Unnecessary type repetition in trait bound

[xd009642]

So I raised this on rust-lang/rust#58471 and it was suggested this is better as a clippy lint so pasting the issue text here for quick reference:

My where clauses in traits recently had some unnecessary noise that the compiler could warn against:

What I was doing:

impl<T> SomeTrait<T> where T: Clone, T: Debug

What I should have been doing:

impl<T> SomeTrait<T> where T: Clone + Debug

I'd be happy to implement this myself if there's a consensus on adding this as a warning, though I may > need some mentoring 👍

The offer to help with a PR myself still stands, I had a quick check of the lints and didn't spot it.

[oli-obk]

There are theoretically infinte ways to write where bounds. So as a first step just doing unification of T: Foo, T: Bar to T: Foo + Bar seems like a good idea.

This lint should not trigger inside macros (as it doesn't matter there and some macros might get hard to write), so you should use the in_macro function to bail out if we're inside a macro.

You can implement the check_generics method and access the where_clauses field on the generics argument. If you want any help with the implementation, don't hesitate to ask here.

[xd009642]

I've just done a draft pull request. It's roughly there and builds but doesn't yet work.