Limits of Lifetimes -

# 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

This currently 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_variables)]
    #fn main() {
    # use std::collections::HashMap;
    # use std::cmp::Eq;
    # 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()
            }
        }
    }
    #}