Detect duplicate trait bounds

[pierwill]

We might look for a way to detect, and possibly remove, repeated trait bounds. Maybe as a lint? Or maybe this would be better in rustfmt or clippy?

An example from #87880:

pub trait ControlFlowGraph:
    DirectedGraph + WithStartNode + WithPredecessors + WithStartNode + WithSuccessors + WithNumNodes

I was surprised this is allowed!

Playground example:

pub trait DirectedGraph {}
pub trait WithStartNode {}
pub trait WithPredecessors {}
pub trait WithSuccessors {}
pub trait WithNumNodes {}

pub trait ControlFlowGraph:
    DirectedGraph + WithStartNode + WithPredecessors + WithStartNode + WithSuccessors + WithNumNodes
{}

[pierwill]

@estebank: Would this make sense as a lint in compiler/rustc_lint/src/traits.rs, or would clippy be better?

[pierwill]

@rustbot label +A-lint

[estebank]

I think this would be ok in the compiler itself (@rust-lang/compiler any reason not to accept such a lint? this would require a proposal?), but it would certainly be reasonable in clippy. You can start working on either codebase that is easiest for you (which might be clippy) and migrate it to the other, as they both use rustc's linting and visitor machinery, so this can be linted almost the same in both.

[pierwill]

@rustbot claim