Forbid implementing Freeze even if the trait is stabilized

compiler/rustc_codegen_cranelift/example/mini_core.rs

@@ -8,6 +8,7 @@
     rustc_attrs,
     transparent_unions,
     auto_traits,
+    freeze_impls,
     thread_local
 )]
 #![no_core]

compiler/rustc_codegen_gcc/example/mini_core.rs

@@ -1,6 +1,6 @@
 #![feature(
     no_core, lang_items, intrinsics, unboxed_closures, type_ascription, extern_types,
-    decl_macro, rustc_attrs, transparent_unions, auto_traits,
+    decl_macro, rustc_attrs, transparent_unions, auto_traits, freeze_impls,
     thread_local
 )]
 #![no_core]

compiler/rustc_feature/src/unstable.rs

@@ -469,6 +469,8 @@ declare_features! (
     (unstable, fn_align, "1.53.0", Some(82232)),
     /// Support delegating implementation of functions to other already implemented functions.
     (incomplete, fn_delegation, "1.76.0", Some(118212)),
+    /// Allows impls for the Freeze trait.
+    (internal, freeze_impls, "CURRENT_RUSTC_VERSION", Some(121675)),
     /// Allows defining gen blocks and `gen fn`.
     (unstable, gen_blocks, "1.75.0", Some(117078)),
     /// Infer generic args for both consts and types.

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);

compiler/rustc_span/src/symbol.rs

@@ -841,6 +841,7 @@ symbols! {
         format_placeholder,
         format_unsafe_arg,
         freeze,
+        freeze_impls,
         freg,
         frem_algebraic,
         frem_fast,

library/core/src/lib.rs

@@ -203,6 +203,7 @@
 // Language features:
 // tidy-alphabetical-start
 #![cfg_attr(bootstrap, feature(platform_intrinsics))]
+#![cfg_attr(not(bootstrap), feature(freeze_impls))]
 #![feature(abi_unadjusted)]
 #![feature(adt_const_params)]
 #![feature(allow_internal_unsafe)]

library/core/src/marker.rs

@@ -818,13 +818,13 @@ pub trait DiscriminantKind {
 /// will not contain interior mutability, and subsequently allow
 /// placing the constant behind references.
 #[lang = "freeze"]
-#[unstable(feature = "freeze", issue = "60715")]
+#[unstable(feature = "freeze", issue = "121675")]
 pub unsafe auto trait Freeze {}
 
-#[unstable(feature = "freeze", issue = "60715")]
+#[unstable(feature = "freeze", issue = "121675")]
 impl<T: ?Sized> !Freeze for UnsafeCell<T> {}
 marker_impls! {
-    #[unstable(feature = "freeze", issue = "60715")]
+    #[unstable(feature = "freeze", issue = "121675")]
     unsafe Freeze for
         {T: ?Sized} PhantomData<T>,
         {T: ?Sized} *const T,

tests/run-make/min-global-align/min_global_align.rs

@@ -1,4 +1,4 @@
-#![feature(no_core, lang_items)]
+#![feature(no_core, lang_items, freeze_impls)]
 #![crate_type = "rlib"]
 #![no_core]
 

tests/ui/feature-gates/feature-gate-freeze-impls.rs

@@ -0,0 +1,15 @@
+#![feature(freeze, negative_impls)]
+
+use std::marker::Freeze;
+
+struct Foo;
+
+unsafe impl Freeze for Foo {}
+//~^ explicit impls for the `Freeze` trait are not permitted
+
+struct Bar;
+
+impl !Freeze for Bar {}
+//~^ explicit impls for the `Freeze` trait are not permitted
+
+fn main() {}

tests/ui/feature-gates/feature-gate-freeze-impls.stderr

@@ -0,0 +1,23 @@
+error[E0658]: explicit impls for the `Freeze` trait are not permitted
+  --> $DIR/feature-gate-freeze-impls.rs:7:1
+   |
+LL | unsafe impl Freeze for Foo {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ impl of `Freeze` not allowed
+   |
+   = note: see issue #121675 <https://github.com/rust-lang/rust/issues/121675> for more information
+   = help: add `#![feature(freeze_impls)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: explicit impls for the `Freeze` trait are not permitted
+  --> $DIR/feature-gate-freeze-impls.rs:12:1
+   |
+LL | impl !Freeze for Bar {}
+   | ^^^^^^^^^^^^^^^^^^^^ impl of `Freeze` not allowed
+   |
+   = note: see issue #121675 <https://github.com/rust-lang/rust/issues/121675> for more information
+   = help: add `#![feature(freeze_impls)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.