Limits of Lifetimes -

# Limits of Lifetimes

Given the following code:

    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();
    }

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:

    <anon>:11:5: 11:8 error: cannot borrow `foo` as immutable because it is also borrowed as mutable
    <anon>:11     foo.share();
                  ^~~
    <anon>:10:16: 10:19 note: previous borrow of `foo` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `foo` until the borrow ends
    <anon>:10     let loan = foo.mutate_and_share();
                             ^~~
    <anon>:12:2: 12:2 note: previous borrow ends here
    <anon>:8 fn main() {
    <anon>:9     let mut foo = Foo;
    <anon>:10     let loan = foo.mutate_and_share();
    <anon>:11     foo.share();
    <anon>:12 }
              ^

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);
                }
            }
        }
    }

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.

TODO: other common problems? SEME regions stuff, mostly?