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, 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 sees that a variable, map, is borrowed twice, and can not infer that the first borrow ceases to be needed before the second one occurs. This is caused by Rust conservatively falling back to using a whole scope for the first borrow. 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`