Constify mem::replace and ptr::replace

Author: usbalbin

library/core/src/mem/mod.rs

@@ -813,7 +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

@@ -584,7 +584,8 @@ 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