Borrow Both

In this case, we have a function where either a or b may be returned. In this case we use the lifetime annotations to tell the compiler that both borrows may flow into the return value.

# // Copyright 2025 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn pick<'a>(c: bool, a: &'a i32, b: &'a i32) -> &'a i32 {
    if c { a } else { b }
}

fn main() {
    let mut a = 5;
    let mut b = 10;

    let r = pick(true, &a, &b);

    // Which one is still borrowed?
    // Should either mutation be allowed?
    // a += 7;
    // b += 7;

    dbg!(r);
}

• The pick function will return either a or b depending on the value of c, which means we can't know at compile time which one will be returned.

• To express this to the compiler, we use the same lifetime for both a and b, along with the return type. This means that the returned reference will borrow BOTH a and b!

• Uncomment both of the commented lines and show that r is borrowing both a and b, even though at runtime it will only point to one of them.

• Change the first argument to pick to show that the result is the same regardless of if a or b is returned.