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

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

non-emoji version of https://github.com/rust-lang/rust/pull/121501

cc #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: https://github.com/google/zerocopy/issues/941

cc ```@RalfJung```

T-lang signed off on reexposing this unstably: https://github.com/rust-lang/rust/pull/121501#issuecomment-1969827742

1 files changed, 14 insertions, 0 deletions

diff --git a/compiler/rustc_hir_analysis/src/coherence/mod.rs b/compiler/rustc_hir_analysis/src/coherence/mod.rs
index fc7a73e12be..054a3af212a 100644
--- a/compiler/rustc_hir_analysis/src/coherence/mod.rs
+++ b/compiler/rustc_hir_analysis/src/coherence/mod.rs
@@ -10,6 +10,7 @@ use rustc_errors::{codes::*, struct_span_code_err};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
+use rustc_session::parse::feature_err;
 use rustc_span::{sym, ErrorGuaranteed};
 use rustc_trait_selection::traits;
 
@@ -49,6 +50,19 @@ fn enforce_trait_manually_implementable(
 ) -> Result<(), ErrorGuaranteed> {
     let impl_header_span = tcx.def_span(impl_def_id);
 
+    if tcx.lang_items().freeze_trait() == Some(trait_def_id) {
+        if !tcx.features().freeze_impls {
+            feature_err(
+                &tcx.sess,
+                sym::freeze_impls,
+                impl_header_span,
+                "explicit impls for the `Freeze` trait are not permitted",
+            )
+            .with_span_label(impl_header_span, format!("impl of `Freeze` not allowed"))
+            .emit();
+        }
+    }
+
     // Disallow *all* explicit impls of traits marked `#[rustc_deny_explicit_impl]`
     if trait_def.deny_explicit_impl {
         let trait_name = tcx.item_name(trait_def_id);