library: Stabilize const_str_from_utf8_unchecked_mut

Const-stabilizes:
- `str::from_utf8_unchecked_mut`

library/core/src/lib.rs

@@ -152,7 +152,6 @@
 #![feature(const_slice_from_ref)]
 #![feature(const_slice_split_at_mut)]
 #![feature(const_str_as_mut)]
-#![feature(const_str_from_utf8_unchecked_mut)]
 #![feature(const_strict_overflow_ops)]
 #![feature(const_swap)]
 #![feature(const_try)]

library/core/src/str/converts.rs

@@ -195,7 +195,10 @@ 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")]
-#[rustc_const_unstable(feature = "const_str_from_utf8_unchecked_mut", issue = "91005")]
+#[rustc_const_stable(
+    feature = "const_str_from_utf8_unchecked_mut",
+    since = "CURRENT_RUSTC_VERSION"
+)]
 #[rustc_diagnostic_item = "str_from_utf8_unchecked_mut"]
 pub const unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str {
     // SAFETY: the caller must guarantee that the bytes `v`