Lifetime Elision

Lifetimes for function arguments and return values must be fully specified, but Rust allows lifetimes to be elided in most cases with a few simple rules. This is not inference -- it is just a syntactic shorthand.

• Each argument which does not have a lifetime annotation is given one.
• If there is only one argument lifetime, it is given to all un-annotated return values.
• If there are multiple argument lifetimes, but the first one is for self, that lifetime is given to all un-annotated return values.

# // Copyright 2024 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn only_args(a: &i32, b: &i32) {
    todo!();
}

fn identity(a: &i32) -> &i32 {
    a
}

struct Foo(i32);
impl Foo {
    fn get(&self, other: &i32) -> &i32 {
        &self.0
    }
}

• Walk through applying the lifetime elision rules to each of the example functions. only_args is completed by the first rule, identity is completed by the second, and Foo::get is completed by the third.

• If all lifetimes have not been filled in by applying the three elision rules then you will get a compiler error telling you to add annotations manually.