New upstream version 1.55.0+dfsg1

[rustc.git] / src / doc / nomicon / src / lifetime-mismatch.md

# Limits of Lifetimes

Given the following code:

```rust,compile_fail
#[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:

```text
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][ex2]. We desugar the program and we get
the following:

<!-- ignore: desugared code -->
```rust,ignore
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.

```rust,compile_fail
# 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:

```text
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`
```

[ex2]: lifetimes.html#example-aliasing-a-mutable-reference