A lifetime parameter of a function definition is called *late-bound* if it both: 1. appears in an argument type 2. does not appear in a generic type constraint You cannot specify lifetime arguments for late-bound lifetime parameters. Erroneous code example: ```compile_fail,E0794 fn foo<'a>(x: &'a str) -> &'a str { x } let _ = foo::<'static>; ``` The type of a concrete instance of a generic function is universally quantified over late-bound lifetime parameters. This is because we want the function to work for any lifetime substituted for the late-bound lifetime parameter, no matter where the function is called. Consequently, it doesn't make sense to specify arguments for late-bound lifetime parameters, since they are not resolved until the function's call site(s). To fix the issue, remove the specified lifetime: ``` fn foo<'a>(x: &'a str) -> &'a str { x } let _ = foo; ``` ### Additional information Lifetime parameters that are not late-bound are called *early-bound*. Confusion may arise from the fact that late-bound and early-bound lifetime parameters are declared the same way in function definitions. When referring to a function pointer type, universal quantification over late-bound lifetime parameters can be made explicit: ``` trait BarTrait<'a> {} struct Bar<'a> { s: &'a str } impl<'a> BarTrait<'a> for Bar<'a> {} fn bar<'a, 'b, T>(x: &'a str, _t: T) -> &'a str where T: BarTrait<'b> { x } let bar_fn: for<'a> fn(&'a str, Bar<'static>) -> &'a str = bar; // OK let bar_fn2 = bar::<'static, Bar>; // Not allowed let bar_fn3 = bar::; // OK ``` In the definition of `bar`, the lifetime parameter `'a` is late-bound, while `'b` is early-bound. This is reflected in the type annotation for `bar_fn`, where `'a` is universally quantified and `'b` is substituted by a specific lifetime. It is not allowed to explicitly specify early-bound lifetime arguments when late-bound lifetime parameters are present (as for `bar_fn2`, see [issue #42868](https://github.com/rust-lang/rust/issues/42868)), although the types that are constrained by early-bound parameters can be specified (as for `bar_fn3`).