impl manual_noop_waker lint

[jaroslawroszyk]

Fixes #16639

Description

This PR introduces a new lint manual_noop_waker that detects manual, empty implementations of the std::task::Wake trait. These can be replaced with std::task::Waker::noop(), which is more performant (avoids Arc allocation) and cleaner.

Example

struct MyWaker;
impl Wake for MyWaker {
    fn wake(self: Arc<Self>) {}
    fn wake_by_ref(self: &Arc<Self>) {}
}

Can be replaced with:

let waker = Waker::noop();

• I've followed the contribution guidelines

• I've added UI tests for the new lint

• I've run cargo dev update_lints

changelog: [manual_noop_waker]: Added a lint to detect manual no-op Wake implementations.

[rustbot]

r? @dswij

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The reviewer was selected based on:

• Owners of files modified in this PR: 7 candidates
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• Random selection from Jarcho, dswij, llogiq, samueltardieu

[samueltardieu]

This will falsely trigger the lint:

trait Wake {
    fn wake(self);
}

impl Wake for () {
    fn wake(self) {}
}

See the Clippy book for good practices. Sorry if this is not the case, but the lint looks vibe-coded by a LLM which is not very efficient.

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[jaroslawroszyk]

This will falsely trigger the lint:

trait Wake {
    fn wake(self);
}

impl Wake for () {
    fn wake(self) {}
}

See the Clippy book for good practices. Sorry if this is not the case, but the lint looks vibe-coded by a LLM which is not very efficient.

View changes since this review

It's not vibe coded. I followed example and on that I'll do implementation. It's my first pr in this repo.

[jaroslawroszyk]

@rustbot ready

[samueltardieu]

You haven't addressed the false positives (see #16687 (review))

View changes since this review

[jaroslawroszyk]

You haven't addressed the false positives (see #16687 (review))

View changes since this review

Did you see?
tests/ui/manual_noop_waker.rs
and stderr of that?

[jaroslawroszyk]

@rustbot ready

[samueltardieu]

Did you see? tests/ui/manual_noop_waker.rs and stderr of that?

It doesn't test the same thing. Did you even try to run Clippy on the exact code I gave?

trait Wake {
    fn wake(self);
}

impl Wake for () {
    fn wake(self) {}
}

warning: manual implementation of a no-op waker
 --> /tmp/t.rs:5:1
  |
5 | / impl Wake for () {
6 | |     fn wake(self) {}
7 | | }
  | |_^
  |
  = help: use `std::task::Waker::noop()` instead

This is wrong.

@rustbot author

[samueltardieu]

The proper way to check that this is the right trait is to use a diagnostic item when available (it is not the case here), or a type path. All this is described in the Clippy book in the trait checking chapter.