Revert "Revert effects of PRs 81167 and 83091."

This reverts commit 9d96b0e.

Author: usbalbin

library/core/src/mem/mod.rs

@@ -682,7 +682,8 @@ pub unsafe fn uninitialized<T>() -> T {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub fn swap<T>(x: &mut T, y: &mut T) {
+#[rustc_const_unstable(feature = "const_swap", issue = "83163")]
+pub const fn swap<T>(x: &mut T, y: &mut T) {
     // SAFETY: the raw pointers have been created from safe mutable references satisfying all the
     // constraints on `ptr::swap_nonoverlapping_one`
     unsafe {
@@ -812,7 +813,8 @@ 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"]
-pub fn replace<T>(dest: &mut T, src: T) -> T {
+#[rustc_const_unstable(feature = "const_replace", issue = "83164")]
+pub const fn replace<T>(dest: &mut T, src: T) -> T {
     // SAFETY: We read from `dest` but directly write `src` into it afterwards,
     // such that the old value is not duplicated. Nothing is dropped and
     // nothing here can panic.

library/core/src/ptr/mod.rs

@@ -430,7 +430,8 @@ pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
 }
 
 #[inline]
-pub(crate) unsafe fn swap_nonoverlapping_one<T>(x: *mut T, y: *mut T) {
+#[rustc_const_unstable(feature = "const_swap", issue = "83163")]
+pub(crate) const unsafe fn swap_nonoverlapping_one<T>(x: *mut T, y: *mut T) {
     // NOTE(eddyb) SPIR-V's Logical addressing model doesn't allow for arbitrary
     // reinterpretation of values as (chunkable) byte arrays, and the loop in the
     // block optimization in `swap_nonoverlapping_bytes` is hard to rewrite back
@@ -563,7 +564,8 @@ const unsafe fn swap_nonoverlapping_bytes(x: *mut u8, y: *mut u8, len: usize) {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
+#[rustc_const_unstable(feature = "const_replace", issue = "83164")]
+pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
     // SAFETY: the caller must guarantee that `dst` is valid to be
     // cast to a mutable reference (valid for writes, aligned, initialized),
     // and cannot overlap `src` since `dst` must point to a distinct
@@ -869,10 +871,12 @@ pub const unsafe fn read_unaligned<T>(src: *const T) -> T {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub unsafe fn write<T>(dst: *mut T, src: T) {
+#[rustc_const_unstable(feature = "const_ptr_write", issue = "none")]
+pub const unsafe fn write<T>(dst: *mut T, src: T) {
     // We are calling the intrinsics directly to avoid function calls in the generated code
     // as `intrinsics::copy_nonoverlapping` is a wrapper function.
     extern "rust-intrinsic" {
+        #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "none")]
         fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
     }
 

library/core/src/ptr/mut_ptr.rs

@@ -1002,8 +1002,9 @@ 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 = "none")]
     #[inline(always)]
-    pub unsafe fn write(self, val: T)
+    pub const unsafe fn write(self, val: T)
     where
         T: Sized,
     {