Compiler indefinitely takes resources when HRTB is used in conjunction with lifetime in GAT
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Description
I tried this code:
pub trait A {
type Item<'a>
where
Self: 'a;
fn run(&mut self) -> Self::Item<'_>;
}
#[derive(Debug)]
pub struct S<F>(F);
impl<F> A for S<F>
where
for<'a> F: FnMut(&Self::Item<'a>),
{
type Item<'a> = i32
where
Self: 'a;
fn run(&mut self) -> Self::Item<'_> {
todo!()
}
}
I expected to see this happen: the code should compile or return a compilation error in a short time.
Instead, this happened: the compiler start working and taking resources, apparently indefinitely.
Notice that removing the bound for F stops the issue to occur.
EDIT: the code triggers infinite recursion, the bound should be FnMut(&I::Item<'a>). As suggested by others, the issue of infinite recursion is managed by the next solver (using -Znext-solver), and thanks to that is also easy to spot the problem in the code.
Meta
rustc --version --verbose:
Stable:
rustc 1.76.0 (07dca489a 2024-02-04)
binary: rustc
commit-hash: 07dca489ac2d933c78d3c5158e3f43beefeb02ce
commit-date: 2024-02-04
host: x86_64-unknown-linux-gnu
release: 1.76.0
LLVM version: 17.0.6
Nightly:
rustc 1.78.0-nightly (c475e2303 2024-02-28)
binary: rustc
commit-hash: c475e2303b551d726307c646181e0677af1e0069
commit-date: 2024-02-28
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0
The issue occurs both on stable and nightly.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the provided HRTB and GAT example with the listed stable and nightly rustc versions, then compare behavior with -Znext-solver. Investigate the compiler's trait-solver handling of the recursive bound; done means compilation terminates with an appropriate diagnostic or succeeds without indefinitely consuming resources, with a regression test for the reproducer.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100