Regression in nightly-2024-09-07: hang affecting `nalgebra`

[dtolnay]

[dependencies]
nalgebra = "=0.33.0"

cargo clean && cargo +nightly-2024-09-07 check hangs for at least 6 hours.

The previous nightly completes in 9.9 seconds on my machine.

Bisects to #121848. FYI @lcnr @compiler-errors

rustc 1.83.0-nightly (26b5599e4 2024-09-06)
binary: rustc
commit-hash: 26b5599e4d6ed2b45152c60493c1788c0a27533d
commit-date: 2024-09-06
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 19.1.0

[theemathas]

Somewhat minimized reproduction

Cargo.toml

[package]
name = "nalgebra"
version = "0.33.0"
edition = "2021"

[dependencies]
simba = { version = "0.9", default-features = false }

Cargo.lock

# This file is automatically @generated by Cargo.
# It is not intended for manual editing.
version = 3

[[package]]
name = "approx"
version = "0.5.1"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "cab112f0a86d568ea0e627cc1d6be74a1e9cd55214684db5561995f6dad897c6"
dependencies = [
 "num-traits",
]

[[package]]
name = "autocfg"
version = "1.3.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "0c4b4d0bd25bd0b74681c0ad21497610ce1b7c91b1022cd21c80c6fbdd9476b0"

[[package]]
name = "nalgebra"
version = "0.33.0"
dependencies = [
 "simba",
]

[[package]]
name = "num-complex"
version = "0.4.6"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "73f88a1307638156682bada9d7604135552957b7818057dcef22705b4d509495"
dependencies = [
 "num-traits",
]

[[package]]
name = "num-traits"
version = "0.2.19"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "071dfc062690e90b734c0b2273ce72ad0ffa95f0c74596bc250dcfd960262841"
dependencies = [
 "autocfg",
]

[[package]]
name = "paste"
version = "1.0.15"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "57c0d7b74b563b49d38dae00a0c37d4d6de9b432382b2892f0574ddcae73fd0a"

[[package]]
name = "simba"
version = "0.9.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "b3a386a501cd104797982c15ae17aafe8b9261315b5d07e3ec803f2ea26be0fa"
dependencies = [
 "approx",
 "num-complex",
 "num-traits",
 "paste",
]

src/lib.rs (105 lines)

use simba::simd::PrimitiveSimdValue;
use simba::simd::SimdValue;
use std::any::Any;
use std::convert::From;
use std::fmt::{self, Debug, Formatter};
use std::marker::PhantomData;

#[repr(transparent)]
#[derive(Copy, Clone, PartialEq, Eq, Hash)]
pub struct ArrayStorage<T, const R: usize, const C: usize>(pub(crate) [[T; R]; C]);

impl<T: Debug, const R: usize, const C: usize> Debug for ArrayStorage<T, R, C> {
    #[inline]
    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
        self.0.fmt(fmt)
    }
}

#[derive(Copy, Clone, Debug)]
pub struct DefaultAllocator;

impl<const R: usize, const C: usize> Allocator<Const<R>, Const<C>> for DefaultAllocator {
    type Buffer<T> = ArrayStorage<T, R, C>;
}

pub trait Allocator<R, C = U1>: Any + Sized {
    type Buffer<T>;
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub struct Const<const R: usize>;

pub(crate) type U1 = Const<1>;

pub type Owned<T, R, C = U1> = <DefaultAllocator as Allocator<R, C>>::Buffer<T>;

pub type OMatrix<T, R, C> = Matrix<T, R, C, Owned<T, R, C>>;

/// A statically sized D-dimensional column vector.
pub(crate) type SVector<T, const D: usize> = Matrix<T, Const<D>, U1, ArrayStorage<T, D, 1>>; // Owned<T, Const<D>, U1>>;

impl<T, R, C> SimdValue for OMatrix<T, R, C>
where
    T: SimdValue,
    DefaultAllocator: Allocator<R, C>,
{
    const LANES: usize = T::LANES;
    type Element = OMatrix<T::Element, R, C>;
    type SimdBool = T::SimdBool;

    #[inline]
    fn splat(_: Self::Element) -> Self {
        unimplemented!()
    }

    #[inline]
    fn extract(&self, _: usize) -> Self::Element {
        unimplemented!()
    }

    #[inline]
    unsafe fn extract_unchecked(&self, _: usize) -> Self::Element {
        unimplemented!()
    }

    #[inline]
    fn replace(&mut self, _: usize, _: Self::Element) {
        unimplemented!()
    }

    #[inline]
    unsafe fn replace_unchecked(&mut self, _: usize, _: Self::Element) {
        unimplemented!()
    }

    fn select(self, _: Self::SimdBool, _: Self) -> Self {
        unimplemented!()
    }
}

impl<T, const D: usize> From<[T; D]> for SVector<T, D> {
    #[inline]
    fn from(_: [T; D]) -> Self {
        unimplemented!()
    }
}

impl<T: PrimitiveSimdValue, R, C> From<[OMatrix<T::Element, R, C>; 16]> for OMatrix<T, R, C>
where
    T: From<[<T as SimdValue>::Element; 16]>,
    T::Element: SimdValue,
    DefaultAllocator: Allocator<R, C>,
{
    fn from(_: [OMatrix<T::Element, R, C>; 16]) -> Self {
        unimplemented!()
    }
}

#[repr(C)]
#[derive(Clone, Copy)]
pub struct Matrix<T, R, C, S> {
    pub(crate) data: S,
    _phantoms: PhantomData<(T, R, C)>,
}

[theemathas]

Minimized further (no dependencies):

pub struct Dummy;
pub trait DummyTrait {
    type DummyType<T>;
}
impl DummyTrait for Dummy {
    type DummyType<T> = T;
}
type AliasOf<T> = <Dummy as DummyTrait>::DummyType<T>;

pub struct Matrix<T, S>(T, S);
pub type OMatrix<T> = Matrix<T, AliasOf<T>>;

pub trait SimdValue {
    type Element;
}
impl<T> SimdValue for OMatrix<T>
where
    T: SimdValue,
{
    type Element = OMatrix<T::Element>;
}

pub trait Generic<T> {}

impl<T> Generic<T> for Matrix<T, T> {}

pub trait PrimitiveSimdValue: SimdValue {}
impl<T: PrimitiveSimdValue> Generic<OMatrix<T::Element>> for OMatrix<T> where
    T: Generic<<T as SimdValue>::Element>
{
}

The DummyTrait construction is similar to the one in #126944

[dtolnay]

Update: nalgebra does eventually compile, but it took 6h24m. So this is "just" slow and not stuck.