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:
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:
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.