Rollup merge of rust-lang#131463 - bjoernager:const-char-encode-utf8, r=RalfJung

Stabilise `const_char_encode_utf8`.

Closes: rust-lang#130512

This PR stabilises the `const_char_encode_utf8` feature gate (i.e. support for `char::encode_utf8` in const scenarios).

Note that the linked tracking issue is currently awaiting FCP.

Author: tgross35

library/core/src/char/methods.rs

@@ -674,8 +674,9 @@ impl char {
     /// 'ß'.encode_utf8(&mut b);
     /// ```
     #[stable(feature = "unicode_encode_char", since = "1.15.0")]
-    #[rustc_const_unstable(feature = "const_char_encode_utf8", issue = "130512")]
+    #[rustc_const_stable(feature = "const_char_encode_utf8", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
     pub const fn encode_utf8(self, dst: &mut [u8]) -> &mut str {
         // SAFETY: `char` is not a surrogate, so this is valid UTF-8.
         unsafe { from_utf8_unchecked_mut(encode_utf8_raw(self as u32, dst)) }
@@ -1770,9 +1771,11 @@ const fn len_utf16(code: u32) -> usize {
 /// Panics if the buffer is not large enough.
 /// A buffer of length four is large enough to encode any `char`.
 #[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
-#[rustc_const_unstable(feature = "const_char_encode_utf8", issue = "130512")]
+#[rustc_const_stable(feature = "const_char_encode_utf8", since = "CURRENT_RUSTC_VERSION")]
 #[doc(hidden)]
 #[inline]
+#[rustc_allow_const_fn_unstable(const_eval_select)]
+#[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
 pub const fn encode_utf8_raw(code: u32, dst: &mut [u8]) -> &mut [u8] {
     const fn panic_at_const(_code: u32, _len: usize, _dst_len: usize) {
         // Note that we cannot format in constant expressions.

library/core/src/lib.rs

@@ -118,7 +118,6 @@
 #![feature(const_bigint_helper_methods)]
 #![feature(const_black_box)]
 #![feature(const_char_encode_utf16)]
-#![feature(const_char_encode_utf8)]
 #![feature(const_eval_select)]
 #![feature(const_exact_div)]
 #![feature(const_fmt_arguments_new)]