Auto merge of #122520 - scottmcm:stabilize_unchecked_math_basics, r=jhpratt

Stabilize `unchecked_{add,sub,mul}`

Tracking issue: #85122

I think we might as well just stabilize these basic three.  They're the ones that have `nuw`/`nsw` flags in LLVM.

Notably, this doesn't include the potentially-more-complex or -more-situational things like `unchecked_neg` or `unchecked_shr` that are under different feature flags.

To quote Ralf https://github.com/rust-lang/rust/issues/85122#issuecomment-1681669646,

> Are there any objections to stabilizing at least `unchecked_{add,sub,mul}`? For those there shouldn't be any surprises about what their safety requirements are.

*Semantially* these are [already available on stable, even in `const`, via](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=bdb1ff889b61950897f1e9f56d0c9a36) `checked_*`+`unreachable_unchecked`.  So IMHO we might as well just let people write them directly, rather than try to go through a `let Some(x) = x.checked_add(y) else { unsafe { hint::unreachable_unchecked() }};` dance.

I added additional text to each method to attempt to better describe the behaviour and encourage `wrapping_*` instead.

r? rust-lang/libs-api

3 files changed, 0 insertions, 6 deletions

diff --git a/library/core/src/ptr/const_ptr.rs b/library/core/src/ptr/const_ptr.rs
index 4081ad124d3..68ce80ee321 100644
--- a/library/core/src/ptr/const_ptr.rs
+++ b/library/core/src/ptr/const_ptr.rs
@@ -1029,8 +1029,6 @@ impl<T: ?Sized> *const T {
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
-    // We could always go back to wrapping if unchecked becomes unacceptable
-    #[rustc_allow_const_fn_unstable(const_int_unchecked_arith)]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn sub(self, count: usize) -> Self
diff --git a/library/core/src/ptr/mut_ptr.rs b/library/core/src/ptr/mut_ptr.rs
index 2ff7d829918..3dde83541d4 100644
--- a/library/core/src/ptr/mut_ptr.rs
+++ b/library/core/src/ptr/mut_ptr.rs
@@ -1119,8 +1119,6 @@ impl<T: ?Sized> *mut T {
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
-    // We could always go back to wrapping if unchecked becomes unacceptable
-    #[rustc_allow_const_fn_unstable(const_int_unchecked_arith)]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn sub(self, count: usize) -> Self
diff --git a/library/core/src/ptr/non_null.rs b/library/core/src/ptr/non_null.rs
index 70e6f3f9e79..c0a36bb405e 100644
--- a/library/core/src/ptr/non_null.rs
+++ b/library/core/src/ptr/non_null.rs
@@ -702,8 +702,6 @@ impl<T: ?Sized> NonNull<T> {
     #[unstable(feature = "non_null_convenience", issue = "117691")]
     #[rustc_const_unstable(feature = "non_null_convenience", issue = "117691")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
-    // We could always go back to wrapping if unchecked becomes unacceptable
-    #[rustc_allow_const_fn_unstable(const_int_unchecked_arith)]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn sub(self, count: usize) -> Self