Report same/similar errors only once

[tyoc213]

It seems that #20692 is not only a case for duplicate error messages as shown by

fn main() {
    let array = [1, 2, 3];
    for a in array {
        println!( "{}", a);
    }
}

Which print the error 3 times.

It is the same as the other case about map

rust/src/librustc/middle/traits/object_safety.rs

Lines 78 to 85 in 7ae4a8e

	pub fn object_safety_violations<'tcx>(tcx: &ty::ctxt<'tcx>,
	trait_def_id: ast::DefId)
	-> Vec<ObjectSafetyViolation<'tcx>>
	{
	traits::supertrait_def_ids(tcx, trait_def_id)
	.flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id).into_iter())
	.collect()
	}

originally spot at https://users.rust-lang.org/t/iterator-and-differences-between-array-and-array/2672

---

I think implementing distinctso that you can collect only the different items could be a good way to start solving this in the board way (all across scala error reporting).

[eddyb]

Minimal examples:

fn main() {
    Iterator::next(&mut false); // 2 errors
}

fn main() {
    for _ in false {} // 3 errors
}

Interestingly enough, when put in the same program, the total number of errors is 4, not 5 (due to some global cache?).
cc @arielb1