How a `case` binding holds the payload

Decided

D-0043, settled by the language’s author: part of spec §18. Older text cites it as 18.43.

Exactly as a for binding (18.42). case Circle(c) is a copy that cannot be changed, &c a read-only reference, &mut c a writable one that needs a let mut, &mut or ^mut subject. One rule for both places; a binding that was always a reference would make &mut c the only mode worth writing. The binding had been writable always, which let a switch change a value declared immutable and failed in C++ behind a & (§10.3)