Wrong suggestion for struct's type-param that is not iter::Iterator but should

[lukaslueg]

In the following example, Rust 1.22.1 and nightly complain about T of Foobar not being iter::Iterator, which IterWrapper demands.

use std::iter;

struct IterWrapper<T:iter::Iterator> {
    iter: T
}

struct Foobar<T> {
    w: IterWrapper<T>
}

The suggestion consider adding a "where T: std::iter::Iterator" bound is correct, the main suggestion maybe try calling ".iter()" or a similar method is not applicable to this situation, though.

error[E0277]: the trait bound `T: std::iter::Iterator` is not satisfied
 --> src/main.rs:8:5
  |
8 |     w: IterWrapper<T>
  |     ^^^^^^^^^^^^^^^^^ `T` is not an iterator; maybe try calling `.iter()` or a similar method
  |
  = help: the trait `std::iter::Iterator` is not implemented for `T`
  = help: consider adding a `where T: std::iter::Iterator` bound
  = note: required by `IterWrapper`

error: aborting due to previous error

Same problem for an impl<T> Foobar<T>: Rustc suggest to call .iter() on the type-parameter, which is nonsense.

[zackmdavis]

#44755 could help with this. (Currently, the "maybe try calling .iter()" language is hardcoded rather than depending on what we know about T.)

[estebank]

To accomplish this correctly, we need to modify on_unimplemented_note to be able to query wether Self is a type argument, so that we can suggest a trait bound, instead of using iter. We would still be unable to tell the compiler to point Selfs span (that might be another worthwhile improvement to rustc_on_unimplemented).