How a `for` binding holds its element

Decided

D-0042, settled by the language’s author: part of spec §18. Older text cites it as 18.42.

On the binding, as C++ does it: e a copy, &e a reference, &mut e a writable reference. e cannot be changed, so a write meant for the element cannot silently land on a copy. A copy is a real copy, never a reference in disguise, since a copy or drop with side effects would otherwise run or not depending on the compiler. Replaces for e in &xs / for e in &mut xs, where the sequence expression said it. Several names always take a tuple element apart, with no parentheses — for id, &btn, &mut state in query — and a pass count comes from xs.indexed(), so for i, e in xs no longer means an index: two readings of two names was the conflict this settles. What an iterator’s Item is decides which bindings its loops may use (§10.4)