Rollup merge of rust-lang#131177 - workingjubilee:stabilize-const-mut-referees, r=tgross35

 Stabilize 5 `const_mut_refs`-dependent API

Since `const_mut_refs` and `const_refs_to_cell` have been stabilized, we now may create mutable references inside our library API. Thus we now stabilize the `const fn` version of these public library APIs which required such in their implementation:
- const `NonNull::as_mut` rust-lang#91822 (comment)
- const `slice::{first,last}_mut`: rust-lang#83570 (comment)
- const `str::as_{mut_ptr,bytes_mut}`: rust-lang#130086 (comment)
- const `str::from_utf8_unchecked_mut`: rust-lang#91005 (comment)
- const `UnsafeCell::get_mut`: rust-lang#88836 (comment)

Author: workingjubilee

Diff for: core/src/cell.rs

@@ -2175,7 +2175,8 @@ impl<T: ?Sized> UnsafeCell<T> {
     /// ```
     #[inline(always)]
     #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
-    #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_unsafecell_get_mut", since = "CURRENT_RUSTC_VERSION")]
     pub const fn get_mut(&mut self) -> &mut T {
         &mut self.value
     }

Diff for: core/src/lib.rs

@@ -138,7 +138,6 @@
 #![feature(const_option_ext)]
 #![feature(const_pin)]
 #![feature(const_pointer_is_aligned)]
-#![feature(const_ptr_as_ref)]
 #![feature(const_ptr_is_null)]
 #![feature(const_ptr_sub_ptr)]
 #![feature(const_ptr_write)]
@@ -149,8 +148,6 @@
 #![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_str_from_utf8_unchecked_mut)]
 #![feature(const_strict_overflow_ops)]
 #![feature(const_swap)]
 #![feature(const_try)]
@@ -159,7 +156,6 @@
 #![feature(const_typed_swap)]
 #![feature(const_ub_checks)]
 #![feature(const_unicode_case_lookup)]
-#![feature(const_unsafecell_get_mut)]
 #![feature(coverage_attribute)]
 #![feature(do_not_recommend)]
 #![feature(duration_consts_float)]

Diff for: core/src/ptr/non_null.rs

@@ -394,7 +394,8 @@ impl<T: ?Sized> NonNull<T> {
     ///
     /// [the module documentation]: crate::ptr#safety
     #[stable(feature = "nonnull", since = "1.25.0")]
-    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_ptr_as_ref", since = "CURRENT_RUSTC_VERSION")]
     #[must_use]
     #[inline(always)]
     pub const unsafe fn as_mut<'a>(&mut self) -> &'a mut T {

Diff for: core/src/slice/mod.rs

@@ -171,7 +171,8 @@ impl<T> [T] {
     /// assert_eq!(None, y.first_mut());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_slice_first_last", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
     #[must_use]
     pub const fn first_mut(&mut self) -> Option<&mut T> {
@@ -213,7 +214,8 @@ impl<T> [T] {
     /// assert_eq!(x, &[3, 4, 5]);
     /// ```
     #[stable(feature = "slice_splits", since = "1.5.0")]
-    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_slice_first_last", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
     #[must_use]
     pub const fn split_first_mut(&mut self) -> Option<(&mut T, &mut [T])> {
@@ -255,7 +257,8 @@ impl<T> [T] {
     /// assert_eq!(x, &[4, 5, 3]);
     /// ```
     #[stable(feature = "slice_splits", since = "1.5.0")]
-    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_slice_first_last", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
     #[must_use]
     pub const fn split_last_mut(&mut self) -> Option<(&mut T, &mut [T])> {
@@ -297,7 +300,8 @@ impl<T> [T] {
     /// assert_eq!(None, y.last_mut());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_slice_first_last", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
     #[must_use]
     pub const fn last_mut(&mut self) -> Option<&mut T> {

Diff for: core/src/str/converts.rs

@@ -195,7 +195,11 @@ pub const unsafe fn from_utf8_unchecked(v: &[u8]) -> &str {
 #[inline]
 #[must_use]
 #[stable(feature = "str_mut_extras", since = "1.20.0")]
-#[rustc_const_unstable(feature = "const_str_from_utf8_unchecked_mut", issue = "91005")]
+#[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"
+)]
 #[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`

Diff for: core/src/str/mod.rs

@@ -339,7 +339,8 @@ impl str {
     /// 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 @@ impl str {
     /// 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)]

Diff for: core/tests/lib.rs

@@ -29,7 +29,6 @@
 #![feature(const_option_ext)]
 #![feature(const_pin)]
 #![feature(const_pointer_is_aligned)]
-#![feature(const_ptr_as_ref)]
 #![feature(const_ptr_write)]
 #![feature(const_result)]
 #![feature(const_slice_from_ref)]