A lifetime parameter of a function definition is called late-bound if it both:
You cannot specify lifetime arguments for late-bound lifetime parameters.
Erroneous code example:
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:
fn foo<'a>(x: &'a str) -> &'a str { x } let _ = foo;
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::<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
).