"The following implementations were found" should also mention the trait bounds.

Test case:

struct Index;
fn iterate(s: std::collections::HashSet<Index>) {
    for _ in s {}
}

This errors with:

error[E0277]: the trait bound `std::collections::HashSet<Index>: std::iter::IntoIterator` is not satisfied
 --> src/main.rs:3:5
  |
3 |     for _ in s {}
  |     ^^^^^^^^^^^^^ the trait `std::iter::IntoIterator` is not implemented for `std::collections::HashSet<Index>`
  |
  = help: the following implementations were found:
            <std::collections::HashSet<T, S> as std::iter::IntoIterator>
            <&'a std::collections::HashSet<T, S> as std::iter::IntoIterator>
  = note: required by `std::iter::IntoIterator::into_iter`

So, the error is HashSet<Index> does not implement IntoIterator, but the help also suggests that HashSet<T, S> does implement IntoIterator, so what is it??

The help should also mention the bounds T: Eq + Hash. Or even better, points out that this impl is not chosen because Index does not implement Eq + Hash.

(Original source code: https://stackoverflow.com/revisions/45717092/2. Tested on playground, rustc 1.21.0-nightly (df511d554 2017-08-14) running on x86_64-unknown-linux-gnu)

1. It could also mention where they were defined (file and line n°)

   Thomas B at 2019-01-03 02:34:22