library: Stabilize const_str_as_mut

Const-stabilizes:
- `str::as_bytes_mut`
- `str::as_mut_ptr`

library/core/src/lib.rs

@@ -151,7 +151,6 @@
 #![feature(const_slice_from_raw_parts_mut)]
 #![feature(const_slice_from_ref)]
 #![feature(const_slice_split_at_mut)]
-#![feature(const_str_as_mut)]
 #![feature(const_strict_overflow_ops)]
 #![feature(const_swap)]
 #![feature(const_try)]

library/core/src/str/converts.rs

@@ -195,6 +195,7 @@ pub const fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
 #[inline]
 #[must_use]
 #[stable(feature = "str_mut_extras", since = "1.20.0")]
+#[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
 #[rustc_const_stable(
     feature = "const_str_from_utf8_unchecked_mut",
     since = "CURRENT_RUSTC_VERSION"

library/core/src/str/mod.rs

@@ -339,7 +339,8 @@ pub const fn as_bytes(&self) -> &[u8] {
     /// assert_eq!("🍔∈🌏", s);
     /// ```
     #[stable(feature = "str_mut_extras", since = "1.20.0")]
-    #[rustc_const_unstable(feature = "const_str_as_mut", issue = "130086")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_str_as_mut", since = "CURRENT_RUSTC_VERSION")]
     #[must_use]
     #[inline(always)]
     pub const unsafe fn as_bytes_mut(&mut self) -> &mut [u8] {
@@ -385,7 +386,8 @@ pub const fn as_ptr(&self) -> *const u8 {
     /// It is your responsibility to make sure that the string slice only gets
     /// modified in a way that it remains valid UTF-8.
     #[stable(feature = "str_as_mut_ptr", since = "1.36.0")]
-    #[rustc_const_unstable(feature = "const_str_as_mut", issue = "130086")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_str_as_mut", since = "CURRENT_RUSTC_VERSION")]
     #[rustc_never_returns_null_ptr]
     #[must_use]
     #[inline(always)]