Improve error message for "for x in [1,2,3] {"

[kmcallister]

Beginners are often confused by the error message

error: `for` loop expression has type `[<generic integer #0>, .. 3]` which does not implement the `Iterator` trait

Perhaps if there's an iter() method we should suggest calling it. This involves hardcoding the name "iter", but only for the purposes of a hint that can be wrong sometimes.

[kmcallister]

Or we just say "did you mean to call a method to get an iterator?" unconditionally. Or only when there's no method call in the expr?

[SiegeLord]

Crazy idea... maybe make it possible to annotate types with error hints? E.g.

#[error_hint(E1234, "Use the `.iter()` method")]
struct Foo{}

[bstrie]

@SiegeLord I'd suggest proposing that as its own issue so it doesn't get buried here forever.