Rollup merge of #130954 - workingjubilee:stabilize-const-mut-fn, r=RalfJung

Stabilize const `ptr::write*` and `mem::replace`

Since `const_mut_refs` and `const_refs_to_cell` have been stabilized, we may now also stabilize the ability to write to places during const evaluation inside our library API. So, we now propose the `const fn` version of `ptr::write` and its variants. This allows us to also stabilize `mem::replace` and `ptr::replace`.
- const `mem::replace`: #83164 (comment)
- const `ptr::write{,_bytes,_unaligned}`: #86302 (comment)

Their implementation requires an additional internal stabilization of `const_intrinsic_forget`, which is required for `*::write*` and thus `*::replace`. Thus we const-stabilize the internal intrinsics `forget`, `write_bytes`, and `write_via_move`.

Author: tgross35

Diff for: library/alloc/src/lib.rs

@@ -173,7 +173,6 @@
 #![feature(allow_internal_unstable)]
 #![feature(cfg_sanitize)]
 #![feature(const_precise_live_drops)]
-#![feature(const_ptr_write)]
 #![feature(const_try)]
 #![feature(decl_macro)]
 #![feature(dropck_eyepatch)]

Diff for: library/alloc/tests/lib.rs

@@ -7,7 +7,6 @@
 #![feature(const_cow_is_borrowed)]
 #![feature(const_heap)]
 #![cfg_attr(bootstrap, feature(const_mut_refs))]
-#![feature(const_ptr_write)]
 #![feature(const_try)]
 #![feature(core_intrinsics)]
 #![feature(extract_if)]

Diff for: library/core/src/intrinsics.rs

@@ -1084,7 +1084,7 @@ extern "rust-intrinsic" {
     /// it does not require an `unsafe` block.
     /// Therefore, implementations must not require the user to uphold
     /// any safety invariants.
-    #[rustc_const_unstable(feature = "const_intrinsic_forget", issue = "none")]
+    #[rustc_const_stable(feature = "const_intrinsic_forget", since = "CURRENT_RUSTC_VERSION")]
     #[rustc_safe_intrinsic]
     #[rustc_nounwind]
     pub fn forget<T: ?Sized>(_: T);
@@ -2688,7 +2688,7 @@ extern "rust-intrinsic" {
     /// This intrinsic can *only* be called where the pointer is a local without
     /// projections (`write_via_move(ptr, x)`, not `write_via_move(*ptr, x)`) so
     /// that it trivially obeys runtime-MIR rules about derefs in operands.
-    #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+    #[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
     #[rustc_nounwind]
     pub fn write_via_move<T>(ptr: *mut T, value: T);
 
@@ -3525,13 +3525,13 @@ pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
 #[doc(alias = "memset")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_allowed_through_unstable_modules]
-#[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+#[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
 #[inline(always)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 #[rustc_diagnostic_item = "ptr_write_bytes"]
 pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
     extern "rust-intrinsic" {
-        #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+        #[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
         #[rustc_nounwind]
         fn write_bytes<T>(dst: *mut T, val: u8, count: usize);
     }

Diff for: library/core/src/lib.rs

@@ -124,7 +124,6 @@
 #![feature(const_hash)]
 #![feature(const_heap)]
 #![feature(const_index_range_slice_index)]
-#![feature(const_intrinsic_forget)]
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]
 #![feature(const_likely)]
@@ -138,9 +137,7 @@
 #![feature(const_pointer_is_aligned)]
 #![feature(const_ptr_is_null)]
 #![feature(const_ptr_sub_ptr)]
-#![feature(const_ptr_write)]
 #![feature(const_raw_ptr_comparison)]
-#![feature(const_replace)]
 #![feature(const_size_of_val)]
 #![feature(const_size_of_val_raw)]
 #![feature(const_strict_overflow_ops)]

Diff for: library/core/src/mem/mod.rs

@@ -857,7 +857,8 @@ pub fn take<T: Default>(dest: &mut T) -> T {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "if you don't need the old value, you can just assign the new value directly"]
-#[rustc_const_unstable(feature = "const_replace", issue = "83164")]
+#[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+#[rustc_const_stable(feature = "const_replace", since = "CURRENT_RUSTC_VERSION")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_replace")]
 pub const fn replace<T>(dest: &mut T, src: T) -> T {
     // It may be tempting to use `swap` to avoid `unsafe` here. Don't!

Diff for: library/core/src/ptr/mod.rs

@@ -1263,7 +1263,8 @@ const unsafe fn swap_nonoverlapping_simple_untyped<T>(x: *mut T, y: *mut T, coun
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature = "const_replace", issue = "83164")]
+#[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+#[rustc_const_stable(feature = "const_replace", since = "CURRENT_RUSTC_VERSION")]
 #[rustc_diagnostic_item = "ptr_replace"]
 pub const unsafe fn replace<T>(dst: *mut T, src: T) -> T {
     // SAFETY: the caller must guarantee that `dst` is valid to be
@@ -1611,7 +1612,7 @@ pub const unsafe fn read_unaligned<T>(src: *const T) -> T {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+#[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
 #[rustc_diagnostic_item = "ptr_write"]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 pub const unsafe fn write<T>(dst: *mut T, src: T) {
@@ -1719,7 +1720,8 @@ pub const unsafe fn write<T>(dst: *mut T, src: T) {
 /// ```
 #[inline]
 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
-#[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+#[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_refs_to_cell))]
+#[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
 #[rustc_diagnostic_item = "ptr_write_unaligned"]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 pub const unsafe fn write_unaligned<T>(dst: *mut T, src: T) {

Diff for: library/core/src/ptr/mut_ptr.rs

@@ -1449,7 +1449,7 @@ impl<T: ?Sized> *mut T {
     ///
     /// [`ptr::write`]: crate::ptr::write()
     #[stable(feature = "pointer_methods", since = "1.26.0")]
-    #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+    #[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn write(self, val: T)
@@ -1468,7 +1468,7 @@ impl<T: ?Sized> *mut T {
     /// [`ptr::write_bytes`]: crate::ptr::write_bytes()
     #[doc(alias = "memset")]
     #[stable(feature = "pointer_methods", since = "1.26.0")]
-    #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+    #[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn write_bytes(self, val: u8, count: usize)
@@ -1509,7 +1509,7 @@ impl<T: ?Sized> *mut T {
     ///
     /// [`ptr::write_unaligned`]: crate::ptr::write_unaligned()
     #[stable(feature = "pointer_methods", since = "1.26.0")]
-    #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+    #[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn write_unaligned(self, val: T)

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

@@ -1013,7 +1013,7 @@ impl<T: ?Sized> NonNull<T> {
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     #[stable(feature = "non_null_convenience", since = "1.80.0")]
-    #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+    #[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
     pub const unsafe fn write(self, val: T)
     where
         T: Sized,
@@ -1032,7 +1032,7 @@ impl<T: ?Sized> NonNull<T> {
     #[doc(alias = "memset")]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     #[stable(feature = "non_null_convenience", since = "1.80.0")]
-    #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+    #[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
     pub const unsafe fn write_bytes(self, val: u8, count: usize)
     where
         T: Sized,
@@ -1073,7 +1073,7 @@ impl<T: ?Sized> NonNull<T> {
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     #[stable(feature = "non_null_convenience", since = "1.80.0")]
-    #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
+    #[rustc_const_stable(feature = "const_ptr_write", since = "CURRENT_RUSTC_VERSION")]
     pub const unsafe fn write_unaligned(self, val: T)
     where
         T: Sized,

Diff for: library/core/tests/lib.rs

@@ -28,7 +28,6 @@
 #![feature(const_option_ext)]
 #![feature(const_pin)]
 #![feature(const_pointer_is_aligned)]
-#![feature(const_ptr_write)]
 #![feature(const_three_way_compare)]
 #![feature(const_trait_impl)]
 #![feature(core_intrinsics)]

Diff for: tests/ui/consts/load-preserves-partial-init.rs

@@ -1,6 +1,5 @@
 //@ run-pass
 
-#![feature(const_ptr_write)]
 // issue: https://github.com/rust-lang/rust/issues/69488
 // Loads of partially-initialized data could produce completely-uninitialized results.
 // Test to make sure that we no longer do such a "deinitializing" load.