Clever recursive types + deref = seemingly infinite compile time

[meithecatte]

The following takes an unknown, potentially infinite amount of time greater than 10 minutes to compile:

struct S<T>(T);
impl<T> std::ops::Deref for S<T> {
    type Target = S<(T,T)>;
    fn deref(&self) -> &Self::Target {
        self.thing()
    }
}

As @rkruppe pointed out on Discord,

Most places in rustc are supposed to have limits to stop them from going in circles forever (or just for a very long time), so at first glance I'd consider this a bug

[sugar700]

This is technically not infinite, but resolving such a type takes a long time (I did calculations and got a value that is about 10²⁴ years assuming infinite memory to avoid running out of memory, although keep in mind this is an estimate) with default recursion limit set to 128. When using #![recursion_limit="16"] or so, a proper error message is returned.

[meithecatte]

Not that it matters, but in my tests the memory usage stayed constant.

[steveklabnik]

This seems to fail to compile pretty quickly for me; is anyone else still seeing this hang?

[meithecatte]

Looks like the code in question got improved in the meantime. Do we want to add a test?

[sugar700]

Seems like this was fixed to take a reasonable amount of time in Rust 1.47.

[Enselic]

Triage: The reproducer is small so it would be nice to turn that into a regression test or make sure there is one already.