#![feature(plugin)] #![plugin(clippy)] #![allow(unused)] #![deny(mut_from_ref)] struct Foo; impl Foo { fn this_wont_hurt_a_bit(&self) -> &mut Foo { unimplemented!() } } trait Ouch { fn ouch(x: &Foo) -> &mut Foo; } impl Ouch for Foo { fn ouch(x: &Foo) -> &mut Foo { unimplemented!() } } fn fail(x: &u32) -> &mut u16 { unimplemented!() } // this is OK, because the result borrows y fn works<'a>(x: &u32, y: &'a mut u32) -> &'a mut u32 { unimplemented!() } // this is also OK, because the result could borrow y fn also_works<'a>(x: &'a u32, y: &'a mut u32) -> &'a mut u32 { unimplemented!() } fn main() { //TODO }