Machine applicable suggestion ignores empty where bounds

[oli-obk]

I tried this code:

(from https://github.com/rust-lang/rust/blob/cf2dff2b1e3fa55fa5415d524200070d0d7aacfe/tests/ui/trait-impl-bound-suggestions.rs)

use std::fmt::Debug;

struct ConstrainedStruct<X: Copy> {
    x: X
}

trait InsufficientlyConstrainedGeneric<X=()> where {
    fn return_the_constrained_type(&self, x: X) -> ConstrainedStruct<X> {
        ConstrainedStruct { x }
    }
}

fn main() { }

I expected to see this happen:

10 | trait InsufficientlyConstrainedGeneric<X=()> where X: std::marker::Copy {
   |                                                    ++++++++++++++++++++

just add the new predicate

Instead, this happened:

10 | trait InsufficientlyConstrainedGeneric<X=()> where where X: std::marker::Copy {
   |                                                    ++++++++++++++++++++++++++

I noticed this because a change to the compiler caused two predicates to get added, so even without an empty where clause (so with none), you'd get where X: std::marker::Copy where X: std::marker::Copy, because the usual suggestion application logic does not see that there is a conflict, because the spans don't actually overlap.

Meta

rustc --version --verbose:

1.78 (nightly, but also happens on earlier stables)

[gurry]

@rustbot claim