How `for` gets an iterator from a container

Decided

D-0051, settled by the language’s author: part of spec §18. Older text cites it as 18.51.

Two prelude traits, Iterable and IterableMut, each naming its iterator as an associated type: fn iter(&self) -> Iter and fn iter_mut(&mut self) -> IterMut. for over a value that is not itself Iterate calls iter(), or iter_mut() when a binding is &mut. Chosen over finding iter() by name, which states no contract and cannot be a bound: fn total<C: Iterable>(c: &C) must be writable (§8.5, §10.4)