Compiling this code caused the compiler to block

[umoho]

I tried this code:

pub trait Trait<T> {
    fn f(&self) -> T;
}

pub struct S1<'a>(pub &'a Box<S3<'a>>);

pub struct S2<'a>(pub Vec<S1<'a>>);

pub struct S3<'a>(pub Vec<S2<'a>>);

impl<T> Trait<T> for S1<'_>
where
    for<'a> S1<'a>: Trait<T>,
    for<'a> S3<'a>: Trait<T>,
{
    fn f(&self) -> T {
        self.0.f()
    }
}

impl<T> Trait<T> for S2<'_>
where
    for<'a> S1<'a>: Trait<T>,
{
    fn f(&self) -> T {
        _ = self.0.iter().map(|s| s.f());
        todo!()
    }
}

impl<T> Trait<T> for S3<'_>
where
    for<'a> S2<'a>: Trait<T>,
{
    fn f(&self) -> T {
        _ = self.0.iter().map(|s| s.f());
        todo!()
    }
}

fn main() {
    S3(vec![]).f();
}

I expected to see this happen: Compilation completed to obtain binary file, or compiler returns an error.

Instead, this happened: Increased CPU and memory usage, unable to compile or exit for a long time.

Meta

rustc --version --verbose:

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: aarch64-apple-darwin
release: 1.78.0
LLVM version: 18.1.2

rustc +nightly --version --verbose:

rustc 1.80.0-nightly (791adf759 2024-05-21)
binary: rustc
commit-hash: 791adf759cc065316f054961875052d5bc03e16c
commit-date: 2024-05-21
host: aarch64-apple-darwin
release: 1.80.0-nightly
LLVM version: 18.1.6

Backtrace

RUST_BACKTRACE=1 cargo build:

   Compiling rs_block_compile v0.1.0 (/Users/umoho/Devs/rs_block_compile)
    Building [                             ] 0/1: rs_block_compile(bin) 

[bjorn3]

ctrl-c not working is likely #124212. Since #111769 we catch ctrl-c and only check if it happens during const eval. If you get stuck in a loop that doesn't contain any const eval, you can't exit anymore.

[saethlin]

unable to compile or exit

Hunh. I expected that people would instinctively send SIGINT again if the compiler stays running. Are you using some system that prevents you from doing this, like an editor? On the command line, rustc will eat your first ctrl+c then instantly exit if you press ctrl+c again.