`mut fn`

Decided

D-0034, settled by the language’s author: part of spec §18. Older text cites it as 18.34.

Removed — the receiver says it. A method mutates when it takes &mut self and reads when it takes &self; fn resize(&mut self, n: u32) is the one spelling. mut fn stated the same fact twice, and the two could disagree: fn f(&mut self) was accepted, and the compiler went on to treat the method as mutating in one place and as read-only in another. The parser now rejects mut fn with a help saying what to write (§7.3)