Multiple Borrows

But what about when there are multiple borrows passed into a function and one being returned?

# // Copyright 2025 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn multiple(a: &i32, b: &i32) -> &i32 {
    todo!("Return either `a` or `b`")
}

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

    let r = multiple(&a, &b);

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

    dbg!(r);
}

• This code does not compile right now because it is missing lifetime annotations. Before we get it to compile, use this opportunity to have students to think about which of our argument borrows should be extended by the return value.

• We pass two borrows into multiple and one is going to come back out, which means we will need to extend the borrow of one of the argument lifetimes. Which one should be extended? Do we need to see the body of multiple to figure this out?

• When borrow checking, the compiler doesn't look at the body of multiple to reason about the borrows flowing out, instead it looks only at the signature of the function for borrow analysis.

• In this case there is not enough information to determine if a or b will be borrowed by the returned reference. Show students the compiler errors and introduce the lifetime syntax:

  rust,ignoreCopy

  # // Copyright 2025 Google LLC
  # // SPDX-License-Identifier: Apache-2.0
  #
  fn multiple<'a>(a: &'a i32, b: &'a i32) -> &'a i32 { ... }