rustc hangs/spins on example involving associated types

[cdisselkoen]

The following 5-line example causes rustc to spin:

pub struct Chicken<'a, T: Trait>(&'a Frog<'a, T::Item>);

pub struct Frog<'a, T: Trait>(&'a Chicken<'a, T>);

pub trait Trait {
    type Item;
}

The spin happens with or without the --edition=2018 flag.

This may be the same issue as #62430, just an even simpler reproduction case.

Output of rustc --version --verbose:

rustc 1.36.0 (a53f9df32 2019-07-03)
binary: rustc
commit-hash: a53f9df32fbb0b5f4382caaad8f1a46f36ea887c
commit-date: 2019-07-03
host: x86_64-apple-darwin
release: 1.36.0
LLVM version: 8.0

Using the Playground, I also reproduced the issue with stable Rust 1.37.0, beta Rust 1.38.0-beta.1, and nightly Rust 1.39.0 2019-08-15.

[Centril]

Reduced:

struct Chicken<'a, T> where T: Trait { f: &'a Frog<'a, T::Item> }

struct Frog<'a, T> where T: Trait  { f: &'a Chicken<'a, T> }

trait Trait {
    type Item;
}

cc @nagisa

[estebank]

Triage, no change. It seems like we get stuck evaluating the following over and over again:

├─  7s  DEBUG rustc_infer::infer::outlives::components components(Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [T/#1], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })) = [Alias(AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [Alias(Projection, AliasTy { args: [T/#1], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })], def_id: DefId(0:12 ~ f79[dbd1]::Trait::Item) })]