Auto merge of #97710 - RalfJung:ptr-addr, r=thomcc

implement ptr.addr() via transmute

As per the discussion in rust-lang/unsafe-code-guidelines#286, the semantics for ptr-to-int transmutes that we are going with for now is to make them strip provenance without exposing it. That's exactly what `ptr.addr()` does! So we can implement `ptr.addr()` via `transmute`. This also means that once #97684 lands, Miri can distinguish `ptr.addr()` from `ptr.expose_addr()`, and the following code will correctly be called out as having UB (if permissive provenance mode is enabled, which will become the default once the [implementation is complete](rust-lang/miri#2133)):

```rust
fn main() {
    let x: i32 = 3;
    let x_ptr = &x as *const i32;

    let x_usize: usize = x_ptr.addr();
    // Cast back an address that did *not* get exposed.
    let ptr = std::ptr::from_exposed_addr::<i32>(x_usize);
    assert_eq!(unsafe { *ptr }, 3); //~ ERROR Undefined Behavior: dereferencing pointer failed
}
```

This completes the Miri implementation of the new distinctions introduced by strict provenance. :)

Cc `@Gankra` -- for now I left in your `FIXME(strict_provenance_magic)` saying these should be intrinsics, but I do not necessarily agree that they should be. Or if we have an intrinsic, I think it should behave exactly like the `transmute` does, which makes one wonder why the intrinsic should be needed.

library/core/src/ptr/const_ptr.rs

@@ -180,7 +180,9 @@ impl<T: ?Sized> *const T {
         T: Sized,
     {
         // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
-        self as usize
+        // SAFETY: Pointer-to-integer transmutes are valid (if you are okay with losing the
+        // provenance).
+        unsafe { mem::transmute(self) }
     }
 
     /// Gets the "address" portion of the pointer, and 'exposes' the "provenance" part for future

library/core/src/ptr/mut_ptr.rs

@@ -184,7 +184,9 @@ impl<T: ?Sized> *mut T {
         T: Sized,
     {
         // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
-        self as usize
+        // SAFETY: Pointer-to-integer transmutes are valid (if you are okay with losing the
+        // provenance).
+        unsafe { mem::transmute(self) }
     }
 
     /// Gets the "address" portion of the pointer, and 'exposes' the "provenance" part for future