Auto merge of #76538 - fusion-engineering-forks:check-useless-unstable-trait-impl, r=lcnr

Warn for #[unstable] on trait impls when it has no effect.

Earlier today I sent a PR with an `#[unstable]` attribute on a trait `impl`, but was informed that this attribute has no effect there. (comment: https://github.com/rust-lang/rust/pull/76525#issuecomment-689678895, issue: https://github.com/rust-lang/rust/issues/55436)

This PR adds a warning for this situation. Trait `impl` blocks with `#[unstable]` where both the type and the trait are stable will result in a warning:

```
warning: An `#[unstable]` annotation here has no effect. See issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information.
   --> library/std/src/panic.rs:235:1
    |
235 | #[unstable(feature = "integer_atomics", issue = "32976")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

---

It detects three problems in the existing code:

1. A few `RefUnwindSafe` implementations for the atomic integer types in `library/std/src/panic.rs`. Example:
https://github.com/rust-lang/rust/blob/d92155bf6ae0b7d79fc83cbeeb0cc0c765353471/library/std/src/panic.rs#L235-L236
2. An implementation of `Error` for `LayoutErr` in `library/std/srd/error.rs`:
https://github.com/rust-lang/rust/blob/d92155bf6ae0b7d79fc83cbeeb0cc0c765353471/library/std/src/error.rs#L392-L397
3. `From` implementations for `Waker` and `RawWaker` in `library/alloc/src/task.rs`. Example:
https://github.com/rust-lang/rust/blob/d92155bf6ae0b7d79fc83cbeeb0cc0c765353471/library/alloc/src/task.rs#L36-L37

Case 3 interesting: It has a bound with an `#[unstable]` trait (`W: Wake`), so appears to have much effect on stable code. It does however break similar blanket implementations. It would also have immediate effect if `Wake` was implemented for any stable type. (Which is not the case right now, but there are no warnings in place to prevent it.) Whether this case is a problem or not is not clear to me. If it isn't, adding a simple `c.visit_generics(..);` to this PR will stop the warning for this case.

2 files changed, 79 insertions, 3 deletions

diff --git a/compiler/rustc_passes/src/stability.rs b/compiler/rustc_passes/src/stability.rs
index 91edc7d9db7..4ca52f405fb 100644
--- a/compiler/rustc_passes/src/stability.rs
+++ b/compiler/rustc_passes/src/stability.rs
@@ -9,13 +9,14 @@ use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
-use rustc_hir::{Generics, HirId, Item, StructField, Variant};
+use rustc_hir::{Generics, HirId, Item, StructField, TraitRef, Ty, TyKind, Variant};
 use rustc_middle::hir::map::Map;
 use rustc_middle::middle::privacy::AccessLevels;
 use rustc_middle::middle::stability::{DeprecationEntry, Index};
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::lint;
+use rustc_session::lint::builtin::INEFFECTIVE_UNSTABLE_TRAIT_IMPL;
 use rustc_session::parse::feature_err;
 use rustc_session::Session;
 use rustc_span::symbol::{sym, Symbol};
@@ -538,7 +539,37 @@ impl Visitor<'tcx> for Checker<'tcx> {
             // For implementations of traits, check the stability of each item
             // individually as it's possible to have a stable trait with unstable
             // items.
-            hir::ItemKind::Impl { of_trait: Some(ref t), items, .. } => {
+            hir::ItemKind::Impl { of_trait: Some(ref t), self_ty, items, .. } => {
+                if self.tcx.features().staged_api {
+                    // If this impl block has an #[unstable] attribute, give an
+                    // error if all involved types and traits are stable, because
+                    // it will have no effect.
+                    // See: https://github.com/rust-lang/rust/issues/55436
+                    if let (Some(Stability { level: attr::Unstable { .. }, .. }), _) =
+                        attr::find_stability(&self.tcx.sess, &item.attrs, item.span)
+                    {
+                        let mut c = CheckTraitImplStable { tcx: self.tcx, fully_stable: true };
+                        c.visit_ty(self_ty);
+                        c.visit_trait_ref(t);
+                        if c.fully_stable {
+                            let span = item
+                                .attrs
+                                .iter()
+                                .find(|a| a.has_name(sym::unstable))
+                                .map_or(item.span, |a| a.span);
+                            self.tcx.struct_span_lint_hir(
+                                INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
+                                item.hir_id,
+                                span,
+                                |lint| lint
+                                    .build("an `#[unstable]` annotation here has no effect")
+                                    .note("see issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information")
+                                    .emit()
+                            );
+                        }
+                    }
+                }
+
                 if let Res::Def(DefKind::Trait, trait_did) = t.path.res {
                     for impl_item_ref in items {
                         let impl_item = self.tcx.hir().impl_item(impl_item_ref.id);
@@ -598,6 +629,44 @@ impl Visitor<'tcx> for Checker<'tcx> {
     }
 }
 
+struct CheckTraitImplStable<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    fully_stable: bool,
+}
+
+impl Visitor<'tcx> for CheckTraitImplStable<'tcx> {
+    type Map = Map<'tcx>;
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+
+    fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, _id: hir::HirId) {
+        if let Some(def_id) = path.res.opt_def_id() {
+            if let Some(stab) = self.tcx.lookup_stability(def_id) {
+                self.fully_stable &= stab.level.is_stable();
+            }
+        }
+        intravisit::walk_path(self, path)
+    }
+
+    fn visit_trait_ref(&mut self, t: &'tcx TraitRef<'tcx>) {
+        if let Res::Def(DefKind::Trait, trait_did) = t.path.res {
+            if let Some(stab) = self.tcx.lookup_stability(trait_did) {
+                self.fully_stable &= stab.level.is_stable();
+            }
+        }
+        intravisit::walk_trait_ref(self, t)
+    }
+
+    fn visit_ty(&mut self, t: &'tcx Ty<'tcx>) {
+        if let TyKind::Never = t.kind {
+            self.fully_stable = false;
+        }
+        intravisit::walk_ty(self, t)
+    }
+}
+
 /// Given the list of enabled features that were not language features (i.e., that
 /// were expected to be library features), and the list of features used from
 /// libraries, identify activated features that don't exist and error about them.
diff --git a/compiler/rustc_session/src/lint/builtin.rs b/compiler/rustc_session/src/lint/builtin.rs
index a9deaaae0da..0fd6cc10382 100644
--- a/compiler/rustc_session/src/lint/builtin.rs
+++ b/compiler/rustc_session/src/lint/builtin.rs
@@ -5,7 +5,7 @@
 //! lints are all available in `rustc_lint::builtin`.
 
 use crate::lint::FutureIncompatibleInfo;
-use crate::{declare_lint, declare_lint_pass};
+use crate::{declare_lint, declare_lint_pass, declare_tool_lint};
 use rustc_span::edition::Edition;
 use rustc_span::symbol::sym;
 
@@ -555,6 +555,12 @@ declare_lint! {
     };
 }
 
+declare_tool_lint! {
+    pub rustc::INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
+    Deny,
+    "detects `#[unstable]` on stable trait implementations for stable types"
+}
+
 declare_lint_pass! {
     /// Does nothing as a lint pass, but registers some `Lint`s
     /// that are used by other parts of the compiler.
@@ -630,6 +636,7 @@ declare_lint_pass! {
         INCOMPLETE_INCLUDE,
         CENUM_IMPL_DROP_CAST,
         CONST_EVALUATABLE_UNCHECKED,
+        INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
     ]
 }