blob: ce5d42b3c7f0d39a9a351d95057d0e83e55cbe43 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
|
#### Note: this error code is no longer emitted by the compiler.
Return type involving a trait did not require `'static` lifetime.
Erroneous code examples:
```compile_fail
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:
```
# 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.
```
# 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:
```
# 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)
}
```
[`dyn Trait`]: https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types
[`impl Trait`]: https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits
|
