new lint: implied_bounds_in_impls

[y21]

Closes #10849

A new lint that looks for explicitly specified bounds that are already implied by other bounds in impl Trait return position types.
One example, as shown in the linked example, would be

fn foo<T>(x: T) -> impl Deref<Target = T> + DerefMut<Target = T> {
    Box::new(x)
}

DerefMut<Target = T> requires Deref<Target = T> to be wellformed, so specifying Deref there is unnecessary.

This currently "ignores" (i.e., does not lint at all) transitive supertrait bounds (e.g. trait A {} trait B: A {} trait C: B {}, then having an impl A + C type), because that seems a bit more difficult and I think this isn't technically a blocker. It leads to FNs, but shouldn't bring any FPs

changelog: new lint [implied_bounds_in_impls]

[rustbot]

r? @xFrednet

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[xFrednet]

Really solid start. I love how you documented the steps in the implementation. There are a few nits, mostly related to documentation and the diagnostic messages, but then it should be ready to be merged. The logic itself looks good to me :)

Sorry, for the delay w

[bors]

☔ The latest upstream changes (presumably #11373) made this pull request unmergeable. Please resolve the merge conflicts.

[xFrednet]

LGTM, thank you for the new lint and swift update :)

@bors r+

[bors]

📌 Commit 1227571 has been approved by xFrednet

It is now in the queue for this repository.

[bors]

⌛ Testing commit 1227571 with merge df68b71...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing df68b71 to master...