Rollup merge of #129091 - RalfJung:box_as_ptr, r=Amanieu

add Box::as_ptr and Box::as_mut_ptr methods

Unstably implements https://github.com/rust-lang/libs-team/issues/355. Tracking issue: https://github.com/rust-lang/rust/issues/129090.

r? libs-api

compiler/rustc_middle/src/lib.rs

@@ -34,6 +34,7 @@
 #![feature(allocator_api)]
 #![feature(array_windows)]
 #![feature(assert_matches)]
+#![feature(box_as_ptr)]
 #![feature(box_patterns)]
 #![feature(closure_track_caller)]
 #![feature(const_option)]

compiler/rustc_middle/src/mir/interpret/allocation.rs

@@ -62,13 +62,11 @@ impl AllocBytes for Box<[u8]> {
     }
 
     fn as_mut_ptr(&mut self) -> *mut u8 {
-        // Carefully avoiding any intermediate references.
-        ptr::addr_of_mut!(**self).cast()
+        Box::as_mut_ptr(self).cast()
     }
 
     fn as_ptr(&self) -> *const u8 {
-        // Carefully avoiding any intermediate references.
-        ptr::addr_of!(**self).cast()
+        Box::as_ptr(self).cast()
     }
 }