Rollup merge of #129812 - RalfJung:box-custom-alloc, r=compiler-errors

interpret, codegen: tweak some comments and checks regarding Box with custom allocator

Cc https://github.com/rust-lang/rust/issues/95453

compiler/rustc_middle/src/ty/sty.rs

@@ -1149,7 +1149,10 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
-    /// Tests whether this is a Box using the global allocator.
+    /// Tests whether this is a Box definitely using the global allocator.
+    ///
+    /// If the allocator is still generic, the answer is `false`, but it may
+    /// later turn out that it does use the global allocator.
     #[inline]
     pub fn is_box_global(self, tcx: TyCtxt<'tcx>) -> bool {
         match self.kind() {