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

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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
This commit is contained in:
Jacob Pratt 2024-03-11 03:47:19 -04:00 • committed by GitHub
commit 05f22c3614
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15 changed files with 89 additions and 11 deletions
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14
compiler/rustc_hir_analysis/src/coherence/mod.rs
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@ -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);