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);
}

This slide should take about 5 minutes.

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.

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Comprehensive Rust 🦀

Borrow Both