Lint on duplicate trait bound

[Aaron1011]

The following code compiles without any warnings:

pub trait Foo {}

pub fn bar<T: Foo + Foo>() {}

We should lint on the duplicate Foo bound.

[ibraheemdev]

Clippy catches this:

#![deny(clippy::pedantic)]

pub trait Foo {}

pub fn bar<T: Foo>() where T: Foo {}

error: this trait bound is already specified in the where clause
 --> src/lib.rs:5:15
  |
5 | pub fn bar<T: Foo>() where T: Foo {}
  |               ^^^
  |
note: the lint level is defined here
 --> src/lib.rs:1:9
  |
1 | #![deny(clippy::pedantic)]
  |         ^^^^^^^^^^^^^^^^
  = note: `#[deny(clippy::trait_duplication_in_bounds)]` implied by `#[deny(clippy::pedantic)]`
  = help: consider removing this trait bound
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trait_duplication_in_bounds

[Aaron1011]

I think it would be useful to uplift this lint into rustc.

[ibraheemdev]

It catches when multiple syntaxes are being used to specify a bound, but not when the same is being used twice. ie: fn foo<T: Clone>() where T: Clone fails, but fn foo<T: Clone + Clone>() does not. Should the lint be modified to catch this as well, or should a new lint be created separately?

[Aaron1011]

Including both in the same lint sounds reasonable.

[ibraheemdev]

@rustbot claim