How can I swap in a new value for a field in a mutable reference to a structure?

Use std::mem::swap().

fn foo(a: &mut A) {
    let mut my_local_var = SomeType::new();
    mem::swap(&mut a.field, &mut my_local_var);
}

Or std::mem::replace().

fn foo(a: &mut A) {
    let mut my_local_var = mem::replace(&mut a.field, SomeType::new());
}