Misleading error message on traits after #8229

[HollayHorvath]

Summary

I have a simple trait that extends the iterators (just like itertools does). This trait has Iterator bound, but my function also has Iterator in its Self argument (just a more precise one).

Before #8229 it was fine, now it hits trait_duplication_in_bounds (which is great!) but the error message is misleading and points to the function's where-clause, however removing that bound does not solve this issue.

Reproducer

I tried this code:

pub trait Iter: Iterator {
    fn into_group_btreemap<K, V>(self) -> BTreeMap<K, Vec<V>>
    where
        Self: Iterator<Item = (K, V)> + Sized,
        K: Ord + Eq,
    {
        into_group_btreemap(self)
    }
}

I expected to see this happen:
The lint should show that the trait bound is superfluous:

3 |     pub trait Iter: Iterator {
  |                     ^^^^^^^^

Instead, this happened:
The lint shows the real bound:

6 |         Self: Iterator<Item = (K, V)> + Sized,
  |               ^^^^^^^^^^^^^^^^^^^^^^^

Version

rustc 1.60.0-nightly (bd3cb5256 2022-01-16)
binary: rustc
commit-hash: bd3cb52565faab2755ff1bdb54d88bc91f47b4b9
commit-date: 2022-01-16
host: x86_64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0

Additional Labels

No response

[Jarcho]

Clippy should definitely not be suggesting to remove the super trait as it's not superfluous. It adds a constraint to the type implementing Iter which would otherwise not exist. The where clause is only restricting the default function implementation so other implementations of the trait are only bound by them if they do not provide an implementation. Any type constrained by Iter will be able to make use of any of Iterator's items. e.g.

fn f<I: Iter>(mut i: I) -> Option<I::Item>  {
    i.next()
}

Removing the Iterator bound on the trait would make the previous code fail to compile.

With that said this is a false positive. The bound Self: Iterator<Item = (K, V)> can not be expressed without duplicating the bound on the trait.

[dswij]

@rustbot claim