endless loop by running cargo test with toolchain 2024-05-14

[xMAC94x]

I tried this code:

(sorry, i couldn't extract a minimal example and reproduce it)

git clone https://gitlab.com/veloren/veloren
cd veloren
git checkout e48fc4c2b235ceab3c1a551292c45c46d47e7130
cd common
cargo test

I expected to see this happen: ElimCase doctest in line 86 to finish after some time

Instead, this happened: the tests takes endless (i waited 2 hours in CI)

Meta

rustc --version --verbose:

rustc 1.80.0-nightly (ab14f944a 2024-05-13)
binary: rustc
commit-hash: ab14f944afe4234db378ced3801e637eae6c0f30
commit-date: 2024-05-13
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.4

cargo test:

   Doc-tests veloren_common

running 14 tests
test common/src/terrain/block.rs - terrain::block::Block (line 120) ... ignored
...
test common/src/typed.rs - typed::ElimCase (line 107) ... ok
...
test common/src/typed.rs - typed::ElimCase (line 86) has been running for over 60 seconds

code in the doc-test in question:

/// ```
/// veloren_common::make_case_elim!(
///     my_type_module,
///     #[repr(u32)]
///     #[derive(Clone,Copy)]
///     pub enum MyType {
///         Constr1 = 0,
///         #[typed(pure)] Constr2(arg : u8) = 1,
///         /* ..., */
///     }
/// );
/// ```

Workaround

As workaround we added a main function to the doctest, as the test doesnt execute code so far (only defines stuff) it then runs immediately again
https://gitlab.com/veloren/veloren/-/commit/3cc4b947ebd049dcdf82ca4a66a815d32d4d43f8

[lukas-code]

This regressed in #123594, where the new solver got enabled for the non_local_definitions lint.

It looks like the core issue here is some kind of exponential runtime in the new solver when there are many bounds on associated types.

Also, this is unrelated to rustdoc and can be reproduced with just rustc.

"minimal" repro

pub trait Typed {}

pub struct SpeciesCases<Elim>(Elim);

pub trait SpeciesPackedElim {
    type Ogre;
    type Cyclops;
    type Wendigo;
    type Cavetroll;
    type Mountaintroll;
    type Swamptroll;
    type Dullahan;
    type Werewolf;
    type Occultsaurok;
    type Mightysaurok;
    type Slysaurok;
    type Mindflayer;
    type Minotaur;
    type Tidalwarrior;
    type Yeti;
    type Harvester;
    type Blueoni;
    type Redoni;
    type Cultistwarlord;
    type Cultistwarlock;
    type Huskbrute;
    type Tursus;
    type Gigasfrost;
    type AdletElder;
    type SeaBishop;
    type HaniwaGeneral;
    type TerracottaBesieger;
    type TerracottaDemolisher;
    type TerracottaPunisher;
    type TerracottaPursuer;
    type Cursekeeper;
}

impl<'b, Elim: SpeciesPackedElim> Typed for &'b SpeciesCases<Elim>
where
    &'b Elim::Ogre: Typed,
    &'b Elim::Cyclops: Typed,
    &'b Elim::Wendigo: Typed,
    &'b Elim::Cavetroll: Typed,
    &'b Elim::Mountaintroll: Typed,
    &'b Elim::Swamptroll: Typed,
    &'b Elim::Dullahan: Typed,
    &'b Elim::Werewolf: Typed,
    &'b Elim::Occultsaurok: Typed,
    &'b Elim::Mightysaurok: Typed,
    &'b Elim::Slysaurok: Typed,
    &'b Elim::Mindflayer: Typed,
    &'b Elim::Minotaur: Typed,
    &'b Elim::Tidalwarrior: Typed,
    &'b Elim::Yeti: Typed,
    &'b Elim::Harvester: Typed,
    &'b Elim::Blueoni: Typed,
    &'b Elim::Redoni: Typed,
    &'b Elim::Cultistwarlord: Typed,
    &'b Elim::Cultistwarlock: Typed,
    &'b Elim::Huskbrute: Typed,
    &'b Elim::Tursus: Typed,
    &'b Elim::Gigasfrost: Typed,
    &'b Elim::AdletElder: Typed,
    &'b Elim::SeaBishop: Typed,
    &'b Elim::HaniwaGeneral: Typed,
    &'b Elim::TerracottaBesieger: Typed,
    &'b Elim::TerracottaDemolisher: Typed,
    &'b Elim::TerracottaPunisher: Typed,
    &'b Elim::TerracottaPursuer: Typed,
    &'b Elim::Cursekeeper: Typed,
{}

fn main() {
    pub struct Cases;
    impl<'b> Typed for &'b Cases {}
}

[lqd]

cc @lcnr could the couple search graph PRs helping on fuchsia also help here?

[lcnr]

looking into it, thanks @lukas-code for the minimization.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high