Rust by Example

13.4.1 Explicit annotation

The borrow checker uses explicit lifetime annotations to determine how long references should be valid. In cases where lifetimes are not elided1, 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:

foo<'a>
// `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:

foo<'a, 'b>
// `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:

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

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

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

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

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

    failed_borrow();
    // `failed_borrow` contains no references to force `'a` to be 
    // longer than the lifetime of the function, but `'a` is longer.
    // Because the lifetime is never constrained, it defaults to `'static`.
}
1. elision implicitly annotates lifetimes and so is different.

See also:

generics and closures