Require Pointee::Metadata to be Freeze

So pointee metadata can be used in anonymous statics.

This is prerequisite for implementing ThinBox without allocation for
ZST.

See #123184 (comment)

Author: stepancheg

library/core/src/ptr/metadata.rs

@@ -2,6 +2,7 @@
 
 use crate::fmt;
 use crate::hash::{Hash, Hasher};
+use crate::marker::Freeze;
 
 /// Provides the pointer metadata type of any pointed-to type.
 ///
@@ -57,7 +58,7 @@ pub trait Pointee {
     // NOTE: Keep trait bounds in `static_assert_expected_bounds_for_metadata`
     // in `library/core/src/ptr/metadata.rs`
     // in sync with those here:
-    type Metadata: Copy + Send + Sync + Ord + Hash + Unpin;
+    type Metadata: Copy + Send + Sync + Ord + Hash + Unpin + Freeze;
 }
 
 /// Pointers to types implementing this trait alias are “thin”.

library/core/tests/lib.rs

@@ -36,6 +36,7 @@
 #![feature(duration_constructors)]
 #![feature(exact_size_is_empty)]
 #![feature(extern_types)]
+#![feature(freeze)]
 #![feature(flt2dec)]
 #![feature(fmt_internals)]
 #![feature(float_minimum_maximum)]

library/core/tests/ptr.rs

@@ -1,4 +1,5 @@
 use core::cell::RefCell;
+use core::marker::Freeze;
 use core::mem::{self, MaybeUninit};
 use core::num::NonZero;
 use core::ptr;
@@ -841,7 +842,7 @@ fn ptr_metadata_bounds() {
     fn static_assert_expected_bounds_for_metadata<Meta>()
     where
         // Keep this in sync with the associated type in `library/core/src/ptr/metadata.rs`
-        Meta: Copy + Send + Sync + Ord + std::hash::Hash + Unpin,
+        Meta: Copy + Send + Sync + Ord + std::hash::Hash + Unpin + Freeze,
     {
     }
 }