How an `if let` binding holds the value

Decided

D-0061, settled by the language’s author: part of spec §18. Older text cites it as 18.61.

As a for or case binding does (18.42, 18.43): v a copy that cannot be changed, &v a read-only reference, &mut v a writable one — for an optional, a pointer and a handle alike. &mut needs a writable pointer, reference or place (DZ0507); v needs a copyable value wherever the if let does not own it (DZ0403). This reverses the pointer half of 18.36, where if let p = ptr gave a &mut T: with handles added, one rule in four places was chosen over three rules and an exception, and p meaning a reference on a pointer but a copy on everything else would have been that exception. A &T? is read as its optional by or and if let both, as 18.35 reads a &T as a T (§9.4)