…ess-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: rust-lang#76525 (comment), issue: rust-lang#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 rust-lang#55436 <rust-lang#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.

…s-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: rust-lang#76525 (comment), issue: rust-lang#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 rust-lang#55436 <rust-lang#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.

Rollup of 14 pull requests

Successful merges:

 - rust-lang#73963 (deny(unsafe_op_in_unsafe_fn) in libstd/path.rs)
 - rust-lang#75099 (lint/ty: move fns to avoid abstraction violation)
 - rust-lang#75502 (Use implicit (not explicit) rules for promotability by default in `const fn`)
 - rust-lang#75580 (Add test for checking duplicated branch or-patterns)
 - rust-lang#76310 (Add `[T; N]: TryFrom<Vec<T>>` (insta-stable))
 - rust-lang#76400 (Clean up vec benches bench_in_place style)
 - rust-lang#76434 (do not inline black_box when building for Miri)
 - rust-lang#76492 (Add associated constant `BITS` to all integer types)
 - rust-lang#76525 (Add as_str() to string::Drain.)
 - rust-lang#76636 (assert ScalarMaybeUninit size)
 - rust-lang#76749 (give *even better* suggestion when matching a const range)
 - rust-lang#76757 (don't convert types to the same type with try_into (clippy::useless_conversion))
 - rust-lang#76796 (Give a better error message when x.py uses the wrong stage for CI)
 - rust-lang#76798 (Build fixes for RISC-V 32-bit Linux support)

Failed merges:

r? `@ghost`