trait_duplication_in_bounds lint doesn't consider generics

[aldhsu]

Summary

@giraffate caught this issue in #8703 PR which is based on the TRAIT_DUPLICATION_IN_BOUNDS lint.

Lint Name

TRAIT_DUPLICATION_IN_BOUNDS

Reproducer

I tried this code:

trait BoundWithGeneric<T> {}

fn good_bounds_with_generic<T: BoundWithGeneric<u32>>(arg0: T, arg1: T)
where
    T: BoundWithGeneric<u64>,
{
    unimplemented!();
}

I saw this happen:

 --> $DIR/trait_duplication_in_bounds.rs:16:32
+   |
+LL | fn good_bounds_with_generic<T: BoundWithGeneric<u32>>(arg0: T, arg1: T)
+   |                                ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider removing this trait bound

I expected to see this happen:

No linting error as while the same trait is used, the concrete type argument it is applied to is different.

Version

rustc 1.62.0-nightly (8f36334ca 2022-04-06)
binary: rustc
commit-hash: 8f36334ca939a67cce3f37f24953ff6f2d3f3d33
commit-date: 2022-04-06
host: x86_64-apple-darwin
release: 1.62.0-nightly
LLVM version: 14.0.0

[digama0]

Here are some more test cases for this issue:

#![warn(clippy::trait_duplication_in_bounds)]

trait Trait<A> {}
struct Foo;

// to avoid trivial bound lint
impl Trait<u8> for Foo {}
impl Trait<i8> for Foo {}
impl Trait<u8> for () {}
impl Trait<i8> for () {}

fn ok1() where Foo: Trait<u8> + Trait<i8> {}
fn bad2<A>() where Foo: Trait<u8> + Trait<i8> {} // bad
fn ok3<A>() where (): Trait<u8> + Trait<i8> {}
fn bad3<A>() where Foo: Trait<A> + Trait<i8> {} // bad
fn bad4<A, B>() where Foo: Trait<A> + Trait<B> {} // bad
impl<A> Trait<(A,)> for Foo where Foo: Trait<A> + Trait<u8> {} // bad
impl<A, B> Trait<(A, B)> for Foo where Foo: Trait<A> + Trait<B> {} // bad

fn main() {}

[aldhsu]

I've been poking at this problem since it is related to my PR.

Reproducer

I tried this code:

fn duplicate_trait<T: Clone + Clone + Default, Z: Copy>(arg0: T, arg1: Z) {}

I saw this happen:

error: this trait bound is already specified in the where clause
  --> $DIR/trait_duplication_in_bounds.rs:110:16
   |
LL | fn duplicate_trait<T: Clone + Clone + Default, Z: Copy>(arg0: T, arg1: Z) {}
   |                       ^^^^^
   |
   = help: consider removing this trait bound

The error message always says this is trait bound is already specified in the where clause.

Two interpretations.

1. The message is correct and the behaviour of the lint should only compare trait bounds to where clauses for the same generic, not within trait bounds or where clauses
2. The message is incorrect and behaviour is expected.

[dswij]

The error message always says this is trait bound is already specified in the where clause.

Two interpretations.

1. The message is correct and the behaviour of the lint should only compare trait bounds to where clauses for the same generic, not within trait bounds or where clauses

2. The message is incorrect and behaviour is expected.

Based on the example in the lint's docs, I think the behavior is unexpected. Point 1 makes more sense, imo