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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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9
compiler/rustc_session/src/lint/builtin.rs
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@ -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,
]
}