Error code E0794

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:

#![allow(unused)]
fn main() {
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 instantiated 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:

#![allow(unused)]
fn main() {
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:

#![allow(unused)]
fn main() {
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::<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 instantiated with 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), although the types that are constrained by early-bound parameters can be specified (as for bar_fn3).