Add a lint to warn about `&(dyn … + Send)`

[m-ou-se]

Adding + Send to a &dyn is a useless restriction. E.g. &(dyn Any + Send) implicitly converts to &dyn Any, but not the other way around, even though it'd be sound to just transmute it in both directions, since the Send isn't of any use for (shared) references.

It'd be nice if we suggest removing the + Send for &(dyn … + Send).

See #110799

[john-h-k]

Would this make the most sense as a rustc lint or a clippy lint?

[m-ou-se]

I think as a rustc lint.

cc @rust-lang/types who probably have an opinion on this.

[john-h-k]

@rustbot claim

[steffahn]

Using the dyn-clone crate, &dyn Trait + Send can be useful, right?