Auto merge of #117658 - RalfJung:ptr-dangling, r=m-ou-se

rename ptr::invalid -> ptr::without_provenance

It has long bothered me that `ptr::invalid` returns a pointer that is actually valid for zero-sized memory accesses. In general, it doesn't even make sense to ask "is this pointer valid", you have to ask "is this pointer valid for a given memory access". We could say that a pointer is invalid if it is not valid for *any* memory access, but [the way this FCP is going](https://github.com/rust-lang/unsafe-code-guidelines/issues/472), it looks like *all* pointers will be valid for zero-sized memory accesses.

Two possible alternative names emerged as people's favorites:
1. Something involving `dangling`, in analogy to `NonNull::dangling`. To avoid inconsistency with the `NonNull` method, the address-taking method could be called `dangling_at(addr: usize) -> *const T`.
2. `without_provenance`, to be symmetric with the inverse operation `ptr.addr_without_provenance()` (currently still called `ptr.addr()` but probably going to be renamed)

I have no idea which one of these is better. I read [this comment](https://github.com/rust-lang/rust/pull/117658#issuecomment-1830934701) as expressing a slight preference for something like the second option, so I went for that. I'm happy to go with `dangling_at` as well.

Cc `@rust-lang/opsem`

1 files changed, 6 insertions, 2 deletions

diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index e1211da4c61..524aa35e045 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -2555,7 +2555,9 @@ impl<T> Weak<T> {
     #[must_use]
     pub const fn new() -> Weak<T> {
         Weak {
-            ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<ArcInner<T>>(usize::MAX)) },
+            ptr: unsafe {
+                NonNull::new_unchecked(ptr::without_provenance_mut::<ArcInner<T>>(usize::MAX))
+            },
             alloc: Global,
         }
     }
@@ -2583,7 +2585,9 @@ impl<T, A: Allocator> Weak<T, A> {
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn new_in(alloc: A) -> Weak<T, A> {
         Weak {
-            ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<ArcInner<T>>(usize::MAX)) },
+            ptr: unsafe {
+                NonNull::new_unchecked(ptr::without_provenance_mut::<ArcInner<T>>(usize::MAX))
+            },
             alloc,
         }
     }