struct Nothing; fn foo<'a, F: FnMut(&'a mut Nothing) -> T, T>(_: F) {} fn bar<'a, F: FnMut(&'a mut Nothing) -> &'a mut Nothing>(_: F) {} fn main() { bar(|x| x); // OK bar(|x: &mut Nothing| x); // OK foo(|x| x); // OK foo(|x: &mut Nothing| x); // Error }

<anon>:11:27: 11:28 error: cannot infer an appropriate lifetime due to conflicting requirements <anon>:11 foo(|x: &mut Nothing| x); // Error ^ <anon>:11:27: 11:28 note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the block at 11:26... <anon>:11 foo(|x: &mut Nothing| x); // Error ^ <anon>:11:27: 11:28 note: ...so that expression is assignable (expected `&mut Nothing`, found `&mut Nothing`) <anon>:11 foo(|x: &mut Nothing| x); // Error ^ <anon>:11:5: 11:8 note: but, the lifetime must be valid for the expression at 11:4... <anon>:11 foo(|x: &mut Nothing| x); // Error ^~~ <anon>:11:5: 11:8 note: ...so that the type `&mut Nothing` will meet its required lifetime bounds <anon>:11 foo(|x: &mut Nothing| x); // Error ^~~ error: aborting due to previous error