diff --git a/library/std/src/sys/windows/os_str.rs b/library/std/src/sys/windows/os_str.rs
index 11883f15022f6..4bdd8c505ff25 100644
--- a/library/std/src/sys/windows/os_str.rs
+++ b/library/std/src/sys/windows/os_str.rs
@@ -164,9 +164,7 @@ impl Slice {
     }
 
     pub fn to_owned(&self) -> Buf {
-        let mut buf = Wtf8Buf::with_capacity(self.inner.len());
-        buf.push_wtf8(&self.inner);
-        Buf { inner: buf }
+        Buf { inner: self.inner.to_owned() }
     }
 
     pub fn clone_into(&self, buf: &mut Buf) {
diff --git a/library/std/src/sys_common/wtf8.rs b/library/std/src/sys_common/wtf8.rs
index 33e20756163fd..dd53767d4521f 100644
--- a/library/std/src/sys_common/wtf8.rs
+++ b/library/std/src/sys_common/wtf8.rs
@@ -89,6 +89,24 @@ impl CodePoint {
         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).
@@ -117,6 +135,14 @@ impl CodePoint {
 #[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 {
@@ -147,13 +173,13 @@ impl Wtf8Buf {
     /// Creates a new, empty WTF-8 string.
     #[inline]
     pub fn new() -> Wtf8Buf {
-        Wtf8Buf { bytes: Vec::new() }
+        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) }
+        Wtf8Buf { bytes: Vec::with_capacity(capacity), is_known_utf8: true }
     }
 
     /// Creates a WTF-8 string from a UTF-8 `String`.
@@ -163,7 +189,7 @@ impl Wtf8Buf {
     /// Since WTF-8 is a superset of UTF-8, this always succeeds.
     #[inline]
     pub fn from_string(string: String) -> Wtf8Buf {
-        Wtf8Buf { bytes: string.into_bytes() }
+        Wtf8Buf { bytes: string.into_bytes(), is_known_utf8: true }
     }
 
     /// Creates a WTF-8 string from a UTF-8 `&str` slice.
@@ -173,11 +199,12 @@ impl Wtf8Buf {
     /// Since WTF-8 is a superset of UTF-8, this always succeeds.
     #[inline]
     pub fn from_str(str: &str) -> Wtf8Buf {
-        Wtf8Buf { bytes: <[_]>::to_vec(str.as_bytes()) }
+        Wtf8Buf { bytes: <[_]>::to_vec(str.as_bytes()), is_known_utf8: true }
     }
 
     pub fn clear(&mut self) {
-        self.bytes.clear()
+        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.
@@ -193,9 +220,11 @@ impl Wtf8Buf {
                     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.push_code_point_unchecked(code_point);
                 }
             }
         }
@@ -203,7 +232,7 @@ impl Wtf8Buf {
     }
 
     /// Copied from String::push
-    /// This does **not** include the WTF-8 concatenation check.
+    /// This does **not** include the WTF-8 concatenation check or `is_known_utf8` check.
     fn push_code_point_unchecked(&mut self, code_point: CodePoint) {
         let mut bytes = [0; 4];
         let bytes = char::encode_utf8_raw(code_point.value, &mut bytes);
@@ -217,6 +246,9 @@ impl Wtf8Buf {
 
     #[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) }
     }
 
@@ -314,7 +346,15 @@ impl Wtf8Buf {
                 self.push_char(decode_surrogate_pair(lead, trail));
                 self.bytes.extend_from_slice(other_without_trail_surrogate);
             }
-            _ => self.bytes.extend_from_slice(&other.bytes),
+            _ => {
+                // If we'll be pushing a string containing a surrogate, we may
+                // no longer have UTF-8.
+                if other.next_surrogate(0).is_some() {
+                    self.is_known_utf8 = false;
+                }
+
+                self.bytes.extend_from_slice(&other.bytes);
+            }
         }
     }
 
@@ -331,13 +371,19 @@ impl Wtf8Buf {
     /// like concatenating ill-formed UTF-16 strings effectively would.
     #[inline]
     pub fn push(&mut self, code_point: CodePoint) {
-        if let trail @ 0xDC00..=0xDFFF = code_point.to_u32() {
+        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 as u16));
+                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.
@@ -364,9 +410,10 @@ impl Wtf8Buf {
     /// (that is, if the string contains surrogates),
     /// the original WTF-8 string is returned instead.
     pub fn into_string(self) -> Result<String, Wtf8Buf> {
-        match self.next_surrogate(0) {
-            None => Ok(unsafe { String::from_utf8_unchecked(self.bytes) }),
-            Some(_) => Err(self),
+        if self.is_known_utf8 || self.next_surrogate(0).is_none() {
+            Ok(unsafe { String::from_utf8_unchecked(self.bytes) })
+        } else {
+            Err(self)
         }
     }
 
@@ -376,6 +423,11 @@ impl Wtf8Buf {
     ///
     /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”)
     pub fn into_string_lossy(mut self) -> String {
+        // Fast path: If we already have UTF-8, we can return it immediately.
+        if self.is_known_utf8 {
+            return unsafe { String::from_utf8_unchecked(self.bytes) };
+        }
+
         let mut pos = 0;
         loop {
             match self.next_surrogate(pos) {
@@ -398,7 +450,7 @@ impl Wtf8Buf {
     /// 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() }
+        Wtf8Buf { bytes: bytes.into_vec(), is_known_utf8: false }
     }
 }
 
@@ -576,6 +628,11 @@ impl Wtf8 {
         }
     }
 
+    /// 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.
     ///
@@ -665,7 +722,8 @@ impl Wtf8 {
     }
 
     pub fn clone_into(&self, buf: &mut Wtf8Buf) {
-        self.bytes.clone_into(&mut buf.bytes)
+        buf.is_known_utf8 = false;
+        self.bytes.clone_into(&mut buf.bytes);
     }
 
     /// Boxes this `Wtf8`.
@@ -705,12 +763,12 @@ impl Wtf8 {
 
     #[inline]
     pub fn to_ascii_lowercase(&self) -> Wtf8Buf {
-        Wtf8Buf { bytes: self.bytes.to_ascii_lowercase() }
+        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() }
+        Wtf8Buf { bytes: self.bytes.to_ascii_uppercase(), is_known_utf8: false }
     }
 
     #[inline]
diff --git a/library/std/src/sys_common/wtf8/tests.rs b/library/std/src/sys_common/wtf8/tests.rs
index 931996791fbe5..1a302d646941b 100644
--- a/library/std/src/sys_common/wtf8/tests.rs
+++ b/library/std/src/sys_common/wtf8/tests.rs
@@ -19,6 +19,36 @@ fn code_point_to_u32() {
     assert_eq!(c(0x10FFFF).to_u32(), 0x10FFFF);
 }
 
+#[test]
+fn code_point_to_lead_surrogate() {
+    fn c(value: u32) -> CodePoint {
+        CodePoint::from_u32(value).unwrap()
+    }
+    assert_eq!(c(0).to_lead_surrogate(), None);
+    assert_eq!(c(0xE9).to_lead_surrogate(), None);
+    assert_eq!(c(0xD800).to_lead_surrogate(), Some(0xD800));
+    assert_eq!(c(0xDBFF).to_lead_surrogate(), Some(0xDBFF));
+    assert_eq!(c(0xDC00).to_lead_surrogate(), None);
+    assert_eq!(c(0xDFFF).to_lead_surrogate(), None);
+    assert_eq!(c(0x1F4A9).to_lead_surrogate(), None);
+    assert_eq!(c(0x10FFFF).to_lead_surrogate(), None);
+}
+
+#[test]
+fn code_point_to_trail_surrogate() {
+    fn c(value: u32) -> CodePoint {
+        CodePoint::from_u32(value).unwrap()
+    }
+    assert_eq!(c(0).to_trail_surrogate(), None);
+    assert_eq!(c(0xE9).to_trail_surrogate(), None);
+    assert_eq!(c(0xD800).to_trail_surrogate(), None);
+    assert_eq!(c(0xDBFF).to_trail_surrogate(), None);
+    assert_eq!(c(0xDC00).to_trail_surrogate(), Some(0xDC00));
+    assert_eq!(c(0xDFFF).to_trail_surrogate(), Some(0xDFFF));
+    assert_eq!(c(0x1F4A9).to_trail_surrogate(), None);
+    assert_eq!(c(0x10FFFF).to_trail_surrogate(), None);
+}
+
 #[test]
 fn code_point_from_char() {
     assert_eq!(CodePoint::from_char('a').to_u32(), 0x61);
@@ -70,35 +100,66 @@ fn wtf8buf_from_string() {
 
 #[test]
 fn wtf8buf_from_wide() {
-    assert_eq!(Wtf8Buf::from_wide(&[]).bytes, b"");
-    assert_eq!(
-        Wtf8Buf::from_wide(&[0x61, 0xE9, 0x20, 0xD83D, 0xD83D, 0xDCA9]).bytes,
-        b"a\xC3\xA9 \xED\xA0\xBD\xF0\x9F\x92\xA9"
-    );
+    let buf = Wtf8Buf::from_wide(&[]);
+    assert_eq!(buf.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!(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!(!buf.is_known_utf8);
+
+    let buf = Wtf8Buf::from_wide(&[0xD800]);
+    assert_eq!(buf.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!(!buf.is_known_utf8);
+
+    let buf = Wtf8Buf::from_wide(&[0xDC00]);
+    assert_eq!(buf.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!(!buf.is_known_utf8);
 }
 
 #[test]
 fn wtf8buf_push_str() {
     let mut string = Wtf8Buf::new();
     assert_eq!(string.bytes, b"");
+    assert!(string.is_known_utf8);
+
     string.push_str("aé 💩");
     assert_eq!(string.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!(string.is_known_utf8);
+
     string.push_char('💩');
     assert_eq!(string.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!(string.is_known_utf8);
+
     string.push(CodePoint::from_char('💩'));
     assert_eq!(string.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert!(string.is_known_utf8);
 
     fn c(value: u32) -> CodePoint {
         CodePoint::from_u32(value).unwrap()
@@ -106,37 +167,46 @@ fn wtf8buf_push() {
 
     let mut string = Wtf8Buf::new();
     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!
 
     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");
 
     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");
 
     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");
 
     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");
 
     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");
 
     let mut string = Wtf8Buf::new();
     string.push(c(0xDC00)); // trail
+    assert!(!string.is_known_utf8);
     assert_eq!(string.bytes, b"\xED\xB0\x80");
 }
 
@@ -146,6 +216,7 @@ fn wtf8buf_push_wtf8() {
     assert_eq!(string.bytes, b"a\xC3\xA9");
     string.push_wtf8(Wtf8::from_str(" 💩"));
     assert_eq!(string.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert!(string.is_known_utf8);
 
     fn w(v: &[u8]) -> &Wtf8 {
         unsafe { Wtf8::from_bytes_unchecked(v) }
@@ -161,37 +232,68 @@ fn wtf8buf_push_wtf8() {
     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!(!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!(!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!(!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!(!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!(!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!(!string.is_known_utf8);
 }
 
 #[test]
 fn wtf8buf_truncate() {
     let mut string = Wtf8Buf::from_str("aé");
+    assert!(string.is_known_utf8);
+
+    string.truncate(3);
+    assert_eq!(string.bytes, b"a\xC3\xA9");
+    assert!(string.is_known_utf8);
+
     string.truncate(1);
     assert_eq!(string.bytes, b"a");
+    assert!(string.is_known_utf8);
+
+    string.truncate(0);
+    assert_eq!(string.bytes, b"");
+    assert!(string.is_known_utf8);
+}
+
+#[test]
+fn wtf8buf_truncate_around_non_bmp() {
+    let mut string = Wtf8Buf::from_str("💩");
+    assert!(string.is_known_utf8);
+
+    string.truncate(4);
+    assert_eq!(string.bytes, b"\xF0\x9F\x92\xA9");
+    assert!(string.is_known_utf8);
+
+    string.truncate(0);
+    assert_eq!(string.bytes, b"");
+    assert!(string.is_known_utf8);
 }
 
 #[test]
@@ -208,11 +310,37 @@ fn wtf8buf_truncate_fail_longer() {
     string.truncate(4);
 }
 
+#[test]
+#[should_panic]
+fn wtf8buf_truncate_splitting_non_bmp3() {
+    let mut string = Wtf8Buf::from_str("💩");
+    assert!(string.is_known_utf8);
+    string.truncate(3);
+}
+
+#[test]
+#[should_panic]
+fn wtf8buf_truncate_splitting_non_bmp2() {
+    let mut string = Wtf8Buf::from_str("💩");
+    assert!(string.is_known_utf8);
+    string.truncate(2);
+}
+
+#[test]
+#[should_panic]
+fn wtf8buf_truncate_splitting_non_bmp1() {
+    let mut string = Wtf8Buf::from_str("💩");
+    assert!(string.is_known_utf8);
+    string.truncate(1);
+}
+
 #[test]
 fn wtf8buf_into_string() {
     let mut string = Wtf8Buf::from_str("aé 💩");
+    assert!(string.is_known_utf8);
     assert_eq!(string.clone().into_string(), Ok(String::from("aé 💩")));
     string.push(CodePoint::from_u32(0xD800).unwrap());
+    assert!(!string.is_known_utf8);
     assert_eq!(string.clone().into_string(), Err(string));
 }
 
@@ -229,15 +357,33 @@ fn wtf8buf_from_iterator() {
     fn f(values: &[u32]) -> Wtf8Buf {
         values.iter().map(|&c| CodePoint::from_u32(c).unwrap()).collect::<Wtf8Buf>()
     }
-    assert_eq!(f(&[0x61, 0xE9, 0x20, 0x1F4A9]).bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(
+        f(&[0x61, 0xE9, 0x20, 0x1F4A9]),
+        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, 0x20, 0xDCA9]).bytes, b"\xED\xA0\xBD \xED\xB2\xA9");
-    assert_eq!(f(&[0xD800, 0xDBFF]).bytes, b"\xED\xA0\x80\xED\xAF\xBF");
-    assert_eq!(f(&[0xD800, 0xE000]).bytes, b"\xED\xA0\x80\xEE\x80\x80");
-    assert_eq!(f(&[0xD7FF, 0xDC00]).bytes, b"\xED\x9F\xBF\xED\xB0\x80");
-    assert_eq!(f(&[0x61, 0xDC00]).bytes, b"\x61\xED\xB0\x80");
-    assert_eq!(f(&[0xDC00]).bytes, b"\xED\xB0\x80");
+    assert_eq!(
+        f(&[0xD83D, 0x20, 0xDCA9]),
+        Wtf8Buf { bytes: b"\xED\xA0\xBD \xED\xB2\xA9".to_vec(), is_known_utf8: false }
+    );
+    assert_eq!(
+        f(&[0xD800, 0xDBFF]),
+        Wtf8Buf { bytes: b"\xED\xA0\x80\xED\xAF\xBF".to_vec(), is_known_utf8: false }
+    );
+    assert_eq!(
+        f(&[0xD800, 0xE000]),
+        Wtf8Buf { bytes: b"\xED\xA0\x80\xEE\x80\x80".to_vec(), is_known_utf8: false }
+    );
+    assert_eq!(
+        f(&[0xD7FF, 0xDC00]),
+        Wtf8Buf { bytes: b"\xED\x9F\xBF\xED\xB0\x80".to_vec(), is_known_utf8: false }
+    );
+    assert_eq!(
+        f(&[0x61, 0xDC00]),
+        Wtf8Buf { bytes: b"\x61\xED\xB0\x80".to_vec(), is_known_utf8: false }
+    );
+    assert_eq!(f(&[0xDC00]), Wtf8Buf { bytes: b"\xED\xB0\x80".to_vec(), is_known_utf8: false });
 }
 
 #[test]
@@ -251,15 +397,36 @@ fn wtf8buf_extend() {
         string
     }
 
-    assert_eq!(e(&[0x61, 0xE9], &[0x20, 0x1F4A9]).bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    assert_eq!(
+        e(&[0x61, 0xE9], &[0x20, 0x1F4A9]),
+        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, 0x20], &[0xDCA9]).bytes, b"\xED\xA0\xBD \xED\xB2\xA9");
-    assert_eq!(e(&[0xD800], &[0xDBFF]).bytes, b"\xED\xA0\x80\xED\xAF\xBF");
-    assert_eq!(e(&[0xD800], &[0xE000]).bytes, b"\xED\xA0\x80\xEE\x80\x80");
-    assert_eq!(e(&[0xD7FF], &[0xDC00]).bytes, b"\xED\x9F\xBF\xED\xB0\x80");
-    assert_eq!(e(&[0x61], &[0xDC00]).bytes, b"\x61\xED\xB0\x80");
-    assert_eq!(e(&[], &[0xDC00]).bytes, b"\xED\xB0\x80");
+    assert_eq!(
+        e(&[0xD83D, 0x20], &[0xDCA9]),
+        Wtf8Buf { bytes: b"\xED\xA0\xBD \xED\xB2\xA9".to_vec(), is_known_utf8: false }
+    );
+    assert_eq!(
+        e(&[0xD800], &[0xDBFF]),
+        Wtf8Buf { bytes: b"\xED\xA0\x80\xED\xAF\xBF".to_vec(), is_known_utf8: false }
+    );
+    assert_eq!(
+        e(&[0xD800], &[0xE000]),
+        Wtf8Buf { bytes: b"\xED\xA0\x80\xEE\x80\x80".to_vec(), is_known_utf8: false }
+    );
+    assert_eq!(
+        e(&[0xD7FF], &[0xDC00]),
+        Wtf8Buf { bytes: b"\xED\x9F\xBF\xED\xB0\x80".to_vec(), is_known_utf8: false }
+    );
+    assert_eq!(
+        e(&[0x61], &[0xDC00]),
+        Wtf8Buf { bytes: b"\x61\xED\xB0\x80".to_vec(), is_known_utf8: false }
+    );
+    assert_eq!(
+        e(&[], &[0xDC00]),
+        Wtf8Buf { bytes: b"\xED\xB0\x80".to_vec(), is_known_utf8: false }
+    );
 }
 
 #[test]
@@ -407,3 +574,93 @@ fn wtf8_encode_wide_size_hint() {
     assert_eq!((0, Some(0)), iter.size_hint());
     assert!(iter.next().is_none());
 }
+
+#[test]
+fn wtf8_clone_into() {
+    let mut string = Wtf8Buf::new();
+    Wtf8::from_str("green").clone_into(&mut string);
+    assert_eq!(string.bytes, b"green");
+
+    let mut string = Wtf8Buf::from_str("green");
+    Wtf8::from_str("").clone_into(&mut string);
+    assert_eq!(string.bytes, b"");
+
+    let mut string = Wtf8Buf::from_str("red");
+    Wtf8::from_str("green").clone_into(&mut string);
+    assert_eq!(string.bytes, b"green");
+
+    let mut string = Wtf8Buf::from_str("green");
+    Wtf8::from_str("red").clone_into(&mut string);
+    assert_eq!(string.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");
+    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"");
+
+    let mut lowercase = Wtf8Buf::from_str("GrEeN gRaPeS! 🍇");
+    lowercase.make_ascii_lowercase();
+    assert_eq!(lowercase.bytes, b"green grapes! \xf0\x9f\x8d\x87");
+
+    let mut lowercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
+    lowercase.make_ascii_lowercase();
+    assert_eq!(lowercase.bytes, b"\xED\xA0\x80");
+    assert!(!lowercase.is_known_utf8);
+}
+
+#[test]
+fn wtf8_make_ascii_uppercase() {
+    let mut uppercase = Wtf8Buf::from_str("");
+    uppercase.make_ascii_uppercase();
+    assert_eq!(uppercase.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");
+
+    let mut uppercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
+    uppercase.make_ascii_uppercase();
+    assert_eq!(uppercase.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");
+    assert!(!string.is_known_utf8);
+}
diff --git a/src/librustdoc/html/render/context.rs b/src/librustdoc/html/render/context.rs
index 6f029c66c0b7d..01b96dc721558 100644
--- a/src/librustdoc/html/render/context.rs
+++ b/src/librustdoc/html/render/context.rs
@@ -71,7 +71,7 @@ pub(crate) struct Context<'tcx> {
 }
 
 // `Context` is cloned a lot, so we don't want the size to grow unexpectedly.
-#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+#[cfg(all(not(windows), target_arch = "x86_64", target_pointer_width = "64"))]
 rustc_data_structures::static_assert_size!(Context<'_>, 128);
 
 /// Shared mutable state used in [`Context`] and elsewhere.