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RPITIT causes incorrect "dropped while still borrowed" error in some cases #131490
Description
From this SO question, the following code does not compile:
trait Iterable {
fn owned_iter(&self) -> impl Iterator<Item = usize> + 'static;
}
struct B<T> {
x: T,
}
impl<T: Iterable + Clone> Iterable for B<T> {
fn owned_iter(&self) -> impl Iterator<Item = usize> + 'static {
let y = self.x.clone();
y.owned_iter() // error[E0597]: `y` does not live long enough
}
}However, boxing the iterator as a trait object allows it to compile:
trait Iterable {
fn owned_iter(&self) -> impl Iterator<Item = usize> + 'static;
}
struct B<T> {
x: T,
}
impl<T: Iterable + Clone> Iterable for B<T> {
fn owned_iter(&self) -> impl Iterator<Item = usize> + 'static {
let y = self.x.clone();
let b: Box<dyn Iterator<Item = usize>> = Box::new(y.owned_iter());
b
}
}Notably, not cloning self.x and simply returning self.x.owned_iter() also works.
I don't see any reason why the first code block shouldn't compile, especially while the boxed version and the non-cloning version do.
Meta
rustc --version --verbose:
rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: x86_64-unknown-linux-gnu
release: 1.81.0
LLVM version: 18.1.7
It also fails to compile on 1.83.0-nightly (2024-10-09 eb4e234).
Activity
compiler-errors commentedon Oct 10, 2024
This was fixed by #116040, but that was not sound. Fixing this is particularly nontrivial.
This may be easier to address soon if #131033 lands and is then stabilized, since you should be able to express the
+'staticas+use<Self>instead.cdhowie commentedon Oct 10, 2024
But the returned iterator in this case doesn't even borrow
self, so it doesn't seem like that should be necessary. It also changes the semantic meaning of the returned type to borrowselfwhen the intent of this code is exactly the opposite. Am I missing something?compiler-errors commentedon Oct 11, 2024
I think so :)
So the RPITIT wouldn't be capturing
self: &Self, but instead it would be capturingSelf, the implicit type parameter of the trait. That is to say, he capture we want to avoid is the elided lifetime'a(which is elided in this case) like:lqd commentedon Oct 11, 2024
Since #131033 landed, this should work on nightly now