Limits of Lifetimes

Given the following code:

#[derive(Debug)]
struct Foo;

impl Foo {
    fn mutate_and_share(&mut self) -> &Self { &*self }
    fn share(&self) {}
}

fn main() {
    let mut foo = Foo;
    let loan = foo.mutate_and_share();
    foo.share();
    println!("{:?}", loan);
}

One might expect it to compile. We call mutate_and_share, which mutably borrows foo temporarily, but then returns only a shared reference. Therefore we would expect foo.share() to succeed as foo shouldn't be mutably borrowed.

However when we try to compile it:

error[E0502]: cannot borrow `foo` as immutable because it is also borrowed as mutable
  --> src/main.rs:12:5
   |
11 |     let loan = foo.mutate_and_share();
   |                --- mutable borrow occurs here
12 |     foo.share();
   |     ^^^ immutable borrow occurs here
13 |     println!("{:?}", loan);

What happened? Well, we got the exact same reasoning as we did for Example 2 in the previous section. We desugar the program and we get the following:

struct Foo;

impl Foo {
    fn mutate_and_share<'a>(&'a mut self) -> &'a Self { &'a *self }
    fn share<'a>(&'a self) {}
}

fn main() {
    'b: {
        let mut foo: Foo = Foo;
        'c: {
            let loan: &'c Foo = Foo::mutate_and_share::<'c>(&'c mut foo);
            'd: {
                Foo::share::<'d>(&'d foo);
            }
            println!("{:?}", loan);
        }
    }
}

The lifetime system is forced to extend the &mut foo to have lifetime 'c, due to the lifetime of loan and mutate_and_share's signature. Then when we try to call share, and it sees we're trying to alias that &'c mut foo and blows up in our face!

This program is clearly correct according to the reference semantics we actually care about, but the lifetime system is too coarse-grained to handle that.

Improperly reduced borrows

The following code fails to compile, because Rust doesn't understand that the borrow is no longer needed and conservatively falls back to using a whole scope for it. This will eventually get fixed.

#![allow(unused)]
fn main() {
use std::collections::HashMap;
use std::hash::Hash;
fn get_default<'m, K, V>(map: &'m mut HashMap<K, V>, key: K) -> &'m mut V
where
    K: Clone + Eq + Hash,
    V: Default,
{
    match map.get_mut(&key) {
        Some(value) => value,
        None => {
            map.insert(key.clone(), V::default());
            map.get_mut(&key).unwrap()
        }
    }
}
}

Because of the lifetime restrictions imposed, &mut map's lifetime overlaps other mutable borrows, resulting in a compile error:

error[E0499]: cannot borrow `*map` as mutable more than once at a time
  --> src/main.rs:12:13
   |
4  |   fn get_default<'m, K, V>(map: &'m mut HashMap<K, V>, key: K) -> &'m mut V
   |                  -- lifetime `'m` defined here
...
9  |       match map.get_mut(&key) {
   |       -     --- first mutable borrow occurs here
   |  _____|
   | |
10 | |         Some(value) => value,
11 | |         None => {
12 | |             map.insert(key.clone(), V::default());
   | |             ^^^ second mutable borrow occurs here
13 | |             map.get_mut(&key).unwrap()
14 | |         }
15 | |     }
   | |_____- returning this value requires that `*map` is borrowed for `'m`