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() } } } #}