Rollup merge of rust-lang#121840 - oli-obk:freeze, r=dtolnay

Expose the Freeze trait again (unstably) and forbid implementing it manually

non-emoji version of rust-lang#121501

cc rust-lang#60715

This trait is useful for generic constants (associated consts of generic traits). See the test (`tests/ui/associated-consts/freeze.rs`) added in this PR for a usage example. The builtin `Freeze` trait is the only way to do it, users cannot work around this issue.

It's also a useful trait for building some very specific abstrations, as shown by the usage by the `zerocopy` crate: google/zerocopy#941

cc ```@RalfJung```

T-lang signed off on reexposing this unstably: rust-lang#121501 (comment)

Author: jhpratt

core/src/lib.rs

@@ -204,6 +204,7 @@
 // tidy-alphabetical-start
 #![cfg_attr(bootstrap, feature(diagnostic_namespace))]
 #![cfg_attr(bootstrap, feature(platform_intrinsics))]
+#![cfg_attr(not(bootstrap), feature(freeze_impls))]
 #![feature(abi_unadjusted)]
 #![feature(adt_const_params)]
 #![feature(allow_internal_unsafe)]

core/src/marker.rs

@@ -810,15 +810,21 @@ pub trait DiscriminantKind {
     type Discriminant: Clone + Copy + Debug + Eq + PartialEq + Hash + Send + Sync + Unpin;
 }
 
-/// Compiler-internal trait used to determine whether a type contains
+/// Used to determine whether a type contains
 /// any `UnsafeCell` internally, but not through an indirection.
 /// This affects, for example, whether a `static` of that type is
 /// placed in read-only static memory or writable static memory.
+/// This can be used to declare that a constant with a generic type
+/// will not contain interior mutability, and subsequently allow
+/// placing the constant behind references.
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[unstable(feature = "freeze", issue = "121675")]
+pub unsafe auto trait Freeze {}
 
+#[unstable(feature = "freeze", issue = "121675")]
 impl<T: ?Sized> !Freeze for UnsafeCell<T> {}
 marker_impls! {
+    #[unstable(feature = "freeze", issue = "121675")]
     unsafe Freeze for
         {T: ?Sized} PhantomData<T>,
         {T: ?Sized} *const T,