7c00d77e8b
Part of issue #16640. I am leaving this issue open to handle parsing of higher-rank lifetimes in traits. This change breaks code that used unboxed closures: * Instead of `F:|&: int| -> int`, write `F:Fn(int) -> int`. * Instead of `F:|&mut: int| -> int`, write `F:FnMut(int) -> int`. * Instead of `F:|: int| -> int`, write `F:FnOnce(int) -> int`. [breaking-change]