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

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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.
This commit is contained in:
bors 2020-09-12 18:01:33 +00:00
commit 989190874f
7 changed files with 129 additions and 16 deletions
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6
library/std/src/error.rs
View file

@ -389,11 +389,7 @@ impl Error for ! {}
)]
impl Error for AllocErr {}
#[unstable(
feature = "allocator_api",
reason = "the precise API and guarantees it provides may be tweaked.",
issue = "32838"
)]
#[stable(feature = "alloc_layout", since = "1.28.0")]
impl Error for LayoutErr {}
#[stable(feature = "rust1", since = "1.0.0")]

16
library/std/src/panic.rs
View file

@ -232,16 +232,16 @@ impl<T: ?Sized> RefUnwindSafe for RwLock<T> {}
#[stable(feature = "unwind_safe_atomic_refs", since = "1.14.0")]
impl RefUnwindSafe for atomic::AtomicIsize {}
#[cfg(target_has_atomic_load_store = "8")]
#[unstable(feature = "integer_atomics", issue = "32976")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for atomic::AtomicI8 {}
#[cfg(target_has_atomic_load_store = "16")]
#[unstable(feature = "integer_atomics", issue = "32976")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for atomic::AtomicI16 {}
#[cfg(target_has_atomic_load_store = "32")]
#[unstable(feature = "integer_atomics", issue = "32976")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for atomic::AtomicI32 {}
#[cfg(target_has_atomic_load_store = "64")]
#[unstable(feature = "integer_atomics", issue = "32976")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for atomic::AtomicI64 {}
#[cfg(target_has_atomic_load_store = "128")]
#[unstable(feature = "integer_atomics", issue = "32976")]
@ -251,16 +251,16 @@ impl RefUnwindSafe for atomic::AtomicI128 {}
#[stable(feature = "unwind_safe_atomic_refs", since = "1.14.0")]
impl RefUnwindSafe for atomic::AtomicUsize {}
#[cfg(target_has_atomic_load_store = "8")]
#[unstable(feature = "integer_atomics", issue = "32976")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for atomic::AtomicU8 {}
#[cfg(target_has_atomic_load_store = "16")]
#[unstable(feature = "integer_atomics", issue = "32976")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for atomic::AtomicU16 {}
#[cfg(target_has_atomic_load_store = "32")]
#[unstable(feature = "integer_atomics", issue = "32976")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for atomic::AtomicU32 {}
#[cfg(target_has_atomic_load_store = "64")]
#[unstable(feature = "integer_atomics", issue = "32976")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for atomic::AtomicU64 {}
#[cfg(target_has_atomic_load_store = "128")]
#[unstable(feature = "integer_atomics", issue = "32976")]