error: internal compiler error: "inference variables in ..." on packed type

[zao]

Given the following source code in both release 1.35.0 and 1.36.0-nightly (e305df184 2019-04-24), I encounter an internal compiler error using nalgebra 0.18.0.

The code is probably not legal to begin with as I ran into it as I was moving from my own homebrewn math code to nalgebra. I have the impression that the compiler probably shouldn't ICE regardless of what junk you feed it, so I'm still reporting this.

use nalgebra::Vector3;

#[derive(Debug, Clone, Copy)]
#[repr(C, packed)]
struct Vertex {
    pos: Vector3<f32>,
    clr: Vector3<f32>,
}

Build output from cargo +nightly build:

   Compiling nalgebra v0.18.0
   Compiling nalgebra-repro v0.1.0 (F:\Temp\nalgebra-repro)
error: internal compiler error: inference variables in nalgebra::base::matrix::Matrix<f32, nalgebra::base::dimension::U3, nalgebra::base::dimension::U1, nalgebra::base::array_storage::ArrayStorage<_, _, _>>
 --> src\lib.rs:5:1
  |
5 | / struct Vertex {
6 | |     pos: Vector3<f32>,
7 | |     clr: Vector3<f32>,
8 | | }
  | |_^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src\librustc_errors\lib.rs:355:17
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.36.0-nightly (e305df184 2019-04-24) running on x86_64-pc-windows-msvc

note: compiler flags: -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `nalgebra-repro`.

To learn more, run the command again with --verbose.

I unfortunately do not have a newer nightly to test with due to the lack of a more recent RLS blocking the rustup attempt.

[197g]

Still encountering this in rustc 1.37.0-nightly (03ee55bb1 2019-06-01).

Indepent of the issue, you can install specific compiler versions with necessary components according to this matrix and this installation process.

[pnkfelix]

This ICE itself arises from this line:

rust/src/librustc_typeck/check/wfcheck.rs

Line 264 in 51dc52b

item.span, &format!("inference variables in {:?}", ty));

[pnkfelix]

triage: P-high, assigning to self. Removing nomination.

[pnkfelix]

Okay I've narrowed the test case down

click for first reduction

Here is a standalone file (play):

#![allow(unused_imports, dead_code)]

pub struct U1;

pub trait ArrayLength {  }
impl ArrayLength for U1 { }

pub trait Trait { type Output; }
impl Trait for U1 { type Output = U1; }

pub trait DimName { type Value; }
impl DimName for U1 { type Value = U1; }

pub struct ArrayStorage<R>
where
    R: DimName,
    R::Value: Trait,
    <R::Value as Trait>::Output: ArrayLength,
{
    data: <R::Value as Trait>::Output,
}

pub struct DefaultAllocator;

pub trait Allocator<R = U1> { type Buffer; }
impl<R: DimName> Allocator<R> for DefaultAllocator
where
    R::Value: Trait,
    <R::Value as Trait>::Output: ArrayLength,
{
    type Buffer = ArrayStorage<R>;
}

#[repr(C, packed)]
struct Vertex {
    pos: <DefaultAllocator as Allocator>::Buffer
}

fn main() { println!("Hello, world!"); }

---

Update: Got a better reduction now (play):

#![allow(unused_imports, dead_code)]

pub struct S;

pub trait Trait<R> { type Assoc; }

impl<X> Trait<X> for S { type Assoc = X; }

#[repr(C, packed)]
struct Packed {
    pos: Box<<S as Trait<usize>>::Assoc>,
}

fn main() { println!("Hello, world!"); }

[pnkfelix]

This is a stable-to-stable regression, injected between rust 1.23 and 1.24.

[pnkfelix]

the fact that the bug goes away if you take away the packed on struct Vertex leads me to hypothesize that the bug is caused by some internal layout code for checking the validity (or just computing the effects) of that use of packed)