Error code E0759
Note: this error code is no longer emitted by the compiler.
Return type involving a trait did not require 'static
lifetime.
Erroneous code examples:
#![allow(unused)] fn main() { use std::fmt::Debug; fn foo(x: &i32) -> impl Debug { // error! x } fn bar(x: &i32) -> Box<dyn Debug> { // error! Box::new(x) } }
Add 'static
requirement to fix them:
#![allow(unused)] fn main() { use std::fmt::Debug; fn foo(x: &'static i32) -> impl Debug + 'static { // ok! x } fn bar(x: &'static i32) -> Box<dyn Debug + 'static> { // ok! Box::new(x) } }
Both 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.
#![allow(unused)] fn main() { use std::fmt::Debug; fn foo(x: &i32) -> impl Debug + '_ { x } fn bar(x: &i32) -> Box<dyn Debug + '_> { Box::new(x) } }
These are equivalent to the following explicit lifetime annotations:
#![allow(unused)] fn main() { use std::fmt::Debug; fn foo<'a>(x: &'a i32) -> impl Debug + 'a { x } fn bar<'a>(x: &'a i32) -> Box<dyn Debug + 'a> { Box::new(x) } }