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:
d92155bf6a/library/std/src/panic.rs (L235-L236)
2. An implementation of `Error` for `LayoutErr` in `library/std/srd/error.rs`:
d92155bf6a/library/std/src/error.rs (L392-L397)
3. `From` implementations for `Waker` and `RawWaker` in `library/alloc/src/task.rs`. Example:
d92155bf6a/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.

src/test/ui/stability-attribute/stability-attribute-trait-impl.rs

@@ -0,0 +1,28 @@
+#![feature(staged_api)]
+
+#[stable(feature = "x", since = "1")]
+struct StableType;
+
+#[unstable(feature = "x", issue = "none")]
+struct UnstableType;
+
+#[stable(feature = "x", since = "1")]
+trait StableTrait {}
+
+#[unstable(feature = "x", issue = "none")]
+trait UnstableTrait {}
+
+#[unstable(feature = "x", issue = "none")]
+impl UnstableTrait for UnstableType {}
+
+#[unstable(feature = "x", issue = "none")]
+impl StableTrait for UnstableType {}
+
+#[unstable(feature = "x", issue = "none")]
+impl UnstableTrait for StableType {}
+
+#[unstable(feature = "x", issue = "none")]
+//~^ ERROR an `#[unstable]` annotation here has no effect [rustc::ineffective_unstable_trait_impl]
+impl StableTrait for StableType {}
+
+fn main() {}

src/test/ui/stability-attribute/stability-attribute-trait-impl.stderr

@@ -0,0 +1,11 @@
+error: an `#[unstable]` annotation here has no effect
+  --> $DIR/stability-attribute-trait-impl.rs:24:1
+   |
+LL | #[unstable(feature = "x", issue = "none")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(rustc::ineffective_unstable_trait_impl)]` on by default
+   = note: see issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information
+
+error: aborting due to previous error
+