lint for redundant trait bounds (due to trait inheritance)

[thestinger]

We should give a warning that the Foo bound is redundant:

trait Foo {}
trait Bar: Foo {}

fn foo<T: Foo + Bar>() {}

[sfackler]

We don't currently warn for even more obvious redundant bounds as well:

trait Foo {}

fn foo<T: Foo+Foo>() {}

[Aatch]

Visiting for triage.

Still doesn't warn.

[pongad]

Interested in trying, will find @thestinger on IRC.

[pongad]

Actually...who's mentoring this?
I'm still at a loss of what to do. xD

[reem]

Triage: This errors now!

Should be closed.

[tamird]

@reem the trivial case errors, but the original case reported here still doesn't

[tamird]

This should either be closed or the E-needstest label removed. Here's a case that demonstrates the problem:

trait Foo {
  fn noop(&self) {}
}

trait Bar: Foo {}

#[allow(dead_code)]
fn bar<T: Foo + Bar>() {}

fn main() {}

[frewsxcv]

It looks like there is no error nor warning for redundant traits

http://is.gd/iqumGL

[steveklabnik]

Since new lints have a big impact on users of rustc, the policy is that they should go through the RFC process like other user-facing changes. As such, I'm going to give this one a close, but if anyone comes across this ticket and wants this lint, consider adding it to clippy and/or writing up an RFC. Thanks!