Lint suggestion: duplicate or redundant trait bounds

What it does

Checks for traits that are bounded by the same trait more than once.

If you want to get fancy with it, you could add a check for implied duplicate bounds, e.g. Hash implies PartialEq, so trait Foo: Hash + PartialEq is pointless.

This was detected in https://github.com/Leafwing-Studios/leafwing-input-manager/pull/545.

Playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e298f6d998b87fe37663e1df073297f4

Advantage

These duplicate bounds are pointless and confusing.

Drawbacks

The more advanced implied redundant trait bound detection could reduce clarity of the code, and is less robust to future refactoring of the traits being relied on, as bounds may be removed in the future.

Example

use core::fmt::Debug;

trait SuperDebug: Debug + Debug + Debug { }

Could be written as:

use core::fmt::Debug;

trait SuperDebug: Debug { }

Start with the Rust Playground example and the duplicate-bound case described in the issue, then inspect rust-clippy's existing lint implementations and tests to find the appropriate entry point. Done means detecting repeated trait bounds with clear diagnostics; implied bounds such as Hash and PartialEq are optional and should be evaluated separately.