Rollup merge of rust-lang#100291 - WaffleLapkin:cstr_const_methods, r…

…=oli-obk

constify some `CStr` methods

This PR marks the following public APIs as `const`:
```rust
impl CStr {
    // feature(const_cstr_from_bytes)
    pub const fn from_bytes_until_nul(bytes: &[u8]) -> Result<&CStr, FromBytesUntilNulError>;
    pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, FromBytesWithNulError>;

    // feature(const_cstr_to_bytes)
    pub const fn to_bytes(&self) -> &[u8];
    pub const fn to_bytes_with_nul(&self) -> &[u8];
    pub const fn to_str(&self) -> Result<&str, str::Utf8Error>;
}
```

r? ``@oli-obk`` (use of `const_eval_select` :P )
cc ``@mina86`` (you've asked for this <3 )

library/core/src/ffi/c_str.rs

@@ -120,10 +120,10 @@ enum FromBytesWithNulErrorKind {
 }
 
 impl FromBytesWithNulError {
- fn interior_nul(pos: usize) -> FromBytesWithNulError {
+ const fn interior_nul(pos: usize) -> FromBytesWithNulError {
  FromBytesWithNulError { kind: FromBytesWithNulErrorKind::InteriorNul(pos) }
  }
- fn not_nul_terminated() -> FromBytesWithNulError {
+ const fn not_nul_terminated() -> FromBytesWithNulError {
  FromBytesWithNulError { kind: FromBytesWithNulErrorKind::NotNulTerminated }
  }
 
@@ -294,7 +294,8 @@ impl CStr {
  /// ```
  ///
  #[unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
- pub fn from_bytes_until_nul(bytes: &[u8]) -> Result<&CStr, FromBytesUntilNulError> {
+ #[rustc_const_unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
+ pub const fn from_bytes_until_nul(bytes: &[u8]) -> Result<&CStr, FromBytesUntilNulError> {
  let nul_pos = memchr::memchr(0, bytes);
  match nul_pos {
  Some(nul_pos) => {
@@ -343,7 +344,8 @@ impl CStr {
  /// assert!(cstr.is_err());
  /// ```
  #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
- pub fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, FromBytesWithNulError> {
+ #[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
+ pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, FromBytesWithNulError> {
  let nul_pos = memchr::memchr(0, bytes);
  match nul_pos {
  Some(nul_pos) if nul_pos + 1 == bytes.len() => {
@@ -493,7 +495,8 @@ impl CStr {
  #[must_use = "this returns the result of the operation, \
  without modifying the original"]
  #[stable(feature = "rust1", since = "1.0.0")]
- pub fn to_bytes(&self) -> &[u8] {
+ #[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
+ pub const fn to_bytes(&self) -> &[u8] {
  let bytes = self.to_bytes_with_nul();
  // SAFETY: to_bytes_with_nul returns slice with length at least 1
  unsafe { bytes.get_unchecked(..bytes.len() - 1) }
@@ -520,7 +523,8 @@ impl CStr {
  #[must_use = "this returns the result of the operation, \
  without modifying the original"]
  #[stable(feature = "rust1", since = "1.0.0")]
- pub fn to_bytes_with_nul(&self) -> &[u8] {
+ #[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
+ pub const fn to_bytes_with_nul(&self) -> &[u8] {
  // SAFETY: Transmuting a slice of `c_char`s to a slice of `u8`s
  // is safe on all supported targets.
  unsafe { &*(&self.inner as *const [c_char] as *const [u8]) }
@@ -543,7 +547,8 @@ impl CStr {
  /// assert_eq!(cstr.to_str(), Ok("foo"));
  /// ```
  #[stable(feature = "cstr_to_str", since = "1.4.0")]
- pub fn to_str(&self) -> Result<&str, str::Utf8Error> {
+ #[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
+ pub const fn to_str(&self) -> Result<&str, str::Utf8Error> {
  // N.B., when `CStr` is changed to perform the length check in `.to_bytes()`
  // instead of in `from_ptr()`, it may be worth considering if this should
  // be rewritten to do the UTF-8 check inline with the length calculation

library/core/src/lib.rs

@@ -159,6 +159,7 @@
 #![feature(const_slice_from_ref)]
 #![feature(const_slice_index)]
 #![feature(const_is_char_boundary)]
+#![feature(const_cstr_methods)]
 //
 // Language features:
 #![feature(abi_unadjusted)]

library/core/src/slice/memchr.rs

@@ -2,6 +2,7 @@
 // Copyright 2015 Andrew Gallant, bluss and Nicolas Koch
 
 use crate::cmp;
+use crate::intrinsics;
 use crate::mem;
 
 const LO_USIZE: usize = usize::repeat_u8(0x01);
@@ -35,13 +36,31 @@ fn repeat_byte(b: u8) -> usize {
 /// Returns the first index matching the byte `x` in `text`.
 #[must_use]
 #[inline]
-pub fn memchr(x: u8, text: &[u8]) -> Option<usize> {
- // Fast path for small slices
- if text.len() < 2 * USIZE_BYTES {
- return text.iter().position(|elt| *elt == x);
+pub const fn memchr(x: u8, text: &[u8]) -> Option<usize> {
+ #[inline]
+ fn rt_impl(x: u8, text: &[u8]) -> Option<usize> {
+ // Fast path for small slices
+ if text.len() < 2 * USIZE_BYTES {
+ return text.iter().position(|elt| *elt == x);
+ }
+
+ memchr_general_case(x, text)
+ }
+
+ const fn const_impl(x: u8, bytes: &[u8]) -> Option<usize> {
+ let mut i = 0;
+ while i < bytes.len() {
+ if bytes[i] == x {
+ return Some(i);
+ }
+ i += 1;
+ }
+
+ None
  }
 
- memchr_general_case(x, text)
+ // SAFETY: The const and runtime versions have identical behavior
+ unsafe { intrinsics::const_eval_select((x, text), const_impl, rt_impl) }
 }
 
 fn memchr_general_case(x: u8, text: &[u8]) -> Option<usize> {