Explicit annotation

The borrow checker uses explicit lifetime annotations to determine how long references should be valid. In cases where lifetimes are not elided[^1], Rust requires explicit annotations to determine what the lifetime of a reference should be. The syntax for explicitly annotating a lifetime uses an apostrophe character as follows:

1. foo<'a>
2. // `foo` has a lifetime parameter `'a`

Similar to closures, using lifetimes requires generics. Additionally, this lifetime syntax indicates that the lifetime of foo may not exceed that of 'a. Explicit annotation of a type has the form &'a T where 'a has already been introduced.

In cases with multiple lifetimes, the syntax is similar:

1. foo<'a, 'b>
2. // `foo` has lifetime parameters `'a` and `'b`

In this case, the lifetime of foo cannot exceed that of either 'a or 'b.

See the following example for explicit lifetime annotation in use:

1. // `print_refs` takes two references to `i32` which have different
2. // lifetimes `'a` and `'b`. These two lifetimes must both be at
3. // least as long as the function `print_refs`.
4. fn print_refs<'a, 'b>(x: &'a i32, y: &'b i32) {
5. println!("x is {} and y is {}", x, y);
6. }

8. // A function which takes no arguments, but has a lifetime parameter `'a`.
9. fn failed_borrow<'a>() {
10. let _x = 12;

12. // ERROR: `_x` does not live long enough
13. //let y: &'a i32 = &_x;
14. // Attempting to use the lifetime `'a` as an explicit type annotation
15. // inside the function will fail because the lifetime of `&_x` is shorter
16. // than that of `y`. A short lifetime cannot be coerced into a longer one.
17. }

19. fn main() {
20. // Create variables to be borrowed below.
21. let (four, nine) = (4, 9);

23. // Borrows (`&`) of both variables are passed into the function.
24. print_refs(&four, &nine);
25. // Any input which is borrowed must outlive the borrower.
26. // In other words, the lifetime of `four` and `nine` must
27. // be longer than that of `print_refs`.

29. failed_borrow();
30. // `failed_borrow` contains no references to force `'a` to be
31. // longer than the lifetime of the function, but `'a` is longer.
32. // Because the lifetime is never constrained, it defaults to `'static`.
33. }

[^1]: elision implicitly annotates lifetimes and so is different.

See also:

generics and closures