Return type involving a trait did not require 'static
lifetime.
Erroneous code examples:
use std::fmt::Debug;
fn foo(x: &i32) -> impl Debug { // error!
x
}
fn bar(x: &i32) -> Box<dyn Debug> { // error!
Box::new(x)
}
RunAdd 'static
requirement to fix them:
fn foo(x: &'static i32) -> impl Debug + 'static { // ok!
x
}
fn bar(x: &'static i32) -> Box<dyn Debug + 'static> { // ok!
Box::new(x)
}
RunBoth dyn Trait
and impl Trait
in return types have an implicit
'static
requirement, meaning that the value implementing them that is being
returned has to be either a 'static
borrow or an owned value.
In order to change the requirement from 'static
to be a lifetime derived from
its arguments, you can add an explicit bound, either to an anonymous lifetime
'_
or some appropriate named lifetime.
fn foo(x: &i32) -> impl Debug + '_ {
x
}
fn bar(x: &i32) -> Box<dyn Debug + '_> {
Box::new(x)
}
RunThese are equivalent to the following explicit lifetime annotations:
fn foo<'a>(x: &'a i32) -> impl Debug + 'a {
x
}
fn bar<'a>(x: &'a i32) -> Box<dyn Debug + 'a> {
Box::new(x)
}
Run