Rollup merge of #144539 - RalfJung:const_with_exposed_provenance, r=oli-obk

constify with_exposed_provenance

We allow `int as ptr` in const, so it only makes sense to also allow this function. Otherwise, `const fn` can't be ported to use the more explicit exposed provenance APIs.

Note that as of today, `with_exposed_provenance` in const is equivalent to `without_provenance`. However, we probably don't want to promise that: if someone does `with_exposed_provenance(MMIO_ADDR)` in const and then uses that pointer at runtime, that is something we should ensure keeps working; if someone does the same with `without_provenance` then I would consider that UB.

Tracking: #144538
Cc `````@rust-lang/wg-const-eval````` `````@rust-lang/opsem`````

Author: Zalathar

library/core/src/ptr/mod.rs

@@ -974,9 +974,10 @@ pub const fn dangling_mut<T>() -> *mut T {
 #[must_use]
 #[inline(always)]
 #[stable(feature = "exposed_provenance", since = "1.84.0")]
+#[rustc_const_unstable(feature = "const_exposed_provenance", issue = "144538")]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 #[allow(fuzzy_provenance_casts)] // this *is* the explicit provenance API one should use instead
-pub fn with_exposed_provenance<T>(addr: usize) -> *const T {
+pub const fn with_exposed_provenance<T>(addr: usize) -> *const T {
     addr as *const T
 }
 
@@ -1014,9 +1015,10 @@ pub fn with_exposed_provenance<T>(addr: usize) -> *const T {
 #[must_use]
 #[inline(always)]
 #[stable(feature = "exposed_provenance", since = "1.84.0")]
+#[rustc_const_unstable(feature = "const_exposed_provenance", issue = "144538")]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 #[allow(fuzzy_provenance_casts)] // this *is* the explicit provenance API one should use instead
-pub fn with_exposed_provenance_mut<T>(addr: usize) -> *mut T {
+pub const fn with_exposed_provenance_mut<T>(addr: usize) -> *mut T {
     addr as *mut T
 }