rustc hangs and leaks memory

[is8ac]

In stable, rustc 1.48.0 (7eac88abb 2020-11-16), rustc demo.rs promptly returns overflow evaluating the requirement and exits. This is expected behavior.

In nightly, rustc 1.50.0-nightly (5be3f9f10 2020-12-03), rustc demo.rs hangs indefinitely, using 100% of one core. Memory usage fluctuates but tends to grow over time at ~800 MB per minute on my machine.

Source

pub trait Baz {}
impl Baz for () {}
impl<T> Baz for [T; 3] {}

pub trait Fiz {}
impl Fiz for bool {}

pub trait Corge<S> {
    type Foo;
}

impl<W: Corge<()>> Corge<[(); 3]> for W {
    type Foo = [<W as Corge<()>>::Foo; 3];
}

pub trait Grault
where
    Self::B: Fiz,
{
    type A;
    type B;
}

impl<T: Grault> Grault for [T; 3]
where
    Self::A: Baz,
    Self::B: Fiz,
{
    type A = [T::A; 3];
    type B = T::B;
}

pub trait Quaks<S, W> {
    fn bar() {}
}

impl<S> Quaks<S, bool> for u8
where
    bool: Corge<S>,
    <bool as Corge<S>>::Foo: Grault,
{
}

fn main() {
    <u8 as Quaks<[(); 3], bool>>::bar();
}

rustc --version --verbose:

rustc 1.50.0-nightly (5be3f9f10 2020-12-03)
binary: rustc
commit-hash: 5be3f9f10e9fd59ea03816840a6051413fbdefae
commit-date: 2020-12-03
host: x86_64-unknown-linux-gnu
release: 1.50.0-nightly

[JohnTitor]

This happens on beta as well.

[JohnTitor]

The regression introduced between nightly-2020-10-06 (a1dfd24) and nightly-2020-10-07 (98edd1f)

Possible culprit PRs: #77606, #73905 (UPDATE: confirmed #73905 is the cause)

[camelid]

Assigning P-high and removing I-prioritize as discussed in the prioritization working group.

[Mark-Simulacrum]

I am going to move this to P-low as this isn't breaking working code, we have more urgent fall out from #73905 that needs to be fixed first.