9 ");
+    assert_eq!(&Wtf8::from_str("aé 💩")[1..4].as_bytes(), b"\xC3\xA9 ");
 }
 
 #[test]
@@ -472,7 +477,7 @@ fn wtf8_slice_not_code_point_boundary() {
 
 #[test]
 fn wtf8_slice_from() {
-    assert_eq!(&Wtf8::from_str("aé 💩")[1..].bytes, b"\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(&Wtf8::from_str("aé 💩")[1..].as_bytes(), b"\xC3\xA9 \xF0\x9F\x92\xA9");
 }
 
 #[test]
@@ -483,7 +488,7 @@ fn wtf8_slice_from_not_code_point_boundary() {
 
 #[test]
 fn wtf8_slice_to() {
-    assert_eq!(&Wtf8::from_str("aé 💩")[..4].bytes, b"a\xC3\xA9 ");
+    assert_eq!(&Wtf8::from_str("aé 💩")[..4].as_bytes(), b"a\xC3\xA9 ");
 }
 
 #[test]
@@ -529,12 +534,12 @@ fn wtf8_as_str() {
 
 #[test]
 fn wtf8_to_string_lossy() {
-    assert_eq!(Wtf8::from_str("").to_string_lossy(), Cow::Borrowed(""));
-    assert_eq!(Wtf8::from_str("aé 💩").to_string_lossy(), Cow::Borrowed("aé 💩"));
+    assert_eq!(to_string_lossy(Wtf8::from_str("")), Cow::Borrowed(""));
+    assert_eq!(to_string_lossy(Wtf8::from_str("aé 💩")), Cow::Borrowed("aé 💩"));
     let mut string = Wtf8Buf::from_str("aé 💩");
     string.push(CodePoint::from_u32(0xD800).unwrap());
     let expected: Cow<'_, str> = Cow::Owned(String::from("aé 💩�"));
-    assert_eq!(string.to_string_lossy(), expected);
+    assert_eq!(to_string_lossy(&string), expected);
 }
 
 #[test]
@@ -548,7 +553,7 @@ fn wtf8_display() {
 
     let mut string = Wtf8Buf::from_str("aé 💩");
     string.push(CodePoint::from_u32(0xD800).unwrap());
-    assert_eq!("aé 💩�", d(string.as_inner()));
+    assert_eq!("aé 💩�", d(string.as_ref()));
 }
 
 #[test]
@@ -577,67 +582,41 @@ fn wtf8_encode_wide_size_hint() {
 #[test]
 fn wtf8_clone_into() {
     let mut string = Wtf8Buf::new();
-    Wtf8::from_str("green").clone_into(&mut string);
-    assert_eq!(string.bytes, b"green");
+    clone_into(Wtf8::from_str("green"), &mut string);
+    assert_eq!(string.as_bytes(), b"green");
 
     let mut string = Wtf8Buf::from_str("green");
-    Wtf8::from_str("").clone_into(&mut string);
-    assert_eq!(string.bytes, b"");
+    clone_into(Wtf8::from_str(""), &mut string);
+    assert_eq!(string.as_bytes(), b"");
 
     let mut string = Wtf8Buf::from_str("red");
-    Wtf8::from_str("green").clone_into(&mut string);
-    assert_eq!(string.bytes, b"green");
+    clone_into(Wtf8::from_str("green"), &mut string);
+    assert_eq!(string.as_bytes(), b"green");
 
     let mut string = Wtf8Buf::from_str("green");
-    Wtf8::from_str("red").clone_into(&mut string);
-    assert_eq!(string.bytes, b"red");
+    clone_into(Wtf8::from_str("red"), &mut string);
+    assert_eq!(string.as_bytes(), b"red");
 
     let mut string = Wtf8Buf::from_str("green");
     assert!(string.is_known_utf8);
-    unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").clone_into(&mut string) };
-    assert_eq!(string.bytes, b"\xED\xA0\x80");
+    clone_into(unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80") }, &mut string);
+    assert_eq!(string.as_bytes(), b"\xED\xA0\x80");
     assert!(!string.is_known_utf8);
 }
 
-#[test]
-fn wtf8_to_ascii_lowercase() {
-    let lowercase = Wtf8::from_str("").to_ascii_lowercase();
-    assert_eq!(lowercase.bytes, b"");
-
-    let lowercase = Wtf8::from_str("GrEeN gRaPeS! 🍇").to_ascii_lowercase();
-    assert_eq!(lowercase.bytes, b"green grapes! \xf0\x9f\x8d\x87");
-
-    let lowercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_ascii_lowercase() };
-    assert_eq!(lowercase.bytes, b"\xED\xA0\x80");
-    assert!(!lowercase.is_known_utf8);
-}
-
-#[test]
-fn wtf8_to_ascii_uppercase() {
-    let uppercase = Wtf8::from_str("").to_ascii_uppercase();
-    assert_eq!(uppercase.bytes, b"");
-
-    let uppercase = Wtf8::from_str("GrEeN gRaPeS! 🍇").to_ascii_uppercase();
-    assert_eq!(uppercase.bytes, b"GREEN GRAPES! \xf0\x9f\x8d\x87");
-
-    let uppercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_ascii_uppercase() };
-    assert_eq!(uppercase.bytes, b"\xED\xA0\x80");
-    assert!(!uppercase.is_known_utf8);
-}
-
 #[test]
 fn wtf8_make_ascii_lowercase() {
     let mut lowercase = Wtf8Buf::from_str("");
     lowercase.make_ascii_lowercase();
-    assert_eq!(lowercase.bytes, b"");
+    assert_eq!(lowercase.as_bytes(), b"");
 
     let mut lowercase = Wtf8Buf::from_str("GrEeN gRaPeS! 🍇");
     lowercase.make_ascii_lowercase();
-    assert_eq!(lowercase.bytes, b"green grapes! \xf0\x9f\x8d\x87");
+    assert_eq!(lowercase.as_bytes(), b"green grapes! \xf0\x9f\x8d\x87");
 
-    let mut lowercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
+    let mut lowercase = to_owned(unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80") });
     lowercase.make_ascii_lowercase();
-    assert_eq!(lowercase.bytes, b"\xED\xA0\x80");
+    assert_eq!(lowercase.as_bytes(), b"\xED\xA0\x80");
     assert!(!lowercase.is_known_utf8);
 }
 
@@ -645,22 +624,22 @@ fn wtf8_make_ascii_lowercase() {
 fn wtf8_make_ascii_uppercase() {
     let mut uppercase = Wtf8Buf::from_str("");
     uppercase.make_ascii_uppercase();
-    assert_eq!(uppercase.bytes, b"");
+    assert_eq!(uppercase.as_bytes(), b"");
 
     let mut uppercase = Wtf8Buf::from_str("GrEeN gRaPeS! 🍇");
     uppercase.make_ascii_uppercase();
-    assert_eq!(uppercase.bytes, b"GREEN GRAPES! \xf0\x9f\x8d\x87");
+    assert_eq!(uppercase.as_bytes(), b"GREEN GRAPES! \xf0\x9f\x8d\x87");
 
-    let mut uppercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
+    let mut uppercase = to_owned(unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80") });
     uppercase.make_ascii_uppercase();
-    assert_eq!(uppercase.bytes, b"\xED\xA0\x80");
+    assert_eq!(uppercase.as_bytes(), b"\xED\xA0\x80");
     assert!(!uppercase.is_known_utf8);
 }
 
 #[test]
 fn wtf8_to_owned() {
-    let string = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
-    assert_eq!(string.bytes, b"\xED\xA0\x80");
+    let string = to_owned(unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80") });
+    assert_eq!(string.as_bytes(), b"\xED\xA0\x80");
     assert!(!string.is_known_utf8);
 }
 
@@ -669,44 +648,44 @@ fn wtf8_valid_utf8_boundaries() {
     let mut string = Wtf8Buf::from_str("aé 💩");
     string.push(CodePoint::from_u32(0xD800).unwrap());
     string.push(CodePoint::from_u32(0xD800).unwrap());
-    check_utf8_boundary(&string, 0);
-    check_utf8_boundary(&string, 1);
-    check_utf8_boundary(&string, 3);
-    check_utf8_boundary(&string, 4);
-    check_utf8_boundary(&string, 8);
-    check_utf8_boundary(&string, 14);
+    string.check_utf8_boundary(0);
+    string.check_utf8_boundary(1);
+    string.check_utf8_boundary(3);
+    string.check_utf8_boundary(4);
+    string.check_utf8_boundary(8);
+    string.check_utf8_boundary(14);
     assert_eq!(string.len(), 14);
 
     string.push_char('a');
-    check_utf8_boundary(&string, 14);
-    check_utf8_boundary(&string, 15);
+    string.check_utf8_boundary(14);
+    string.check_utf8_boundary(15);
 
     let mut string = Wtf8Buf::from_str("a");
     string.push(CodePoint::from_u32(0xD800).unwrap());
-    check_utf8_boundary(&string, 1);
+    string.check_utf8_boundary(1);
 
     let mut string = Wtf8Buf::from_str("\u{D7FF}");
     string.push(CodePoint::from_u32(0xD800).unwrap());
-    check_utf8_boundary(&string, 3);
+    string.check_utf8_boundary(3);
 
     let mut string = Wtf8Buf::new();
     string.push(CodePoint::from_u32(0xD800).unwrap());
     string.push_char('\u{D7FF}');
-    check_utf8_boundary(&string, 3);
+    string.check_utf8_boundary(3);
 }
 
 #[test]
 #[should_panic(expected = "byte index 4 is out of bounds")]
 fn wtf8_utf8_boundary_out_of_bounds() {
     let string = Wtf8::from_str("aé");
-    check_utf8_boundary(&string, 4);
+    string.check_utf8_boundary(4);
 }
 
 #[test]
 #[should_panic(expected = "byte index 1 is not a codepoint boundary")]
 fn wtf8_utf8_boundary_inside_codepoint() {
     let string = Wtf8::from_str("é");
-    check_utf8_boundary(&string, 1);
+    string.check_utf8_boundary(1);
 }
 
 #[test]
@@ -714,7 +693,7 @@ fn wtf8_utf8_boundary_inside_codepoint() {
 fn wtf8_utf8_boundary_inside_surrogate() {
     let mut string = Wtf8Buf::new();
     string.push(CodePoint::from_u32(0xD800).unwrap());
-    check_utf8_boundary(&string, 1);
+    string.check_utf8_boundary(1);
 }
 
 #[test]
@@ -723,20 +702,22 @@ fn wtf8_utf8_boundary_between_surrogates() {
     let mut string = Wtf8Buf::new();
     string.push(CodePoint::from_u32(0xD800).unwrap());
     string.push(CodePoint::from_u32(0xD800).unwrap());
-    check_utf8_boundary(&string, 3);
+    string.check_utf8_boundary(3);
 }
 
 #[test]
 fn wobbled_wtf8_plus_bytes_isnt_utf8() {
-    let mut string: Wtf8Buf = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
+    let mut string: Wtf8Buf = to_owned(unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80") });
     assert!(!string.is_known_utf8);
-    string.extend_from_slice(b"some utf-8");
+    unsafe {
+        string.extend_from_slice_unchecked(b"some utf-8");
+    }
     assert!(!string.is_known_utf8);
 }
 
 #[test]
 fn wobbled_wtf8_plus_str_isnt_utf8() {
-    let mut string: Wtf8Buf = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
+    let mut string: Wtf8Buf = to_owned(unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80") });
     assert!(!string.is_known_utf8);
     string.push_str("some utf-8");
     assert!(!string.is_known_utf8);
diff --git a/library/alloctests/lib.rs b/library/alloctests/lib.rs
index 3241b4b00454b..97de7d6a701ca 100644
--- a/library/alloctests/lib.rs
+++ b/library/alloctests/lib.rs
@@ -18,6 +18,8 @@
 #![feature(allocator_api)]
 #![feature(array_into_iter_constructors)]
 #![feature(assert_matches)]
+#![feature(char_internals)]
+#![feature(char_max_len)]
 #![feature(core_intrinsics)]
 #![feature(exact_size_is_empty)]
 #![feature(extend_one)]
@@ -41,6 +43,7 @@
 #![feature(trusted_random_access)]
 #![feature(try_reserve_kind)]
 #![feature(try_trait_v2)]
+#![feature(wtf8_internals)]
 // tidy-alphabetical-end
 //
 // Language features:
@@ -68,15 +71,18 @@ extern crate test;
 mod testing;
 use realalloc::*;
 
-// We are directly including collections and raw_vec here as both use non-public
-// methods and fields in tests and as such need to have the types to test in the
-// same crate as the tests themself.
+// We are directly including collections, raw_vec, and wtf8 here as they use non-public
+// methods and fields in tests and as such need to have the types to test in the same
+// crate as the tests themself.
 #[path = "../alloc/src/collections/mod.rs"]
 mod collections;
 
 #[path = "../alloc/src/raw_vec/mod.rs"]
 mod raw_vec;
 
+#[path = "../alloc/src/wtf8/mod.rs"]
+mod wtf8;
+
 #[allow(dead_code)] // Not used in all configurations
 pub(crate) mod test_helpers {
     /// Copied from `std::test_helpers::test_rng`, since these tests rely on the
diff --git a/library/alloctests/tests/lib.rs b/library/alloctests/tests/lib.rs
index fcfc7f8dd296d..447af240a4b9d 100644
--- a/library/alloctests/tests/lib.rs
+++ b/library/alloctests/tests/lib.rs
@@ -3,6 +3,7 @@
 #![feature(iter_array_chunks)]
 #![feature(assert_matches)]
 #![feature(btree_extract_if)]
+#![feature(wtf8_internals)]
 #![feature(char_max_len)]
 #![feature(cow_is_borrowed)]
 #![feature(core_intrinsics)]
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index f69d6a98592ae..82506379cac7b 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -363,6 +363,8 @@ pub mod slice;
 pub mod str;
 pub mod time;
 
+pub mod wtf8;
+
 pub mod unicode;
 
 /* Async */
diff --git a/library/core/src/num/niche_types.rs b/library/core/src/num/niche_types.rs
index d57b1d433e51c..610d9d8cf92e0 100644
--- a/library/core/src/num/niche_types.rs
+++ b/library/core/src/num/niche_types.rs
@@ -178,3 +178,18 @@ impl NotAllOnesHelper for u64 {
 impl NotAllOnesHelper for i64 {
     type Type = I64NotAllOnes;
 }
+
+define_valid_range_type! {
+    pub struct CodePointInner(u32 as u32 in 0..=0x10ffff);
+}
+
+impl CodePointInner {
+    pub const ZERO: Self = CodePointInner::new(0).unwrap();
+}
+
+impl Default for CodePointInner {
+    #[inline]
+    fn default() -> Self {
+        Self::ZERO
+    }
+}
diff --git a/library/core/src/ptr/metadata.rs b/library/core/src/ptr/metadata.rs
index 0deac3621e84c..dc3ec3fd19945 100644
--- a/library/core/src/ptr/metadata.rs
+++ b/library/core/src/ptr/metadata.rs
@@ -106,6 +106,9 @@ pub const fn metadata<T: PointeeSized>(ptr: *const T) -> <T as Pointee>::Metadat
 /// For slices, see the documentation of [`slice::from_raw_parts`] for safety requirements.
 /// For trait objects, the metadata must come from a pointer to the same underlying erased type.
 ///
+/// If you are attempting to deconstruct a DST in a generic context to be reconstructed later,
+/// a thin pointer can always be obtained by casting `*const T` to `*const ()`.
+///
 /// [`slice::from_raw_parts`]: crate::slice::from_raw_parts
 #[unstable(feature = "ptr_metadata", issue = "81513")]
 #[inline]
diff --git a/library/core/src/wtf8.rs b/library/core/src/wtf8.rs
new file mode 100644
index 0000000000000..de0dfa560a3f3
--- /dev/null
+++ b/library/core/src/wtf8.rs
@@ -0,0 +1,597 @@
+//! Implementation of [the WTF-8 encoding](https://simonsapin.github.io/wtf-8/).
+//!
+//! This library uses Rust’s type system to maintain
+//! [well-formedness](https://simonsapin.github.io/wtf-8/#well-formed),
+//! like the `String` and `&str` types do for UTF-8.
+//!
+//! Since [WTF-8 must not be used
+//! for interchange](https://simonsapin.github.io/wtf-8/#intended-audience),
+//! this library deliberately does not provide access to the underlying bytes
+//! of WTF-8 strings,
+//! nor can it decode WTF-8 from arbitrary bytes.
+//! WTF-8 strings can be obtained from UTF-8, UTF-16, or code points.
+#![unstable(
+    feature = "wtf8_internals",
+    issue = "none",
+    reason = "this is internal code for representing OsStr on some platforms and not a public API"
+)]
+// rustdoc bug: doc(hidden) on the module won't stop types in the module from showing up in trait
+// implementations, so, we'll have to add more doc(hidden)s anyway
+#![doc(hidden)]
+
+use crate::char::{MAX_LEN_UTF16, encode_utf16_raw};
+use crate::clone::CloneToUninit;
+use crate::fmt::{self, Write};
+use crate::hash::{Hash, Hasher};
+use crate::iter::FusedIterator;
+use crate::num::niche_types::CodePointInner;
+use crate::str::next_code_point;
+use crate::{ops, slice, str};
+
+/// A Unicode code point: from U+0000 to U+10FFFF.
+///
+/// Compares with the `char` type,
+/// which represents a Unicode scalar value:
+/// a code point that is not a surrogate (U+D800 to U+DFFF).
+#[derive(Eq, PartialEq, Ord, PartialOrd, Clone, Copy)]
+#[doc(hidden)]
+pub struct CodePoint(CodePointInner);
+
+/// Format the code point as `U+` followed by four to six hexadecimal digits.
+/// Example: `U+1F4A9`
+impl fmt::Debug for CodePoint {
+    #[inline]
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(formatter, "U+{:04X}", self.0.as_inner())
+    }
+}
+
+impl CodePoint {
+    /// Unsafely creates a new `CodePoint` without checking the value.
+    ///
+    /// Only use when `value` is known to be less than or equal to 0x10FFFF.
+    #[inline]
+    pub unsafe fn from_u32_unchecked(value: u32) -> CodePoint {
+        // SAFETY: Guaranteed by caller.
+        CodePoint(unsafe { CodePointInner::new_unchecked(value) })
+    }
+
+    /// Creates a new `CodePoint` if the value is a valid code point.
+    ///
+    /// Returns `None` if `value` is above 0x10FFFF.
+    #[inline]
+    pub fn from_u32(value: u32) -> Option<CodePoint> {
+        Some(CodePoint(CodePointInner::new(value)?))
+    }
+
+    /// Creates a new `CodePoint` from a `char`.
+    ///
+    /// Since all Unicode scalar values are code points, this always succeeds.
+    #[inline]
+    pub fn from_char(value: char) -> CodePoint {
+        // SAFETY: All char are valid for this type.
+        unsafe { CodePoint::from_u32_unchecked(value as u32) }
+    }
+
+    /// Returns the numeric value of the code point.
+    #[inline]
+    pub fn to_u32(&self) -> u32 {
+        self.0.as_inner()
+    }
+
+    /// Returns the numeric value of the code point if it is a leading surrogate.
+    #[inline]
+    pub fn to_lead_surrogate(&self) -> Option<u16> {
+        match self.to_u32() {
+            lead @ 0xD800..=0xDBFF => Some(lead as u16),
+            _ => None,
+        }
+    }
+
+    /// Returns the numeric value of the code point if it is a trailing surrogate.
+    #[inline]
+    pub fn to_trail_surrogate(&self) -> Option<u16> {
+        match self.to_u32() {
+            trail @ 0xDC00..=0xDFFF => Some(trail as u16),
+            _ => None,
+        }
+    }
+
+    /// Optionally returns a Unicode scalar value for the code point.
+    ///
+    /// Returns `None` if the code point is a surrogate (from U+D800 to U+DFFF).
+    #[inline]
+    pub fn to_char(&self) -> Option<char> {
+        match self.to_u32() {
+            0xD800..=0xDFFF => None,
+            // SAFETY: We explicitly check that the char is valid.
+            valid => Some(unsafe { char::from_u32_unchecked(valid) }),
+        }
+    }
+
+    /// Returns a Unicode scalar value for the code point.
+    ///
+    /// Returns `'\u{FFFD}'` (the replacement character “�”)
+    /// if the code point is a surrogate (from U+D800 to U+DFFF).
+    #[inline]
+    pub fn to_char_lossy(&self) -> char {
+        self.to_char().unwrap_or(char::REPLACEMENT_CHARACTER)
+    }
+}
+
+/// A borrowed slice of well-formed WTF-8 data.
+///
+/// Similar to `&str`, but can additionally contain surrogate code points
+/// if they’re not in a surrogate pair.
+#[derive(Eq, Ord, PartialEq, PartialOrd)]
+#[repr(transparent)]
+#[rustc_has_incoherent_inherent_impls]
+#[doc(hidden)]
+pub struct Wtf8 {
+    bytes: [u8],
+}
+
+impl AsRef<[u8]> for Wtf8 {
+    #[inline]
+    fn as_ref(&self) -> &[u8] {
+        &self.bytes
+    }
+}
+
+/// Formats the string in double quotes, with characters escaped according to
+/// [`char::escape_debug`] and unpaired surrogates represented as `\u{xxxx}`,
+/// where each `x` is a hexadecimal digit.
+impl fmt::Debug for Wtf8 {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fn write_str_escaped(f: &mut fmt::Formatter<'_>, s: &str) -> fmt::Result {
+            use crate::fmt::Write;
+            for c in s.chars().flat_map(|c| c.escape_debug()) {
+                f.write_char(c)?
+            }
+            Ok(())
+        }
+
+        formatter.write_str("\"")?;
+        let mut pos = 0;
+        while let Some((surrogate_pos, surrogate)) = self.next_surrogate(pos) {
+            // SAFETY: next_surrogate provides an index for a range of valid UTF-8 bytes.
+            write_str_escaped(formatter, unsafe {
+                str::from_utf8_unchecked(&self.bytes[pos..surrogate_pos])
+            })?;
+            write!(formatter, "\\u{{{:x}}}", surrogate)?;
+            pos = surrogate_pos + 3;
+        }
+
+        // SAFETY: after next_surrogate returns None, the remainder is valid UTF-8.
+        write_str_escaped(formatter, unsafe { str::from_utf8_unchecked(&self.bytes[pos..]) })?;
+        formatter.write_str("\"")
+    }
+}
+
+/// Formats the string with unpaired surrogates substituted with the replacement
+/// character, U+FFFD.
+impl fmt::Display for Wtf8 {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let wtf8_bytes = &self.bytes;
+        let mut pos = 0;
+        loop {
+            match self.next_surrogate(pos) {
+                Some((surrogate_pos, _)) => {
+                    // SAFETY: next_surrogate provides an index for a range of valid UTF-8 bytes.
+                    formatter.write_str(unsafe {
+                        str::from_utf8_unchecked(&wtf8_bytes[pos..surrogate_pos])
+                    })?;
+                    formatter.write_char(char::REPLACEMENT_CHARACTER)?;
+                    pos = surrogate_pos + 3;
+                }
+                None => {
+                    // SAFETY: after next_surrogate returns None, the remainder is valid UTF-8.
+                    let s = unsafe { str::from_utf8_unchecked(&wtf8_bytes[pos..]) };
+                    if pos == 0 { return s.fmt(formatter) } else { return formatter.write_str(s) }
+                }
+            }
+        }
+    }
+}
+
+impl Wtf8 {
+    /// Creates a WTF-8 slice from a UTF-8 `&str` slice.
+    #[inline]
+    pub fn from_str(value: &str) -> &Wtf8 {
+        // SAFETY: Since WTF-8 is a superset of UTF-8, this always is valid.
+        unsafe { Wtf8::from_bytes_unchecked(value.as_bytes()) }
+    }
+
+    /// Creates a WTF-8 slice from a WTF-8 byte slice.
+    ///
+    /// Since the byte slice is not checked for valid WTF-8, this functions is
+    /// marked unsafe.
+    #[inline]
+    pub unsafe fn from_bytes_unchecked(value: &[u8]) -> &Wtf8 {
+        // SAFETY: start with &[u8], end with fancy &[u8]
+        unsafe { &*(value as *const [u8] as *const Wtf8) }
+    }
+
+    /// Creates a mutable WTF-8 slice from a mutable WTF-8 byte slice.
+    ///
+    /// Since the byte slice is not checked for valid WTF-8, this functions is
+    /// marked unsafe.
+    #[inline]
+    pub unsafe fn from_mut_bytes_unchecked(value: &mut [u8]) -> &mut Wtf8 {
+        // SAFETY: start with &mut [u8], end with fancy &mut [u8]
+        unsafe { &mut *(value as *mut [u8] as *mut Wtf8) }
+    }
+
+    /// Returns the length, in WTF-8 bytes.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.bytes.len()
+    }
+
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.bytes.is_empty()
+    }
+
+    /// Returns the code point at `position` if it is in the ASCII range,
+    /// or `b'\xFF'` otherwise.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `position` is beyond the end of the string.
+    #[inline]
+    pub fn ascii_byte_at(&self, position: usize) -> u8 {
+        match self.bytes[position] {
+            ascii_byte @ 0x00..=0x7F => ascii_byte,
+            _ => 0xFF,
+        }
+    }
+
+    /// Returns an iterator for the string’s code points.
+    #[inline]
+    pub fn code_points(&self) -> Wtf8CodePoints<'_> {
+        Wtf8CodePoints { bytes: self.bytes.iter() }
+    }
+
+    /// Access raw bytes of WTF-8 data
+    #[inline]
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.bytes
+    }
+
+    /// Tries to convert the string to UTF-8 and return a `&str` slice.
+    ///
+    /// Returns `None` if the string contains surrogates.
+    ///
+    /// This does not copy the data.
+    #[inline]
+    pub fn as_str(&self) -> Result<&str, str::Utf8Error> {
+        str::from_utf8(&self.bytes)
+    }
+
+    /// Converts the WTF-8 string to potentially ill-formed UTF-16
+    /// and return an iterator of 16-bit code units.
+    ///
+    /// This is lossless:
+    /// calling `Wtf8Buf::from_ill_formed_utf16` on the resulting code units
+    /// would always return the original WTF-8 string.
+    #[inline]
+    pub fn encode_wide(&self) -> EncodeWide<'_> {
+        EncodeWide { code_points: self.code_points(), extra: 0 }
+    }
+
+    #[inline]
+    pub fn next_surrogate(&self, mut pos: usize) -> Option<(usize, u16)> {
+        let mut iter = self.bytes[pos..].iter();
+        loop {
+            let b = *iter.next()?;
+            if b < 0x80 {
+                pos += 1;
+            } else if b < 0xE0 {
+                iter.next();
+                pos += 2;
+            } else if b == 0xED {
+                match (iter.next(), iter.next()) {
+                    (Some(&b2), Some(&b3)) if b2 >= 0xA0 => {
+                        return Some((pos, decode_surrogate(b2, b3)));
+                    }
+                    _ => pos += 3,
+                }
+            } else if b < 0xF0 {
+                iter.next();
+                iter.next();
+                pos += 3;
+            } else {
+                iter.next();
+                iter.next();
+                iter.next();
+                pos += 4;
+            }
+        }
+    }
+
+    #[inline]
+    pub fn final_lead_surrogate(&self) -> Option<u16> {
+        match self.bytes {
+            [.., 0xED, b2 @ 0xA0..=0xAF, b3] => Some(decode_surrogate(b2, b3)),
+            _ => None,
+        }
+    }
+
+    #[inline]
+    pub fn initial_trail_surrogate(&self) -> Option<u16> {
+        match self.bytes {
+            [0xED, b2 @ 0xB0..=0xBF, b3, ..] => Some(decode_surrogate(b2, b3)),
+            _ => None,
+        }
+    }
+
+    #[inline]
+    pub fn make_ascii_lowercase(&mut self) {
+        self.bytes.make_ascii_lowercase()
+    }
+
+    #[inline]
+    pub fn make_ascii_uppercase(&mut self) {
+        self.bytes.make_ascii_uppercase()
+    }
+
+    #[inline]
+    pub fn is_ascii(&self) -> bool {
+        self.bytes.is_ascii()
+    }
+
+    #[inline]
+    pub fn eq_ignore_ascii_case(&self, other: &Self) -> bool {
+        self.bytes.eq_ignore_ascii_case(&other.bytes)
+    }
+}
+
+/// Returns a slice of the given string for the byte range \[`begin`..`end`).
+///
+/// # Panics
+///
+/// Panics when `begin` and `end` do not point to code point boundaries,
+/// or point beyond the end of the string.
+impl ops::Index<ops::Range<usize>> for Wtf8 {
+    type Output = Wtf8;
+
+    #[inline]
+    fn index(&self, range: ops::Range<usize>) -> &Wtf8 {
+        if range.start <= range.end
+            && self.is_code_point_boundary(range.start)
+            && self.is_code_point_boundary(range.end)
+        {
+            // SAFETY: is_code_point_boundary checks that the index is valid
+            unsafe { slice_unchecked(self, range.start, range.end) }
+        } else {
+            slice_error_fail(self, range.start, range.end)
+        }
+    }
+}
+
+/// Returns a slice of the given string from byte `begin` to its end.
+///
+/// # Panics
+///
+/// Panics when `begin` is not at a code point boundary,
+/// or is beyond the end of the string.
+impl ops::Index<ops::RangeFrom<usize>> for Wtf8 {
+    type Output = Wtf8;
+
+    #[inline]
+    fn index(&self, range: ops::RangeFrom<usize>) -> &Wtf8 {
+        if self.is_code_point_boundary(range.start) {
+            // SAFETY: is_code_point_boundary checks that the index is valid
+            unsafe { slice_unchecked(self, range.start, self.len()) }
+        } else {
+            slice_error_fail(self, range.start, self.len())
+        }
+    }
+}
+
+/// Returns a slice of the given string from its beginning to byte `end`.
+///
+/// # Panics
+///
+/// Panics when `end` is not at a code point boundary,
+/// or is beyond the end of the string.
+impl ops::Index<ops::RangeTo<usize>> for Wtf8 {
+    type Output = Wtf8;
+
+    #[inline]
+    fn index(&self, range: ops::RangeTo<usize>) -> &Wtf8 {
+        if self.is_code_point_boundary(range.end) {
+            // SAFETY: is_code_point_boundary checks that the index is valid
+            unsafe { slice_unchecked(self, 0, range.end) }
+        } else {
+            slice_error_fail(self, 0, range.end)
+        }
+    }
+}
+
+impl ops::Index<ops::RangeFull> for Wtf8 {
+    type Output = Wtf8;
+
+    #[inline]
+    fn index(&self, _range: ops::RangeFull) -> &Wtf8 {
+        self
+    }
+}
+
+#[inline]
+fn decode_surrogate(second_byte: u8, third_byte: u8) -> u16 {
+    // The first byte is assumed to be 0xED
+    0xD800 | (second_byte as u16 & 0x3F) << 6 | third_byte as u16 & 0x3F
+}
+
+impl Wtf8 {
+    /// Copied from str::is_char_boundary
+    #[inline]
+    pub fn is_code_point_boundary(&self, index: usize) -> bool {
+        if index == 0 {
+            return true;
+        }
+        match self.bytes.get(index) {
+            None => index == self.len(),
+            Some(&b) => (b as i8) >= -0x40,
+        }
+    }
+
+    /// Verify that `index` is at the edge of either a valid UTF-8 codepoint
+    /// (i.e. a codepoint that's not a surrogate) or of the whole string.
+    ///
+    /// These are the cases currently permitted by `OsStr::self_encoded_bytes`.
+    /// Splitting between surrogates is valid as far as WTF-8 is concerned, but
+    /// we do not permit it in the public API because WTF-8 is considered an
+    /// implementation detail.
+    #[track_caller]
+    #[inline]
+    pub fn check_utf8_boundary(&self, index: usize) {
+        if index == 0 {
+            return;
+        }
+        match self.bytes.get(index) {
+            Some(0xED) => (), // Might be a surrogate
+            Some(&b) if (b as i8) >= -0x40 => return,
+            Some(_) => panic!("byte index {index} is not a codepoint boundary"),
+            None if index == self.len() => return,
+            None => panic!("byte index {index} is out of bounds"),
+        }
+        if self.bytes[index + 1] >= 0xA0 {
+            // There's a surrogate after index. Now check before index.
+            if index >= 3 && self.bytes[index - 3] == 0xED && self.bytes[index - 2] >= 0xA0 {
+                panic!("byte index {index} lies between surrogate codepoints");
+            }
+        }
+    }
+}
+
+/// Copied from core::str::raw::slice_unchecked
+#[inline]
+unsafe fn slice_unchecked(s: &Wtf8, begin: usize, end: usize) -> &Wtf8 {
+    // SAFETY: memory layout of a &[u8] and &Wtf8 are the same
+    unsafe {
+        let len = end - begin;
+        let start = s.as_bytes().as_ptr().add(begin);
+        Wtf8::from_bytes_unchecked(slice::from_raw_parts(start, len))
+    }
+}
+
+/// Copied from core::str::raw::slice_error_fail
+#[inline(never)]
+fn slice_error_fail(s: &Wtf8, begin: usize, end: usize) -> ! {
+    assert!(begin <= end);
+    panic!("index {begin} and/or {end} in `{s:?}` do not lie on character boundary");
+}
+
+/// Iterator for the code points of a WTF-8 string.
+///
+/// Created with the method `.code_points()`.
+#[derive(Clone)]
+#[doc(hidden)]
+pub struct Wtf8CodePoints<'a> {
+    bytes: slice::Iter<'a, u8>,
+}
+
+impl Iterator for Wtf8CodePoints<'_> {
+    type Item = CodePoint;
+
+    #[inline]
+    fn next(&mut self) -> Option<CodePoint> {
+        // SAFETY: `self.bytes` has been created from a WTF-8 string
+        unsafe { next_code_point(&mut self.bytes).map(|c| CodePoint::from_u32_unchecked(c)) }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.bytes.len();
+        (len.saturating_add(3) / 4, Some(len))
+    }
+}
+
+impl fmt::Debug for Wtf8CodePoints<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Wtf8CodePoints")
+            // SAFETY: We always leave the string in a valid state after each iteration.
+            .field(&unsafe { Wtf8::from_bytes_unchecked(self.bytes.as_slice()) })
+            .finish()
+    }
+}
+
+/// Generates a wide character sequence for potentially ill-formed UTF-16.
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Clone)]
+#[doc(hidden)]
+pub struct EncodeWide<'a> {
+    code_points: Wtf8CodePoints<'a>,
+    extra: u16,
+}
+
+// Copied from libunicode/u_str.rs
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Iterator for EncodeWide<'_> {
+    type Item = u16;
+
+    #[inline]
+    fn next(&mut self) -> Option<u16> {
+        if self.extra != 0 {
+            let tmp = self.extra;
+            self.extra = 0;
+            return Some(tmp);
+        }
+
+        let mut buf = [0; MAX_LEN_UTF16];
+        self.code_points.next().map(|code_point| {
+            let n = encode_utf16_raw(code_point.to_u32(), &mut buf).len();
+            if n == 2 {
+                self.extra = buf[1];
+            }
+            buf[0]
+        })
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (low, high) = self.code_points.size_hint();
+        let ext = (self.extra != 0) as usize;
+        // every code point gets either one u16 or two u16,
+        // so this iterator is between 1 or 2 times as
+        // long as the underlying iterator.
+        (low + ext, high.and_then(|n| n.checked_mul(2)).and_then(|n| n.checked_add(ext)))
+    }
+}
+
+impl fmt::Debug for EncodeWide<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("EncodeWide").finish_non_exhaustive()
+    }
+}
+
+#[stable(feature = "encode_wide_fused_iterator", since = "1.62.0")]
+impl FusedIterator for EncodeWide<'_> {}
+
+impl Hash for CodePoint {
+    #[inline]
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.hash(state)
+    }
+}
+
+impl Hash for Wtf8 {
+    #[inline]
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        state.write(&self.bytes);
+        0xfeu8.hash(state)
+    }
+}
+
+#[unstable(feature = "clone_to_uninit", issue = "126799")]
+unsafe impl CloneToUninit for Wtf8 {
+    #[inline]
+    #[cfg_attr(debug_assertions, track_caller)]
+    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
+        // SAFETY: we're just a transparent wrapper around [u8]
+        unsafe { self.bytes.clone_to_uninit(dst) }
+    }
+}
diff --git a/library/coretests/tests/lib.rs b/library/coretests/tests/lib.rs
index d2281b1df2ffc..4415b61481708 100644
--- a/library/coretests/tests/lib.rs
+++ b/library/coretests/tests/lib.rs
@@ -194,6 +194,7 @@ mod time;
 mod tuple;
 mod unicode;
 mod waker;
+mod wtf8;
 
 /// Copied from `std::test_helpers::test_rng`, see that function for rationale.
 #[track_caller]
diff --git a/library/coretests/tests/wtf8.rs b/library/coretests/tests/wtf8.rs
new file mode 100644
index 0000000000000..9f187e7063015
--- /dev/null
+++ b/library/coretests/tests/wtf8.rs
@@ -0,0 +1 @@
+// All `wtf8` tests live in library/alloctests/tests/wtf8.rs
diff --git a/library/std/src/fs.rs b/library/std/src/fs.rs
index 1ed4f2f9f0c66..d9c9606fc1ce0 100644
--- a/library/std/src/fs.rs
+++ b/library/std/src/fs.rs
@@ -3036,6 +3036,9 @@ pub fn remove_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// Entries for the current and parent directories (typically `.` and `..`) are
 /// skipped.
 ///
+/// The order in which `read_dir` returns entries can change between calls. If reproducible
+/// ordering is required, the entries should be explicitly sorted.
+///
 /// # Platform-specific behavior
 ///
 /// This function currently corresponds to the `opendir` function on Unix
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index f0de03c9a2806..30a1b10881784 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -391,6 +391,7 @@
 #![feature(try_with_capacity)]
 #![feature(unique_rc_arc)]
 #![feature(vec_into_raw_parts)]
+#![feature(wtf8_internals)]
 // tidy-alphabetical-end
 //
 // Library features (unwind):
diff --git a/library/std/src/os/windows/ffi.rs b/library/std/src/os/windows/ffi.rs
index 496443dbbc3ac..345d5b74285e1 100644
--- a/library/std/src/os/windows/ffi.rs
+++ b/library/std/src/os/windows/ffi.rs
@@ -53,12 +53,13 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use alloc::wtf8::Wtf8Buf;
+
 use crate::ffi::{OsStr, OsString};
+use crate::fmt;
+use crate::iter::FusedIterator;
 use crate::sealed::Sealed;
 use crate::sys::os_str::Buf;
-#[stable(feature = "rust1", since = "1.0.0")]
-pub use crate::sys_common::wtf8::EncodeWide;
-use crate::sys_common::wtf8::Wtf8Buf;
 use crate::sys_common::{AsInner, FromInner};
 
 /// Windows-specific extensions to [`OsString`].
@@ -130,6 +131,35 @@ pub trait OsStrExt: Sealed {
 impl OsStrExt for OsStr {
     #[inline]
     fn encode_wide(&self) -> EncodeWide<'_> {
-        self.as_inner().inner.encode_wide()
+        EncodeWide { inner: self.as_inner().inner.encode_wide() }
+    }
+}
+
+/// Iterator returned by [`OsStrExt::encode_wide`].
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Clone)]
+pub struct EncodeWide<'a> {
+    inner: alloc::wtf8::EncodeWide<'a>,
+}
+#[stable(feature = "encode_wide_debug", since = "CURRENT_RUSTC_VERSION")]
+impl fmt::Debug for EncodeWide<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.inner, f)
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Iterator for EncodeWide<'_> {
+    type Item = u16;
+
+    #[inline]
+    fn next(&mut self) -> Option<u16> {
+        self.inner.next()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
     }
 }
+#[stable(feature = "encode_wide_fused_iterator", since = "1.62.0")]
+impl FusedIterator for EncodeWide<'_> {}
diff --git a/library/std/src/sync/mpmc/utils.rs b/library/std/src/sync/mpmc/utils.rs
index 0cbc61160f7ee..e3bcb149f648b 100644
--- a/library/std/src/sync/mpmc/utils.rs
+++ b/library/std/src/sync/mpmc/utils.rs
@@ -23,14 +23,13 @@ use crate::ops::{Deref, DerefMut};
     any(target_arch = "x86_64", target_arch = "aarch64", target_arch = "powerpc64",),
     repr(align(128))
 )]
-// arm, mips, mips64, and riscv64 have 32-byte cache line size.
+// arm, mips and mips64 have 32-byte cache line size.
 //
 // Sources:
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_arm.go#L7
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mips.go#L7
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mipsle.go#L7
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mips64x.go#L9
-// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_riscv64.go#L7
 #[cfg_attr(
     any(
         target_arch = "arm",
@@ -38,7 +37,6 @@ use crate::ops::{Deref, DerefMut};
         target_arch = "mips32r6",
         target_arch = "mips64",
         target_arch = "mips64r6",
-        target_arch = "riscv64",
     ),
     repr(align(32))
 )]
@@ -47,11 +45,12 @@ use crate::ops::{Deref, DerefMut};
 // Sources:
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_s390x.go#L7
 #[cfg_attr(target_arch = "s390x", repr(align(256)))]
-// x86 and wasm have 64-byte cache line size.
+// x86, wasm and riscv have 64-byte cache line size.
 //
 // Sources:
 // - https://github.com/golang/go/blob/dda2991c2ea0c5914714469c4defc2562a907230/src/internal/cpu/cpu_x86.go#L9
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_wasm.go#L7
+// - https://github.com/golang/go/blob/5e31f78c8a4ed1b872ddc194f0cd1ae931b37d7e/src/internal/cpu/cpu_riscv64.go#L7
 //
 // All others are assumed to have 64-byte cache line size.
 #[cfg_attr(
@@ -64,7 +63,6 @@ use crate::ops::{Deref, DerefMut};
         target_arch = "mips32r6",
         target_arch = "mips64",
         target_arch = "mips64r6",
-        target_arch = "riscv64",
         target_arch = "s390x",
     )),
     repr(align(64))
diff --git a/library/std/src/sys/os_str/wtf8.rs b/library/std/src/sys/os_str/wtf8.rs
index bbc704ebf8697..96da891874ef0 100644
--- a/library/std/src/sys/os_str/wtf8.rs
+++ b/library/std/src/sys/os_str/wtf8.rs
@@ -1,12 +1,12 @@
 //! The underlying OsString/OsStr implementation on Windows is a
 //! wrapper around the "WTF-8" encoding; see the `wtf8` module for more.
+use alloc::wtf8::{Wtf8, Wtf8Buf};
 use core::clone::CloneToUninit;
 
 use crate::borrow::Cow;
 use crate::collections::TryReserveError;
 use crate::rc::Rc;
 use crate::sync::Arc;
-use crate::sys_common::wtf8::{Wtf8, Wtf8Buf, check_utf8_boundary};
 use crate::sys_common::{AsInner, FromInner, IntoInner};
 use crate::{fmt, mem};
 
@@ -220,7 +220,9 @@ impl Buf {
     /// trailing surrogate half.
     #[inline]
     pub unsafe fn extend_from_slice_unchecked(&mut self, other: &[u8]) {
-        self.inner.extend_from_slice(other);
+        unsafe {
+            self.inner.extend_from_slice_unchecked(other);
+        }
     }
 }
 
@@ -238,7 +240,7 @@ impl Slice {
     #[track_caller]
     #[inline]
     pub fn check_public_boundary(&self, index: usize) {
-        check_utf8_boundary(&self.inner, index);
+        self.inner.check_utf8_boundary(index);
     }
 
     #[inline]
diff --git a/library/std/src/sys_common/mod.rs b/library/std/src/sys_common/mod.rs
index 24b6cff130974..ec45c723e0de5 100644
--- a/library/std/src/sys_common/mod.rs
+++ b/library/std/src/sys_common/mod.rs
@@ -21,7 +21,6 @@
 mod tests;
 
 pub mod wstr;
-pub mod wtf8;
 
 // common error constructors
 
diff --git a/library/std/src/sys_common/wtf8.rs b/library/std/src/sys_common/wtf8.rs
deleted file mode 100644
index 50bde88b5a4c3..0000000000000
--- a/library/std/src/sys_common/wtf8.rs
+++ /dev/null
@@ -1,1083 +0,0 @@
-//! Implementation of [the WTF-8 encoding](https://simonsapin.github.io/wtf-8/).
-//!
-//! This library uses Rust’s type system to maintain
-//! [well-formedness](https://simonsapin.github.io/wtf-8/#well-formed),
-//! like the `String` and `&str` types do for UTF-8.
-//!
-//! Since [WTF-8 must not be used
-//! for interchange](https://simonsapin.github.io/wtf-8/#intended-audience),
-//! this library deliberately does not provide access to the underlying bytes
-//! of WTF-8 strings,
-//! nor can it decode WTF-8 from arbitrary bytes.
-//! WTF-8 strings can be obtained from UTF-8, UTF-16, or code points.
-
-// this module is imported from @SimonSapin's repo and has tons of dead code on
-// unix (it's mostly used on windows), so don't worry about dead code here.
-#![allow(dead_code)]
-
-#[cfg(test)]
-mod tests;
-
-use core::char::{MAX_LEN_UTF8, MAX_LEN_UTF16, encode_utf8_raw, encode_utf16_raw};
-use core::clone::CloneToUninit;
-use core::str::next_code_point;
-
-use crate::borrow::Cow;
-use crate::collections::TryReserveError;
-use crate::hash::{Hash, Hasher};
-use crate::iter::FusedIterator;
-use crate::rc::Rc;
-use crate::sync::Arc;
-use crate::sys_common::AsInner;
-use crate::{fmt, mem, ops, slice, str};
-
-const UTF8_REPLACEMENT_CHARACTER: &str = "\u{FFFD}";
-
-/// A Unicode code point: from U+0000 to U+10FFFF.
-///
-/// Compares with the `char` type,
-/// which represents a Unicode scalar value:
-/// a code point that is not a surrogate (U+D800 to U+DFFF).
-#[derive(Eq, PartialEq, Ord, PartialOrd, Clone, Copy)]
-pub struct CodePoint {
-    value: u32,
-}
-
-/// Format the code point as `U+` followed by four to six hexadecimal digits.
-/// Example: `U+1F4A9`
-impl fmt::Debug for CodePoint {
-    #[inline]
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(formatter, "U+{:04X}", self.value)
-    }
-}
-
-impl CodePoint {
-    /// Unsafely creates a new `CodePoint` without checking the value.
-    ///
-    /// Only use when `value` is known to be less than or equal to 0x10FFFF.
-    #[inline]
-    pub unsafe fn from_u32_unchecked(value: u32) -> CodePoint {
-        CodePoint { value }
-    }
-
-    /// Creates a new `CodePoint` if the value is a valid code point.
-    ///
-    /// Returns `None` if `value` is above 0x10FFFF.
-    #[inline]
-    pub fn from_u32(value: u32) -> Option<CodePoint> {
-        match value {
-            0..=0x10FFFF => Some(CodePoint { value }),
-            _ => None,
-        }
-    }
-
-    /// Creates a new `CodePoint` from a `char`.
-    ///
-    /// Since all Unicode scalar values are code points, this always succeeds.
-    #[inline]
-    pub fn from_char(value: char) -> CodePoint {
-        CodePoint { value: value as u32 }
-    }
-
-    /// Returns the numeric value of the code point.
-    #[inline]
-    pub fn to_u32(&self) -> u32 {
-        self.value
-    }
-
-    /// Returns the numeric value of the code point if it is a leading surrogate.
-    #[inline]
-    pub fn to_lead_surrogate(&self) -> Option<u16> {
-        match self.value {
-            lead @ 0xD800..=0xDBFF => Some(lead as u16),
-            _ => None,
-        }
-    }
-
-    /// Returns the numeric value of the code point if it is a trailing surrogate.
-    #[inline]
-    pub fn to_trail_surrogate(&self) -> Option<u16> {
-        match self.value {
-            trail @ 0xDC00..=0xDFFF => Some(trail as u16),
-            _ => None,
-        }
-    }
-
-    /// Optionally returns a Unicode scalar value for the code point.
-    ///
-    /// Returns `None` if the code point is a surrogate (from U+D800 to U+DFFF).
-    #[inline]
-    pub fn to_char(&self) -> Option<char> {
-        match self.value {
-            0xD800..=0xDFFF => None,
-            _ => Some(unsafe { char::from_u32_unchecked(self.value) }),
-        }
-    }
-
-    /// Returns a Unicode scalar value for the code point.
-    ///
-    /// Returns `'\u{FFFD}'` (the replacement character “�”)
-    /// if the code point is a surrogate (from U+D800 to U+DFFF).
-    #[inline]
-    pub fn to_char_lossy(&self) -> char {
-        self.to_char().unwrap_or('\u{FFFD}')
-    }
-}
-
-/// An owned, growable string of well-formed WTF-8 data.
-///
-/// Similar to `String`, but can additionally contain surrogate code points
-/// if they’re not in a surrogate pair.
-#[derive(Eq, PartialEq, Ord, PartialOrd, Clone)]
-pub struct Wtf8Buf {
-    bytes: Vec<u8>,
-
-    /// Do we know that `bytes` holds a valid UTF-8 encoding? We can easily
-    /// know this if we're constructed from a `String` or `&str`.
-    ///
-    /// It is possible for `bytes` to have valid UTF-8 without this being
-    /// set, such as when we're concatenating `&Wtf8`'s and surrogates become
-    /// paired, as we don't bother to rescan the entire string.
-    is_known_utf8: bool,
-}
-
-impl ops::Deref for Wtf8Buf {
-    type Target = Wtf8;
-
-    fn deref(&self) -> &Wtf8 {
-        self.as_slice()
-    }
-}
-
-impl ops::DerefMut for Wtf8Buf {
-    fn deref_mut(&mut self) -> &mut Wtf8 {
-        self.as_mut_slice()
-    }
-}
-
-/// Formats the string in double quotes, with characters escaped according to
-/// [`char::escape_debug`] and unpaired surrogates represented as `\u{xxxx}`,
-/// where each `x` is a hexadecimal digit.
-///
-/// For example, the code units [U+0061, U+D800, U+000A] are formatted as
-/// `"a\u{D800}\n"`.
-impl fmt::Debug for Wtf8Buf {
-    #[inline]
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&**self, formatter)
-    }
-}
-
-/// Formats the string with unpaired surrogates substituted with the replacement
-/// character, U+FFFD.
-impl fmt::Display for Wtf8Buf {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        if let Some(s) = self.as_known_utf8() {
-            fmt::Display::fmt(s, formatter)
-        } else {
-            fmt::Display::fmt(&**self, formatter)
-        }
-    }
-}
-
-impl Wtf8Buf {
-    /// Creates a new, empty WTF-8 string.
-    #[inline]
-    pub fn new() -> Wtf8Buf {
-        Wtf8Buf { bytes: Vec::new(), is_known_utf8: true }
-    }
-
-    /// Creates a new, empty WTF-8 string with pre-allocated capacity for `capacity` bytes.
-    #[inline]
-    pub fn with_capacity(capacity: usize) -> Wtf8Buf {
-        Wtf8Buf { bytes: Vec::with_capacity(capacity), is_known_utf8: true }
-    }
-
-    /// Creates a WTF-8 string from a WTF-8 byte vec.
-    ///
-    /// Since the byte vec is not checked for valid WTF-8, this function is
-    /// marked unsafe.
-    #[inline]
-    pub unsafe fn from_bytes_unchecked(value: Vec<u8>) -> Wtf8Buf {
-        Wtf8Buf { bytes: value, is_known_utf8: false }
-    }
-
-    /// Creates a WTF-8 string from a UTF-8 `String`.
-    ///
-    /// This takes ownership of the `String` and does not copy.
-    ///
-    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
-    #[inline]
-    pub const fn from_string(string: String) -> Wtf8Buf {
-        Wtf8Buf { bytes: string.into_bytes(), is_known_utf8: true }
-    }
-
-    /// Creates a WTF-8 string from a UTF-8 `&str` slice.
-    ///
-    /// This copies the content of the slice.
-    ///
-    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
-    #[inline]
-    pub fn from_str(s: &str) -> Wtf8Buf {
-        Wtf8Buf { bytes: s.as_bytes().to_vec(), is_known_utf8: true }
-    }
-
-    pub fn clear(&mut self) {
-        self.bytes.clear();
-        self.is_known_utf8 = true;
-    }
-
-    /// Creates a WTF-8 string from a potentially ill-formed UTF-16 slice of 16-bit code units.
-    ///
-    /// This is lossless: calling `.encode_wide()` on the resulting string
-    /// will always return the original code units.
-    pub fn from_wide(v: &[u16]) -> Wtf8Buf {
-        let mut string = Wtf8Buf::with_capacity(v.len());
-        for item in char::decode_utf16(v.iter().cloned()) {
-            match item {
-                Ok(ch) => string.push_char(ch),
-                Err(surrogate) => {
-                    let surrogate = surrogate.unpaired_surrogate();
-                    // Surrogates are known to be in the code point range.
-                    let code_point = unsafe { CodePoint::from_u32_unchecked(surrogate as u32) };
-                    // The string will now contain an unpaired surrogate.
-                    string.is_known_utf8 = false;
-                    // Skip the WTF-8 concatenation check,
-                    // surrogate pairs are already decoded by decode_utf16
-                    string.push_code_point_unchecked(code_point);
-                }
-            }
-        }
-        string
-    }
-
-    /// Appends the given `char` to the end of this string.
-    /// This does **not** include the WTF-8 concatenation check or `is_known_utf8` check.
-    /// Copied from String::push.
-    fn push_code_point_unchecked(&mut self, code_point: CodePoint) {
-        let mut bytes = [0; MAX_LEN_UTF8];
-        let bytes = encode_utf8_raw(code_point.value, &mut bytes);
-        self.bytes.extend_from_slice(bytes)
-    }
-
-    #[inline]
-    pub fn as_slice(&self) -> &Wtf8 {
-        unsafe { Wtf8::from_bytes_unchecked(&self.bytes) }
-    }
-
-    #[inline]
-    pub fn as_mut_slice(&mut self) -> &mut Wtf8 {
-        // Safety: `Wtf8` doesn't expose any way to mutate the bytes that would
-        // cause them to change from well-formed UTF-8 to ill-formed UTF-8,
-        // which would break the assumptions of the `is_known_utf8` field.
-        unsafe { Wtf8::from_mut_bytes_unchecked(&mut self.bytes) }
-    }
-
-    /// Converts the string to UTF-8 without validation, if it was created from
-    /// valid UTF-8.
-    #[inline]
-    fn as_known_utf8(&self) -> Option<&str> {
-        if self.is_known_utf8 {
-            // SAFETY: The buffer is known to be valid UTF-8.
-            Some(unsafe { str::from_utf8_unchecked(self.as_bytes()) })
-        } else {
-            None
-        }
-    }
-
-    /// Reserves capacity for at least `additional` more bytes to be inserted
-    /// in the given `Wtf8Buf`.
-    /// The collection may reserve more space to avoid frequent reallocations.
-    ///
-    /// # Panics
-    ///
-    /// Panics if the new capacity exceeds `isize::MAX` bytes.
-    #[inline]
-    pub fn reserve(&mut self, additional: usize) {
-        self.bytes.reserve(additional)
-    }
-
-    /// Tries to reserve capacity for at least `additional` more bytes to be
-    /// inserted in the given `Wtf8Buf`. The `Wtf8Buf` may reserve more space to
-    /// avoid frequent reallocations. After calling `try_reserve`, capacity will
-    /// be greater than or equal to `self.len() + additional`. Does nothing if
-    /// capacity is already sufficient. This method preserves the contents even
-    /// if an error occurs.
-    ///
-    /// # Errors
-    ///
-    /// If the capacity overflows, or the allocator reports a failure, then an error
-    /// is returned.
-    #[inline]
-    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
-        self.bytes.try_reserve(additional)
-    }
-
-    #[inline]
-    pub fn reserve_exact(&mut self, additional: usize) {
-        self.bytes.reserve_exact(additional)
-    }
-
-    /// Tries to reserve the minimum capacity for exactly `additional` more
-    /// bytes to be inserted in the given `Wtf8Buf`. After calling
-    /// `try_reserve_exact`, capacity will be greater than or equal to
-    /// `self.len() + additional` if it returns `Ok(())`.
-    /// Does nothing if the capacity is already sufficient.
-    ///
-    /// Note that the allocator may give the `Wtf8Buf` more space than it
-    /// requests. Therefore, capacity can not be relied upon to be precisely
-    /// minimal. Prefer [`try_reserve`] if future insertions are expected.
-    ///
-    /// [`try_reserve`]: Wtf8Buf::try_reserve
-    ///
-    /// # Errors
-    ///
-    /// If the capacity overflows, or the allocator reports a failure, then an error
-    /// is returned.
-    #[inline]
-    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
-        self.bytes.try_reserve_exact(additional)
-    }
-
-    #[inline]
-    pub fn shrink_to_fit(&mut self) {
-        self.bytes.shrink_to_fit()
-    }
-
-    #[inline]
-    pub fn shrink_to(&mut self, min_capacity: usize) {
-        self.bytes.shrink_to(min_capacity)
-    }
-
-    #[inline]
-    pub fn leak<'a>(self) -> &'a mut Wtf8 {
-        unsafe { Wtf8::from_mut_bytes_unchecked(self.bytes.leak()) }
-    }
-
-    /// Returns the number of bytes that this string buffer can hold without reallocating.
-    #[inline]
-    pub fn capacity(&self) -> usize {
-        self.bytes.capacity()
-    }
-
-    /// Append a UTF-8 slice at the end of the string.
-    #[inline]
-    pub fn push_str(&mut self, other: &str) {
-        self.bytes.extend_from_slice(other.as_bytes())
-    }
-
-    /// Append a WTF-8 slice at the end of the string.
-    ///
-    /// This replaces newly paired surrogates at the boundary
-    /// with a supplementary code point,
-    /// like concatenating ill-formed UTF-16 strings effectively would.
-    #[inline]
-    pub fn push_wtf8(&mut self, other: &Wtf8) {
-        match ((&*self).final_lead_surrogate(), other.initial_trail_surrogate()) {
-            // Replace newly paired surrogates by a supplementary code point.
-            (Some(lead), Some(trail)) => {
-                let len_without_lead_surrogate = self.len() - 3;
-                self.bytes.truncate(len_without_lead_surrogate);
-                let other_without_trail_surrogate = &other.bytes[3..];
-                // 4 bytes for the supplementary code point
-                self.bytes.reserve(4 + other_without_trail_surrogate.len());
-                self.push_char(decode_surrogate_pair(lead, trail));
-                self.bytes.extend_from_slice(other_without_trail_surrogate);
-            }
-            _ => {
-                // If we'll be pushing a string containing a surrogate, we may
-                // no longer have UTF-8.
-                if self.is_known_utf8 && other.next_surrogate(0).is_some() {
-                    self.is_known_utf8 = false;
-                }
-
-                self.bytes.extend_from_slice(&other.bytes);
-            }
-        }
-    }
-
-    /// Append a Unicode scalar value at the end of the string.
-    #[inline]
-    pub fn push_char(&mut self, c: char) {
-        self.push_code_point_unchecked(CodePoint::from_char(c))
-    }
-
-    /// Append a code point at the end of the string.
-    ///
-    /// This replaces newly paired surrogates at the boundary
-    /// with a supplementary code point,
-    /// like concatenating ill-formed UTF-16 strings effectively would.
-    #[inline]
-    pub fn push(&mut self, code_point: CodePoint) {
-        if let Some(trail) = code_point.to_trail_surrogate() {
-            if let Some(lead) = (&*self).final_lead_surrogate() {
-                let len_without_lead_surrogate = self.len() - 3;
-                self.bytes.truncate(len_without_lead_surrogate);
-                self.push_char(decode_surrogate_pair(lead, trail));
-                return;
-            }
-
-            // We're pushing a trailing surrogate.
-            self.is_known_utf8 = false;
-        } else if code_point.to_lead_surrogate().is_some() {
-            // We're pushing a leading surrogate.
-            self.is_known_utf8 = false;
-        }
-
-        // No newly paired surrogates at the boundary.
-        self.push_code_point_unchecked(code_point)
-    }
-
-    /// Shortens a string to the specified length.
-    ///
-    /// # Panics
-    ///
-    /// Panics if `new_len` > current length,
-    /// or if `new_len` is not a code point boundary.
-    #[inline]
-    pub fn truncate(&mut self, new_len: usize) {
-        assert!(is_code_point_boundary(self, new_len));
-        self.bytes.truncate(new_len)
-    }
-
-    /// Consumes the WTF-8 string and tries to convert it to a vec of bytes.
-    #[inline]
-    pub fn into_bytes(self) -> Vec<u8> {
-        self.bytes
-    }
-
-    /// Consumes the WTF-8 string and tries to convert it to UTF-8.
-    ///
-    /// This does not copy the data.
-    ///
-    /// If the contents are not well-formed UTF-8
-    /// (that is, if the string contains surrogates),
-    /// the original WTF-8 string is returned instead.
-    pub fn into_string(self) -> Result<String, Wtf8Buf> {
-        if self.is_known_utf8 || self.next_surrogate(0).is_none() {
-            Ok(unsafe { String::from_utf8_unchecked(self.bytes) })
-        } else {
-            Err(self)
-        }
-    }
-
-    /// Consumes the WTF-8 string and converts it lossily to UTF-8.
-    ///
-    /// This does not copy the data (but may overwrite parts of it in place).
-    ///
-    /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”)
-    pub fn into_string_lossy(mut self) -> String {
-        if !self.is_known_utf8 {
-            let mut pos = 0;
-            while let Some((surrogate_pos, _)) = self.next_surrogate(pos) {
-                pos = surrogate_pos + 3;
-                // Surrogates and the replacement character are all 3 bytes, so
-                // they can substituted in-place.
-                self.bytes[surrogate_pos..pos]
-                    .copy_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
-            }
-        }
-        unsafe { String::from_utf8_unchecked(self.bytes) }
-    }
-
-    /// Converts this `Wtf8Buf` into a boxed `Wtf8`.
-    #[inline]
-    pub fn into_box(self) -> Box<Wtf8> {
-        // SAFETY: relies on `Wtf8` being `repr(transparent)`.
-        unsafe { mem::transmute(self.bytes.into_boxed_slice()) }
-    }
-
-    /// Converts a `Box<Wtf8>` into a `Wtf8Buf`.
-    pub fn from_box(boxed: Box<Wtf8>) -> Wtf8Buf {
-        let bytes: Box<[u8]> = unsafe { mem::transmute(boxed) };
-        Wtf8Buf { bytes: bytes.into_vec(), is_known_utf8: false }
-    }
-
-    /// Provides plumbing to core `Vec::extend_from_slice`.
-    /// More well behaving alternative to allowing outer types
-    /// full mutable access to the core `Vec`.
-    #[inline]
-    pub(crate) fn extend_from_slice(&mut self, other: &[u8]) {
-        self.bytes.extend_from_slice(other);
-        self.is_known_utf8 = false;
-    }
-}
-
-/// Creates a new WTF-8 string from an iterator of code points.
-///
-/// This replaces surrogate code point pairs with supplementary code points,
-/// like concatenating ill-formed UTF-16 strings effectively would.
-impl FromIterator<CodePoint> for Wtf8Buf {
-    fn from_iter<T: IntoIterator<Item = CodePoint>>(iter: T) -> Wtf8Buf {
-        let mut string = Wtf8Buf::new();
-        string.extend(iter);
-        string
-    }
-}
-
-/// Append code points from an iterator to the string.
-///
-/// This replaces surrogate code point pairs with supplementary code points,
-/// like concatenating ill-formed UTF-16 strings effectively would.
-impl Extend<CodePoint> for Wtf8Buf {
-    fn extend<T: IntoIterator<Item = CodePoint>>(&mut self, iter: T) {
-        let iterator = iter.into_iter();
-        let (low, _high) = iterator.size_hint();
-        // Lower bound of one byte per code point (ASCII only)
-        self.bytes.reserve(low);
-        iterator.for_each(move |code_point| self.push(code_point));
-    }
-
-    #[inline]
-    fn extend_one(&mut self, code_point: CodePoint) {
-        self.push(code_point);
-    }
-
-    #[inline]
-    fn extend_reserve(&mut self, additional: usize) {
-        // Lower bound of one byte per code point (ASCII only)
-        self.bytes.reserve(additional);
-    }
-}
-
-/// A borrowed slice of well-formed WTF-8 data.
-///
-/// Similar to `&str`, but can additionally contain surrogate code points
-/// if they’re not in a surrogate pair.
-#[derive(Eq, Ord, PartialEq, PartialOrd)]
-#[repr(transparent)]
-pub struct Wtf8 {
-    bytes: [u8],
-}
-
-impl AsInner<[u8]> for Wtf8 {
-    #[inline]
-    fn as_inner(&self) -> &[u8] {
-        &self.bytes
-    }
-}
-
-/// Formats the string in double quotes, with characters escaped according to
-/// [`char::escape_debug`] and unpaired surrogates represented as `\u{xxxx}`,
-/// where each `x` is a hexadecimal digit.
-impl fmt::Debug for Wtf8 {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fn write_str_escaped(f: &mut fmt::Formatter<'_>, s: &str) -> fmt::Result {
-            use crate::fmt::Write;
-            for c in s.chars().flat_map(|c| c.escape_debug()) {
-                f.write_char(c)?
-            }
-            Ok(())
-        }
-
-        formatter.write_str("\"")?;
-        let mut pos = 0;
-        while let Some((surrogate_pos, surrogate)) = self.next_surrogate(pos) {
-            write_str_escaped(formatter, unsafe {
-                str::from_utf8_unchecked(&self.bytes[pos..surrogate_pos])
-            })?;
-            write!(formatter, "\\u{{{:x}}}", surrogate)?;
-            pos = surrogate_pos + 3;
-        }
-        write_str_escaped(formatter, unsafe { str::from_utf8_unchecked(&self.bytes[pos..]) })?;
-        formatter.write_str("\"")
-    }
-}
-
-/// Formats the string with unpaired surrogates substituted with the replacement
-/// character, U+FFFD.
-impl fmt::Display for Wtf8 {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let wtf8_bytes = &self.bytes;
-        let mut pos = 0;
-        loop {
-            match self.next_surrogate(pos) {
-                Some((surrogate_pos, _)) => {
-                    formatter.write_str(unsafe {
-                        str::from_utf8_unchecked(&wtf8_bytes[pos..surrogate_pos])
-                    })?;
-                    formatter.write_str(UTF8_REPLACEMENT_CHARACTER)?;
-                    pos = surrogate_pos + 3;
-                }
-                None => {
-                    let s = unsafe { str::from_utf8_unchecked(&wtf8_bytes[pos..]) };
-                    if pos == 0 { return s.fmt(formatter) } else { return formatter.write_str(s) }
-                }
-            }
-        }
-    }
-}
-
-impl Wtf8 {
-    /// Creates a WTF-8 slice from a UTF-8 `&str` slice.
-    ///
-    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
-    #[inline]
-    pub fn from_str(value: &str) -> &Wtf8 {
-        unsafe { Wtf8::from_bytes_unchecked(value.as_bytes()) }
-    }
-
-    /// Creates a WTF-8 slice from a WTF-8 byte slice.
-    ///
-    /// Since the byte slice is not checked for valid WTF-8, this functions is
-    /// marked unsafe.
-    #[inline]
-    pub unsafe fn from_bytes_unchecked(value: &[u8]) -> &Wtf8 {
-        // SAFETY: start with &[u8], end with fancy &[u8]
-        unsafe { &*(value as *const [u8] as *const Wtf8) }
-    }
-
-    /// Creates a mutable WTF-8 slice from a mutable WTF-8 byte slice.
-    ///
-    /// Since the byte slice is not checked for valid WTF-8, this functions is
-    /// marked unsafe.
-    #[inline]
-    unsafe fn from_mut_bytes_unchecked(value: &mut [u8]) -> &mut Wtf8 {
-        // SAFETY: start with &mut [u8], end with fancy &mut [u8]
-        unsafe { &mut *(value as *mut [u8] as *mut Wtf8) }
-    }
-
-    /// Returns the length, in WTF-8 bytes.
-    #[inline]
-    pub fn len(&self) -> usize {
-        self.bytes.len()
-    }
-
-    #[inline]
-    pub fn is_empty(&self) -> bool {
-        self.bytes.is_empty()
-    }
-
-    /// Returns the code point at `position` if it is in the ASCII range,
-    /// or `b'\xFF'` otherwise.
-    ///
-    /// # Panics
-    ///
-    /// Panics if `position` is beyond the end of the string.
-    #[inline]
-    pub fn ascii_byte_at(&self, position: usize) -> u8 {
-        match self.bytes[position] {
-            ascii_byte @ 0x00..=0x7F => ascii_byte,
-            _ => 0xFF,
-        }
-    }
-
-    /// Returns an iterator for the string’s code points.
-    #[inline]
-    pub fn code_points(&self) -> Wtf8CodePoints<'_> {
-        Wtf8CodePoints { bytes: self.bytes.iter() }
-    }
-
-    /// Access raw bytes of WTF-8 data
-    #[inline]
-    pub fn as_bytes(&self) -> &[u8] {
-        &self.bytes
-    }
-
-    /// Tries to convert the string to UTF-8 and return a `&str` slice.
-    ///
-    /// Returns `None` if the string contains surrogates.
-    ///
-    /// This does not copy the data.
-    #[inline]
-    pub fn as_str(&self) -> Result<&str, str::Utf8Error> {
-        str::from_utf8(&self.bytes)
-    }
-
-    /// Creates an owned `Wtf8Buf` from a borrowed `Wtf8`.
-    pub fn to_owned(&self) -> Wtf8Buf {
-        Wtf8Buf { bytes: self.bytes.to_vec(), is_known_utf8: false }
-    }
-
-    /// Lossily converts the string to UTF-8.
-    /// Returns a UTF-8 `&str` slice if the contents are well-formed in UTF-8.
-    ///
-    /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”).
-    ///
-    /// This only copies the data if necessary (if it contains any surrogate).
-    pub fn to_string_lossy(&self) -> Cow<'_, str> {
-        let Some((surrogate_pos, _)) = self.next_surrogate(0) else {
-            return Cow::Borrowed(unsafe { str::from_utf8_unchecked(&self.bytes) });
-        };
-        let wtf8_bytes = &self.bytes;
-        let mut utf8_bytes = Vec::with_capacity(self.len());
-        utf8_bytes.extend_from_slice(&wtf8_bytes[..surrogate_pos]);
-        utf8_bytes.extend_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
-        let mut pos = surrogate_pos + 3;
-        loop {
-            match self.next_surrogate(pos) {
-                Some((surrogate_pos, _)) => {
-                    utf8_bytes.extend_from_slice(&wtf8_bytes[pos..surrogate_pos]);
-                    utf8_bytes.extend_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
-                    pos = surrogate_pos + 3;
-                }
-                None => {
-                    utf8_bytes.extend_from_slice(&wtf8_bytes[pos..]);
-                    return Cow::Owned(unsafe { String::from_utf8_unchecked(utf8_bytes) });
-                }
-            }
-        }
-    }
-
-    /// Converts the WTF-8 string to potentially ill-formed UTF-16
-    /// and return an iterator of 16-bit code units.
-    ///
-    /// This is lossless:
-    /// calling `Wtf8Buf::from_ill_formed_utf16` on the resulting code units
-    /// would always return the original WTF-8 string.
-    #[inline]
-    pub fn encode_wide(&self) -> EncodeWide<'_> {
-        EncodeWide { code_points: self.code_points(), extra: 0 }
-    }
-
-    #[inline]
-    fn next_surrogate(&self, mut pos: usize) -> Option<(usize, u16)> {
-        let mut iter = self.bytes[pos..].iter();
-        loop {
-            let b = *iter.next()?;
-            if b < 0x80 {
-                pos += 1;
-            } else if b < 0xE0 {
-                iter.next();
-                pos += 2;
-            } else if b == 0xED {
-                match (iter.next(), iter.next()) {
-                    (Some(&b2), Some(&b3)) if b2 >= 0xA0 => {
-                        return Some((pos, decode_surrogate(b2, b3)));
-                    }
-                    _ => pos += 3,
-                }
-            } else if b < 0xF0 {
-                iter.next();
-                iter.next();
-                pos += 3;
-            } else {
-                iter.next();
-                iter.next();
-                iter.next();
-                pos += 4;
-            }
-        }
-    }
-
-    #[inline]
-    fn final_lead_surrogate(&self) -> Option<u16> {
-        match self.bytes {
-            [.., 0xED, b2 @ 0xA0..=0xAF, b3] => Some(decode_surrogate(b2, b3)),
-            _ => None,
-        }
-    }
-
-    #[inline]
-    fn initial_trail_surrogate(&self) -> Option<u16> {
-        match self.bytes {
-            [0xED, b2 @ 0xB0..=0xBF, b3, ..] => Some(decode_surrogate(b2, b3)),
-            _ => None,
-        }
-    }
-
-    pub fn clone_into(&self, buf: &mut Wtf8Buf) {
-        buf.is_known_utf8 = false;
-        self.bytes.clone_into(&mut buf.bytes);
-    }
-
-    /// Boxes this `Wtf8`.
-    #[inline]
-    pub fn into_box(&self) -> Box<Wtf8> {
-        let boxed: Box<[u8]> = self.bytes.into();
-        unsafe { mem::transmute(boxed) }
-    }
-
-    /// Creates a boxed, empty `Wtf8`.
-    pub fn empty_box() -> Box<Wtf8> {
-        let boxed: Box<[u8]> = Default::default();
-        unsafe { mem::transmute(boxed) }
-    }
-
-    #[inline]
-    pub fn into_arc(&self) -> Arc<Wtf8> {
-        let arc: Arc<[u8]> = Arc::from(&self.bytes);
-        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const Wtf8) }
-    }
-
-    #[inline]
-    pub fn into_rc(&self) -> Rc<Wtf8> {
-        let rc: Rc<[u8]> = Rc::from(&self.bytes);
-        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const Wtf8) }
-    }
-
-    #[inline]
-    pub fn make_ascii_lowercase(&mut self) {
-        self.bytes.make_ascii_lowercase()
-    }
-
-    #[inline]
-    pub fn make_ascii_uppercase(&mut self) {
-        self.bytes.make_ascii_uppercase()
-    }
-
-    #[inline]
-    pub fn to_ascii_lowercase(&self) -> Wtf8Buf {
-        Wtf8Buf { bytes: self.bytes.to_ascii_lowercase(), is_known_utf8: false }
-    }
-
-    #[inline]
-    pub fn to_ascii_uppercase(&self) -> Wtf8Buf {
-        Wtf8Buf { bytes: self.bytes.to_ascii_uppercase(), is_known_utf8: false }
-    }
-
-    #[inline]
-    pub fn is_ascii(&self) -> bool {
-        self.bytes.is_ascii()
-    }
-
-    #[inline]
-    pub fn eq_ignore_ascii_case(&self, other: &Self) -> bool {
-        self.bytes.eq_ignore_ascii_case(&other.bytes)
-    }
-}
-
-/// Returns a slice of the given string for the byte range \[`begin`..`end`).
-///
-/// # Panics
-///
-/// Panics when `begin` and `end` do not point to code point boundaries,
-/// or point beyond the end of the string.
-impl ops::Index<ops::Range<usize>> for Wtf8 {
-    type Output = Wtf8;
-
-    #[inline]
-    fn index(&self, range: ops::Range<usize>) -> &Wtf8 {
-        // is_code_point_boundary checks that the index is in [0, .len()]
-        if range.start <= range.end
-            && is_code_point_boundary(self, range.start)
-            && is_code_point_boundary(self, range.end)
-        {
-            unsafe { slice_unchecked(self, range.start, range.end) }
-        } else {
-            slice_error_fail(self, range.start, range.end)
-        }
-    }
-}
-
-/// Returns a slice of the given string from byte `begin` to its end.
-///
-/// # Panics
-///
-/// Panics when `begin` is not at a code point boundary,
-/// or is beyond the end of the string.
-impl ops::Index<ops::RangeFrom<usize>> for Wtf8 {
-    type Output = Wtf8;
-
-    #[inline]
-    fn index(&self, range: ops::RangeFrom<usize>) -> &Wtf8 {
-        // is_code_point_boundary checks that the index is in [0, .len()]
-        if is_code_point_boundary(self, range.start) {
-            unsafe { slice_unchecked(self, range.start, self.len()) }
-        } else {
-            slice_error_fail(self, range.start, self.len())
-        }
-    }
-}
-
-/// Returns a slice of the given string from its beginning to byte `end`.
-///
-/// # Panics
-///
-/// Panics when `end` is not at a code point boundary,
-/// or is beyond the end of the string.
-impl ops::Index<ops::RangeTo<usize>> for Wtf8 {
-    type Output = Wtf8;
-
-    #[inline]
-    fn index(&self, range: ops::RangeTo<usize>) -> &Wtf8 {
-        // is_code_point_boundary checks that the index is in [0, .len()]
-        if is_code_point_boundary(self, range.end) {
-            unsafe { slice_unchecked(self, 0, range.end) }
-        } else {
-            slice_error_fail(self, 0, range.end)
-        }
-    }
-}
-
-impl ops::Index<ops::RangeFull> for Wtf8 {
-    type Output = Wtf8;
-
-    #[inline]
-    fn index(&self, _range: ops::RangeFull) -> &Wtf8 {
-        self
-    }
-}
-
-#[inline]
-fn decode_surrogate(second_byte: u8, third_byte: u8) -> u16 {
-    // The first byte is assumed to be 0xED
-    0xD800 | (second_byte as u16 & 0x3F) << 6 | third_byte as u16 & 0x3F
-}
-
-#[inline]
-fn decode_surrogate_pair(lead: u16, trail: u16) -> char {
-    let code_point = 0x10000 + ((((lead - 0xD800) as u32) << 10) | (trail - 0xDC00) as u32);
-    unsafe { char::from_u32_unchecked(code_point) }
-}
-
-/// Copied from str::is_char_boundary
-#[inline]
-pub fn is_code_point_boundary(slice: &Wtf8, index: usize) -> bool {
-    if index == 0 {
-        return true;
-    }
-    match slice.bytes.get(index) {
-        None => index == slice.len(),
-        Some(&b) => (b as i8) >= -0x40,
-    }
-}
-
-/// Verify that `index` is at the edge of either a valid UTF-8 codepoint
-/// (i.e. a codepoint that's not a surrogate) or of the whole string.
-///
-/// These are the cases currently permitted by `OsStr::slice_encoded_bytes`.
-/// Splitting between surrogates is valid as far as WTF-8 is concerned, but
-/// we do not permit it in the public API because WTF-8 is considered an
-/// implementation detail.
-#[track_caller]
-#[inline]
-pub fn check_utf8_boundary(slice: &Wtf8, index: usize) {
-    if index == 0 {
-        return;
-    }
-    match slice.bytes.get(index) {
-        Some(0xED) => (), // Might be a surrogate
-        Some(&b) if (b as i8) >= -0x40 => return,
-        Some(_) => panic!("byte index {index} is not a codepoint boundary"),
-        None if index == slice.len() => return,
-        None => panic!("byte index {index} is out of bounds"),
-    }
-    if slice.bytes[index + 1] >= 0xA0 {
-        // There's a surrogate after index. Now check before index.
-        if index >= 3 && slice.bytes[index - 3] == 0xED && slice.bytes[index - 2] >= 0xA0 {
-            panic!("byte index {index} lies between surrogate codepoints");
-        }
-    }
-}
-
-/// Copied from core::str::raw::slice_unchecked
-#[inline]
-pub unsafe fn slice_unchecked(s: &Wtf8, begin: usize, end: usize) -> &Wtf8 {
-    // SAFETY: memory layout of a &[u8] and &Wtf8 are the same
-    unsafe {
-        let len = end - begin;
-        let start = s.as_bytes().as_ptr().add(begin);
-        Wtf8::from_bytes_unchecked(slice::from_raw_parts(start, len))
-    }
-}
-
-/// Copied from core::str::raw::slice_error_fail
-#[inline(never)]
-pub fn slice_error_fail(s: &Wtf8, begin: usize, end: usize) -> ! {
-    assert!(begin <= end);
-    panic!("index {begin} and/or {end} in `{s:?}` do not lie on character boundary");
-}
-
-/// Iterator for the code points of a WTF-8 string.
-///
-/// Created with the method `.code_points()`.
-#[derive(Clone)]
-pub struct Wtf8CodePoints<'a> {
-    bytes: slice::Iter<'a, u8>,
-}
-
-impl Iterator for Wtf8CodePoints<'_> {
-    type Item = CodePoint;
-
-    #[inline]
-    fn next(&mut self) -> Option<CodePoint> {
-        // SAFETY: `self.bytes` has been created from a WTF-8 string
-        unsafe { next_code_point(&mut self.bytes).map(|c| CodePoint { value: c }) }
-    }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        let len = self.bytes.len();
-        (len.saturating_add(3) / 4, Some(len))
-    }
-}
-
-/// Generates a wide character sequence for potentially ill-formed UTF-16.
-#[stable(feature = "rust1", since = "1.0.0")]
-#[derive(Clone)]
-pub struct EncodeWide<'a> {
-    code_points: Wtf8CodePoints<'a>,
-    extra: u16,
-}
-
-// Copied from libunicode/u_str.rs
-#[stable(feature = "rust1", since = "1.0.0")]
-impl Iterator for EncodeWide<'_> {
-    type Item = u16;
-
-    #[inline]
-    fn next(&mut self) -> Option<u16> {
-        if self.extra != 0 {
-            let tmp = self.extra;
-            self.extra = 0;
-            return Some(tmp);
-        }
-
-        let mut buf = [0; MAX_LEN_UTF16];
-        self.code_points.next().map(|code_point| {
-            let n = encode_utf16_raw(code_point.value, &mut buf).len();
-            if n == 2 {
-                self.extra = buf[1];
-            }
-            buf[0]
-        })
-    }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        let (low, high) = self.code_points.size_hint();
-        let ext = (self.extra != 0) as usize;
-        // every code point gets either one u16 or two u16,
-        // so this iterator is between 1 or 2 times as
-        // long as the underlying iterator.
-        (low + ext, high.and_then(|n| n.checked_mul(2)).and_then(|n| n.checked_add(ext)))
-    }
-}
-
-#[stable(feature = "encode_wide_fused_iterator", since = "1.62.0")]
-impl FusedIterator for EncodeWide<'_> {}
-
-impl Hash for CodePoint {
-    #[inline]
-    fn hash<H: Hasher>(&self, state: &mut H) {
-        self.value.hash(state)
-    }
-}
-
-impl Hash for Wtf8Buf {
-    #[inline]
-    fn hash<H: Hasher>(&self, state: &mut H) {
-        state.write(&self.bytes);
-        0xfeu8.hash(state)
-    }
-}
-
-impl Hash for Wtf8 {
-    #[inline]
-    fn hash<H: Hasher>(&self, state: &mut H) {
-        state.write(&self.bytes);
-        0xfeu8.hash(state)
-    }
-}
-
-#[unstable(feature = "clone_to_uninit", issue = "126799")]
-unsafe impl CloneToUninit for Wtf8 {
-    #[inline]
-    #[cfg_attr(debug_assertions, track_caller)]
-    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
-        // SAFETY: we're just a transparent wrapper around [u8]
-        unsafe { self.bytes.clone_to_uninit(dst) }
-    }
-}
diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md
index 2f815bcd1415f..edfc2db7d6f68 100644
--- a/src/doc/rustc/src/platform-support.md
+++ b/src/doc/rustc/src/platform-support.md
@@ -103,7 +103,6 @@ target | notes
 [`powerpc64le-unknown-linux-gnu`](platform-support/powerpc64le-unknown-linux-gnu.md) | PPC64LE Linux (kernel 3.10+, glibc 2.17)
 [`powerpc64le-unknown-linux-musl`](platform-support/powerpc64le-unknown-linux-musl.md) | PPC64LE Linux (kernel 4.19+, musl 1.2.3)
 [`riscv64gc-unknown-linux-gnu`](platform-support/riscv64gc-unknown-linux-gnu.md) | RISC-V Linux (kernel 4.20+, glibc 2.29)
-[`riscv64gc-unknown-linux-musl`](platform-support/riscv64gc-unknown-linux-musl.md) | RISC-V Linux (kernel 4.20+, musl 1.2.3)
 [`s390x-unknown-linux-gnu`](platform-support/s390x-unknown-linux-gnu.md) | S390x Linux (kernel 3.2+, glibc 2.17)
 [`x86_64-apple-darwin`](platform-support/apple-darwin.md) | 64-bit macOS (10.12+, Sierra+)
 [`x86_64-pc-windows-gnullvm`](platform-support/windows-gnullvm.md) | 64-bit x86 MinGW (Windows 10+), LLVM ABI
@@ -183,6 +182,7 @@ target | std | notes
 [`riscv32imac-unknown-none-elf`](platform-support/riscv32-unknown-none-elf.md) | * | Bare RISC-V (RV32IMAC ISA)
 [`riscv32imafc-unknown-none-elf`](platform-support/riscv32-unknown-none-elf.md) | * | Bare RISC-V (RV32IMAFC ISA)
 [`riscv32imc-unknown-none-elf`](platform-support/riscv32-unknown-none-elf.md) | * | Bare RISC-V (RV32IMC ISA)
+[`riscv64gc-unknown-linux-musl`](platform-support/riscv64gc-unknown-linux-musl.md) | RISC-V Linux (kernel 4.20+, musl 1.2.3)
 `riscv64gc-unknown-none-elf` | * | Bare RISC-V (RV64IMAFDC ISA)
 `riscv64imac-unknown-none-elf` | * | Bare RISC-V (RV64IMAC ISA)
 `sparc64-unknown-linux-gnu` | ✓ | SPARC Linux (kernel 4.4+, glibc 2.23)
diff --git a/src/tools/tidy/src/unit_tests.rs b/src/tools/tidy/src/unit_tests.rs
index 3d14a46731932..7396310ed37c9 100644
--- a/src/tools/tidy/src/unit_tests.rs
+++ b/src/tools/tidy/src/unit_tests.rs
@@ -61,6 +61,7 @@ pub fn check(root_path: &Path, stdlib: bool, bad: &mut bool) {
                 || path.ends_with("library/alloc/src/collections/linked_list/tests.rs")
                 || path.ends_with("library/alloc/src/collections/vec_deque/tests.rs")
                 || path.ends_with("library/alloc/src/raw_vec/tests.rs")
+                || path.ends_with("library/alloc/src/wtf8/tests.rs")
         }
     };
 
diff --git a/tests/mir-opt/building/async_await.rs b/tests/mir-opt/building/async_await.rs
index 6c44570d109c2..2d14a1fb0c6f7 100644
--- a/tests/mir-opt/building/async_await.rs
+++ b/tests/mir-opt/building/async_await.rs
@@ -4,7 +4,7 @@
 // related to `yield` are `&mut Context`, and its return type is `Poll`.
 
 //@ edition:2018
-//@ compile-flags: -C panic=abort
+//@ compile-flags: -Zmir-opt-level=0 -C panic=abort
 
 #![crate_type = "lib"]
 
diff --git a/tests/mir-opt/building/dump_mir_cycle.rs b/tests/mir-opt/building/dump_mir_cycle.rs
index 8e13420aed790..ab0f2ea6db863 100644
--- a/tests/mir-opt/building/dump_mir_cycle.rs
+++ b/tests/mir-opt/building/dump_mir_cycle.rs
@@ -1,3 +1,5 @@
+//@ compile-flags: -Zmir-opt-level=0
+
 #[derive(Debug)]
 pub struct Thing {
     pub next: &'static Thing,
diff --git a/tests/mir-opt/building/enum_cast.rs b/tests/mir-opt/building/enum_cast.rs
index eaf5537e0ab4a..07025f572ab20 100644
--- a/tests/mir-opt/building/enum_cast.rs
+++ b/tests/mir-opt/building/enum_cast.rs
@@ -1,3 +1,4 @@
+//@ compile-flags: -Zmir-opt-level=0
 // skip-filecheck
 // EMIT_MIR enum_cast.foo.built.after.mir
 // EMIT_MIR enum_cast.bar.built.after.mir
diff --git a/tests/mir-opt/building/eq_never_type.rs b/tests/mir-opt/building/eq_never_type.rs
index 90e2e69753531..486e2f9fb8dfa 100644
--- a/tests/mir-opt/building/eq_never_type.rs
+++ b/tests/mir-opt/building/eq_never_type.rs
@@ -1,3 +1,4 @@
+//@ compile-flags: -Zmir-opt-level=0
 // skip-filecheck
 #![feature(never_type)]
 #![allow(unreachable_code)]
diff --git a/tests/mir-opt/building/index_array_and_slice.rs b/tests/mir-opt/building/index_array_and_slice.rs
index 42ede66d92ba0..47d8255ef57f1 100644
--- a/tests/mir-opt/building/index_array_and_slice.rs
+++ b/tests/mir-opt/building/index_array_and_slice.rs
@@ -1,36 +1,42 @@
-//@ compile-flags: -C opt-level=0
+//@ compile-flags: -Zmir-opt-level=0 -C opt-level=0
 
 // EMIT_MIR index_array_and_slice.index_array.built.after.mir
 fn index_array(array: &[i32; 7], index: usize) -> &i32 {
     // CHECK: bb0:
-    // CHECK: [[LT:_.+]] = Lt(copy _2, const 7_usize);
-    // CHECK: assert(move [[LT]], "index out of bounds{{.+}}", const 7_usize, copy _2) -> [success: bb1, unwind
+    // CHECK: _3 = copy _2;
+    // CHECK: [[LT:_.+]] = Lt(copy _3, const 7_usize);
+    // CHECK: assert(move [[LT]], "index out of bounds{{.+}}", const 7_usize, copy _3) -> [success: bb1, unwind
 
     // CHECK: bb1:
-    // CHECK: _0 = &(*_1)[_2];
+    // CHECK: _5 = &(*_1)[_3];
+    // CHECK: _0 = &(*_5);
     &array[index]
 }
 
 // EMIT_MIR index_array_and_slice.index_const_generic_array.built.after.mir
 fn index_const_generic_array<const N: usize>(array: &[i32; N], index: usize) -> &i32 {
     // CHECK: bb0:
-    // CHECK: [[LT:_.+]] = Lt(copy _2, const N);
-    // CHECK: assert(move [[LT]], "index out of bounds{{.+}}", const N, copy _2) -> [success: bb1, unwind
+    // CHECK: _3 = copy _2;
+    // CHECK: [[LT:_.+]] = Lt(copy _3, const N);
+    // CHECK: assert(move [[LT]], "index out of bounds{{.+}}", const N, copy _3) -> [success: bb1, unwind
 
     // CHECK: bb1:
-    // CHECK: _0 = &(*_1)[_2];
+    // CHECK: _5 = &(*_1)[_3];
+    // CHECK: _0 = &(*_5);
     &array[index]
 }
 
 // EMIT_MIR index_array_and_slice.index_slice.built.after.mir
 fn index_slice(slice: &[i32], index: usize) -> &i32 {
     // CHECK: bb0:
+    // CHECK: _3 = copy _2;
     // CHECK: [[LEN:_.+]] = PtrMetadata(copy _1);
-    // CHECK: [[LT:_.+]] = Lt(copy _2, copy [[LEN]]);
-    // CHECK: assert(move [[LT]], "index out of bounds{{.+}}", move [[LEN]], copy _2) -> [success: bb1,
+    // CHECK: [[LT:_.+]] = Lt(copy _3, copy [[LEN]]);
+    // CHECK: assert(move [[LT]], "index out of bounds{{.+}}", move [[LEN]], copy _3) -> [success: bb1,
 
     // CHECK: bb1:
-    // CHECK: _0 = &(*_1)[_2];
+    // CHECK: _6 = &(*_1)[_3];
+    // CHECK: _0 = &(*_6);
     &slice[index]
 }
 
@@ -40,12 +46,15 @@ fn index_mut_slice(slice: &mut [i32], index: usize) -> &i32 {
     // This cannot `copy _1` in the *built* MIR, only in the *runtime* MIR.
 
     // CHECK: bb0:
-    // CHECK: [[LEN:_.+]] = PtrMetadata(copy _1);
-    // CHECK: [[LT:_.+]] = Lt(copy _2, copy [[LEN]]);
-    // CHECK: assert(move [[LT]], "index out of bounds{{.+}}", move [[LEN]], copy _2) -> [success: bb1,
+    // CHECK: _3 = copy _2;
+    // CHECK: _4 = &raw const (fake) (*_1);
+    // CHECK: [[LEN:_.+]] = PtrMetadata(move _4);
+    // CHECK: [[LT:_.+]] = Lt(copy _3, copy [[LEN]]);
+    // CHECK: assert(move [[LT]], "index out of bounds{{.+}}", move [[LEN]], copy _3) -> [success: bb1,
 
     // CHECK: bb1:
-    // CHECK: _0 = &(*_1)[_2];
+    // CHECK: _7 = &(*_1)[_3];
+    // CHECK: _0 = &(*_7);
     &slice[index]
 }
 
@@ -54,13 +63,15 @@ struct WithSliceTail(f64, [i32]);
 // EMIT_MIR index_array_and_slice.index_custom.built.after.mir
 fn index_custom(custom: &WithSliceTail, index: usize) -> &i32 {
     // CHECK: bb0:
+    // CHECK: _3 = copy _2;
     // CHECK: [[PTR:_.+]] = &raw const (fake) ((*_1).1: [i32]);
-    // CHECK: [[LEN:_.+]] = PtrMetadata(copy [[PTR]]);
-    // CHECK: [[LT:_.+]] = Lt(copy _2, copy [[LEN]]);
-    // CHECK: assert(move [[LT]], "index out of bounds{{.+}}", move [[LEN]], copy _2) -> [success: bb1,
+    // CHECK: [[LEN:_.+]] = PtrMetadata(move [[PTR]]);
+    // CHECK: [[LT:_.+]] = Lt(copy _3, copy [[LEN]]);
+    // CHECK: assert(move [[LT]], "index out of bounds{{.+}}", move [[LEN]], copy _3) -> [success: bb1,
 
     // CHECK: bb1:
-    // CHECK: _0 = &((*_1).1: [i32])[_2];
+    // CHECK: _7 = &((*_1).1: [i32])[_3];
+    // CHECK: _0 = &(*_7);
     &custom.1[index]
 }
 
diff --git a/tests/mir-opt/building/issue_101867.main.built.after.mir b/tests/mir-opt/building/issue_101867.main.built.after.mir
index 8a36c901eedec..fa35658a16d4a 100644
--- a/tests/mir-opt/building/issue_101867.main.built.after.mir
+++ b/tests/mir-opt/building/issue_101867.main.built.after.mir
@@ -1,8 +1,8 @@
 // MIR for `main` after built
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: Ty(std::option::Option<u8>), max_universe: U0, variables: [] }, span: $DIR/issue_101867.rs:4:12: 4:22, inferred_ty: std::option::Option<u8>
-| 1: user_ty: Canonical { value: Ty(std::option::Option<u8>), max_universe: U0, variables: [] }, span: $DIR/issue_101867.rs:4:12: 4:22, inferred_ty: std::option::Option<u8>
+| 0: user_ty: Canonical { value: Ty(std::option::Option<u8>), max_universe: U0, variables: [] }, span: $DIR/issue_101867.rs:5:12: 5:22, inferred_ty: std::option::Option<u8>
+| 1: user_ty: Canonical { value: Ty(std::option::Option<u8>), max_universe: U0, variables: [] }, span: $DIR/issue_101867.rs:5:12: 5:22, inferred_ty: std::option::Option<u8>
 |
 fn main() -> () {
     let mut _0: ();
diff --git a/tests/mir-opt/building/issue_101867.rs b/tests/mir-opt/building/issue_101867.rs
index f8a531e8982d5..a00bc13b3fe5f 100644
--- a/tests/mir-opt/building/issue_101867.rs
+++ b/tests/mir-opt/building/issue_101867.rs
@@ -1,3 +1,4 @@
+//@ compile-flags: -Zmir-opt-level=0
 // skip-filecheck
 // EMIT_MIR issue_101867.main.built.after.mir
 fn main() {
diff --git a/tests/mir-opt/building/issue_110508.rs b/tests/mir-opt/building/issue_110508.rs
index e597cd5d06b32..aa8a7b1d08f4a 100644
--- a/tests/mir-opt/building/issue_110508.rs
+++ b/tests/mir-opt/building/issue_110508.rs
@@ -1,3 +1,4 @@
+//@ compile-flags: -Zmir-opt-level=0
 // skip-filecheck
 // EMIT_MIR issue_110508.{impl#0}-BAR.built.after.mir
 // EMIT_MIR issue_110508.{impl#0}-SELF_BAR.built.after.mir
diff --git a/tests/mir-opt/building/issue_110508.{impl#0}-BAR.built.after.mir b/tests/mir-opt/building/issue_110508.{impl#0}-BAR.built.after.mir
index c3d28fae51833..0cefb9c8fd1b7 100644
--- a/tests/mir-opt/building/issue_110508.{impl#0}-BAR.built.after.mir
+++ b/tests/mir-opt/building/issue_110508.{impl#0}-BAR.built.after.mir
@@ -1,6 +1,6 @@
-// MIR for `<impl at $DIR/issue_110508.rs:9:1: 9:9>::BAR` after built
+// MIR for `<impl at $DIR/issue_110508.rs:10:1: 10:9>::BAR` after built
 
-const <impl at $DIR/issue_110508.rs:9:1: 9:9>::BAR: Foo = {
+const <impl at $DIR/issue_110508.rs:10:1: 10:9>::BAR: Foo = {
     let mut _0: Foo;
     let mut _1: ();
 
diff --git a/tests/mir-opt/building/issue_110508.{impl#0}-SELF_BAR.built.after.mir b/tests/mir-opt/building/issue_110508.{impl#0}-SELF_BAR.built.after.mir
index 177518c30af7f..0e0e914938519 100644
--- a/tests/mir-opt/building/issue_110508.{impl#0}-SELF_BAR.built.after.mir
+++ b/tests/mir-opt/building/issue_110508.{impl#0}-SELF_BAR.built.after.mir
@@ -1,6 +1,6 @@
-// MIR for `<impl at $DIR/issue_110508.rs:9:1: 9:9>::SELF_BAR` after built
+// MIR for `<impl at $DIR/issue_110508.rs:10:1: 10:9>::SELF_BAR` after built
 
-const <impl at $DIR/issue_110508.rs:9:1: 9:9>::SELF_BAR: Foo = {
+const <impl at $DIR/issue_110508.rs:10:1: 10:9>::SELF_BAR: Foo = {
     let mut _0: Foo;
     let mut _1: ();
 
diff --git a/tests/mir-opt/building/issue_49232.rs b/tests/mir-opt/building/issue_49232.rs
index ac06e02778f0d..f17e3e955a41c 100644
--- a/tests/mir-opt/building/issue_49232.rs
+++ b/tests/mir-opt/building/issue_49232.rs
@@ -1,3 +1,4 @@
+//@ compile-flags: -Zmir-opt-level=0
 // skip-filecheck
 // We must mark a variable whose initialization fails due to an
 // abort statement as StorageDead.
diff --git a/tests/mir-opt/building/logical_or_in_conditional.rs b/tests/mir-opt/building/logical_or_in_conditional.rs
index 249ccf728044c..d1c1ea2896aaf 100644
--- a/tests/mir-opt/building/logical_or_in_conditional.rs
+++ b/tests/mir-opt/building/logical_or_in_conditional.rs
@@ -1,5 +1,5 @@
 // skip-filecheck
-//@ compile-flags: -Z validate-mir
+//@ compile-flags: -Zmir-opt-level=0 -Z validate-mir
 //@ edition: 2024
 struct Droppy(u8);
 impl Drop for Droppy {
diff --git a/tests/mir-opt/building/receiver_ptr_mutability.main.built.after.mir b/tests/mir-opt/building/receiver_ptr_mutability.main.built.after.mir
index 744553c7cd266..5cf182c21c319 100644
--- a/tests/mir-opt/building/receiver_ptr_mutability.main.built.after.mir
+++ b/tests/mir-opt/building/receiver_ptr_mutability.main.built.after.mir
@@ -1,10 +1,10 @@
 // MIR for `main` after built
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: Ty(*mut Test), max_universe: U0, variables: [] }, span: $DIR/receiver_ptr_mutability.rs:15:14: 15:23, inferred_ty: *mut Test
-| 1: user_ty: Canonical { value: Ty(*mut Test), max_universe: U0, variables: [] }, span: $DIR/receiver_ptr_mutability.rs:15:14: 15:23, inferred_ty: *mut Test
-| 2: user_ty: Canonical { value: Ty(&&&&*mut Test), max_universe: U0, variables: [Region(U0), Region(U0), Region(U0), Region(U0)] }, span: $DIR/receiver_ptr_mutability.rs:19:18: 19:31, inferred_ty: &&&&*mut Test
-| 3: user_ty: Canonical { value: Ty(&&&&*mut Test), max_universe: U0, variables: [Region(U0), Region(U0), Region(U0), Region(U0)] }, span: $DIR/receiver_ptr_mutability.rs:19:18: 19:31, inferred_ty: &&&&*mut Test
+| 0: user_ty: Canonical { value: Ty(*mut Test), max_universe: U0, variables: [] }, span: $DIR/receiver_ptr_mutability.rs:16:14: 16:23, inferred_ty: *mut Test
+| 1: user_ty: Canonical { value: Ty(*mut Test), max_universe: U0, variables: [] }, span: $DIR/receiver_ptr_mutability.rs:16:14: 16:23, inferred_ty: *mut Test
+| 2: user_ty: Canonical { value: Ty(&&&&*mut Test), max_universe: U0, variables: [Region(U0), Region(U0), Region(U0), Region(U0)] }, span: $DIR/receiver_ptr_mutability.rs:20:18: 20:31, inferred_ty: &&&&*mut Test
+| 3: user_ty: Canonical { value: Ty(&&&&*mut Test), max_universe: U0, variables: [Region(U0), Region(U0), Region(U0), Region(U0)] }, span: $DIR/receiver_ptr_mutability.rs:20:18: 20:31, inferred_ty: &&&&*mut Test
 |
 fn main() -> () {
     let mut _0: ();
diff --git a/tests/mir-opt/building/receiver_ptr_mutability.rs b/tests/mir-opt/building/receiver_ptr_mutability.rs
index 1ddb8b71a5a3c..37b142f1d8ad2 100644
--- a/tests/mir-opt/building/receiver_ptr_mutability.rs
+++ b/tests/mir-opt/building/receiver_ptr_mutability.rs
@@ -1,3 +1,4 @@
+//@ compile-flags: -Zmir-opt-level=0
 // skip-filecheck
 // EMIT_MIR receiver_ptr_mutability.main.built.after.mir
 
diff --git a/tests/mir-opt/building/shifts.rs b/tests/mir-opt/building/shifts.rs
index d7747bb2f7844..165b388bc18b5 100644
--- a/tests/mir-opt/building/shifts.rs
+++ b/tests/mir-opt/building/shifts.rs
@@ -1,5 +1,5 @@
 // skip-filecheck
-//@ compile-flags: -C debug-assertions=yes
+//@ compile-flags: -Zmir-opt-level=0 -C debug-assertions=yes
 
 // EMIT_MIR shifts.shift_signed.built.after.mir
 fn shift_signed(small: i8, big: u128, a: i8, b: i32, c: i128) -> ([i8; 3], [u128; 3]) {
diff --git a/tests/mir-opt/building/storage_live_dead_in_statics.rs b/tests/mir-opt/building/storage_live_dead_in_statics.rs
index 7cb74acbf0606..868f4a8f55974 100644
--- a/tests/mir-opt/building/storage_live_dead_in_statics.rs
+++ b/tests/mir-opt/building/storage_live_dead_in_statics.rs
@@ -1,3 +1,4 @@
+//@ compile-flags: -Zmir-opt-level=0
 // skip-filecheck
 // Check that when we compile the static `XXX` into MIR, we do not
 // generate `StorageStart` or `StorageEnd` statements.
diff --git a/tests/mir-opt/building/uniform_array_move_out.rs b/tests/mir-opt/building/uniform_array_move_out.rs
index aff5996d0b64b..36245273fe1c1 100644
--- a/tests/mir-opt/building/uniform_array_move_out.rs
+++ b/tests/mir-opt/building/uniform_array_move_out.rs
@@ -1,3 +1,4 @@
+//@ compile-flags: -Zmir-opt-level=0
 // skip-filecheck
 #![feature(liballoc_internals, rustc_attrs)]
 
diff --git a/tests/mir-opt/building/user_type_annotations.let_else.built.after.mir b/tests/mir-opt/building/user_type_annotations.let_else.built.after.mir
index 3d26fe24ac9ca..bbf504d311f6d 100644
--- a/tests/mir-opt/building/user_type_annotations.let_else.built.after.mir
+++ b/tests/mir-opt/building/user_type_annotations.let_else.built.after.mir
@@ -1,8 +1,8 @@
 // MIR for `let_else` after built
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:35:20: 35:45, inferred_ty: (u32, u64, &char)
-| 1: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:35:20: 35:45, inferred_ty: (u32, u64, &char)
+| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:36:20: 36:45, inferred_ty: (u32, u64, &char)
+| 1: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:36:20: 36:45, inferred_ty: (u32, u64, &char)
 |
 fn let_else() -> () {
     let mut _0: ();
diff --git a/tests/mir-opt/building/user_type_annotations.let_else_bindless.built.after.mir b/tests/mir-opt/building/user_type_annotations.let_else_bindless.built.after.mir
index b2a06ae53a8ae..7bf2551e99f23 100644
--- a/tests/mir-opt/building/user_type_annotations.let_else_bindless.built.after.mir
+++ b/tests/mir-opt/building/user_type_annotations.let_else_bindless.built.after.mir
@@ -1,8 +1,8 @@
 // MIR for `let_else_bindless` after built
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:40:20: 40:45, inferred_ty: (u32, u64, &char)
-| 1: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:40:20: 40:45, inferred_ty: (u32, u64, &char)
+| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:41:20: 41:45, inferred_ty: (u32, u64, &char)
+| 1: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:41:20: 41:45, inferred_ty: (u32, u64, &char)
 |
 fn let_else_bindless() -> () {
     let mut _0: ();
diff --git a/tests/mir-opt/building/user_type_annotations.let_init.built.after.mir b/tests/mir-opt/building/user_type_annotations.let_init.built.after.mir
index d1b8f823e9bc5..0cf681d8ab2c0 100644
--- a/tests/mir-opt/building/user_type_annotations.let_init.built.after.mir
+++ b/tests/mir-opt/building/user_type_annotations.let_init.built.after.mir
@@ -1,8 +1,8 @@
 // MIR for `let_init` after built
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:25:20: 25:45, inferred_ty: (u32, u64, &char)
-| 1: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:25:20: 25:45, inferred_ty: (u32, u64, &char)
+| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:26:20: 26:45, inferred_ty: (u32, u64, &char)
+| 1: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:26:20: 26:45, inferred_ty: (u32, u64, &char)
 |
 fn let_init() -> () {
     let mut _0: ();
diff --git a/tests/mir-opt/building/user_type_annotations.let_init_bindless.built.after.mir b/tests/mir-opt/building/user_type_annotations.let_init_bindless.built.after.mir
index 6702f9300607f..968813c826e45 100644
--- a/tests/mir-opt/building/user_type_annotations.let_init_bindless.built.after.mir
+++ b/tests/mir-opt/building/user_type_annotations.let_init_bindless.built.after.mir
@@ -1,8 +1,8 @@
 // MIR for `let_init_bindless` after built
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:30:20: 30:45, inferred_ty: (u32, u64, &char)
-| 1: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:30:20: 30:45, inferred_ty: (u32, u64, &char)
+| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:31:20: 31:45, inferred_ty: (u32, u64, &char)
+| 1: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:31:20: 31:45, inferred_ty: (u32, u64, &char)
 |
 fn let_init_bindless() -> () {
     let mut _0: ();
diff --git a/tests/mir-opt/building/user_type_annotations.let_uninit.built.after.mir b/tests/mir-opt/building/user_type_annotations.let_uninit.built.after.mir
index 76b5938b87d2a..b6fdc4ff46dc5 100644
--- a/tests/mir-opt/building/user_type_annotations.let_uninit.built.after.mir
+++ b/tests/mir-opt/building/user_type_annotations.let_uninit.built.after.mir
@@ -1,7 +1,7 @@
 // MIR for `let_uninit` after built
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:15:20: 15:45, inferred_ty: (u32, u64, &char)
+| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:16:20: 16:45, inferred_ty: (u32, u64, &char)
 |
 fn let_uninit() -> () {
     let mut _0: ();
diff --git a/tests/mir-opt/building/user_type_annotations.let_uninit_bindless.built.after.mir b/tests/mir-opt/building/user_type_annotations.let_uninit_bindless.built.after.mir
index 0cd125587714d..472dbfb630432 100644
--- a/tests/mir-opt/building/user_type_annotations.let_uninit_bindless.built.after.mir
+++ b/tests/mir-opt/building/user_type_annotations.let_uninit_bindless.built.after.mir
@@ -1,7 +1,7 @@
 // MIR for `let_uninit_bindless` after built
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:20:20: 20:45, inferred_ty: (u32, u64, &char)
+| 0: user_ty: Canonical { value: Ty((u32, u64, &'static char)), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:21:20: 21:45, inferred_ty: (u32, u64, &char)
 |
 fn let_uninit_bindless() -> () {
     let mut _0: ();
diff --git a/tests/mir-opt/building/user_type_annotations.match_assoc_const.built.after.mir b/tests/mir-opt/building/user_type_annotations.match_assoc_const.built.after.mir
index c0ce6f1d06b5e..ff4b0bf7600df 100644
--- a/tests/mir-opt/building/user_type_annotations.match_assoc_const.built.after.mir
+++ b/tests/mir-opt/building/user_type_annotations.match_assoc_const.built.after.mir
@@ -1,8 +1,8 @@
 // MIR for `match_assoc_const` after built
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:54:9: 54:44, inferred_ty: u32
-| 1: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:54:9: 54:44, inferred_ty: u32
+| 0: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:55:9: 55:44, inferred_ty: u32
+| 1: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:55:9: 55:44, inferred_ty: u32
 |
 fn match_assoc_const() -> () {
     let mut _0: ();
diff --git a/tests/mir-opt/building/user_type_annotations.match_assoc_const_range.built.after.mir b/tests/mir-opt/building/user_type_annotations.match_assoc_const_range.built.after.mir
index 3a6aa5b7c2ca4..4cc433f475f60 100644
--- a/tests/mir-opt/building/user_type_annotations.match_assoc_const_range.built.after.mir
+++ b/tests/mir-opt/building/user_type_annotations.match_assoc_const_range.built.after.mir
@@ -1,10 +1,10 @@
 // MIR for `match_assoc_const_range` after built
 
 | User Type Annotations
-| 0: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:62:11: 62:46, inferred_ty: u32
-| 1: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:62:11: 62:46, inferred_ty: u32
-| 2: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:63:9: 63:44, inferred_ty: u32
-| 3: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:63:9: 63:44, inferred_ty: u32
+| 0: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:63:11: 63:46, inferred_ty: u32
+| 1: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:63:11: 63:46, inferred_ty: u32
+| 2: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:64:9: 64:44, inferred_ty: u32
+| 3: user_ty: Canonical { value: TypeOf(DefId(0:11 ~ user_type_annotations[ee8e]::MyTrait::FOO), UserArgs { args: [MyStruct, 'static], user_self_ty: None }), max_universe: U0, variables: [] }, span: $DIR/user_type_annotations.rs:64:9: 64:44, inferred_ty: u32
 |
 fn match_assoc_const_range() -> () {
     let mut _0: ();
diff --git a/tests/mir-opt/building/user_type_annotations.rs b/tests/mir-opt/building/user_type_annotations.rs
index d55c678d5ae35..527bf62f7dc05 100644
--- a/tests/mir-opt/building/user_type_annotations.rs
+++ b/tests/mir-opt/building/user_type_annotations.rs
@@ -1,3 +1,4 @@
+//@ compile-flags: -Zmir-opt-level=0
 //@ edition: 2024
 // skip-filecheck