broken MIR in Item (nightly)

[ArthurKValladares]

Code

#[derive(Debug)]
pub struct HalSetLayouts {
    vertex_layout: <B as hal::Backend>::DescriptorSetLayout,
    fragment_layout: <B as hal::Backend>::DescriptorSetLayout,
    fragment_sampler_layout: <B as hal::Backend>::DescriptorSetLayout,
}

impl HalSetLayouts {
    fn iter<DSL>(&self) -> impl ExactSizeIterator<Item = &DSL> + '_
    where
        // rust 1.37 fails to typecheck `iter` without this. This is fixed in nightly.
        B: hal::Backend<DescriptorSetLayout = DSL>,
        DSL: std::fmt::Debug + Send + Sync + 'static,
    {
        std::iter::once(&self.vertex_layout)
            .chain(std::iter::once(&self.fragment_layout))
            .chain(std::iter::once(&self.fragment_sampler_layout))
            .exact(3)
    }
}

Meta

rustc --version --verbose:

rustc 1.64.0-nightly (2643b1646 2022-07-27)
binary: rustc
commit-hash: 2643b16468fda787470340890212591d8bc832b7
commit-date: 2022-07-27
host: x86_64-apple-darwin
release: 1.64.0-nightly
LLVM version: 14.0.6

Error output

error: internal compiler error: no errors encountered even though `delay_span_bug` issued

error: internal compiler error: broken MIR in Item(WithOptConstParam { did: DefId(0:3021 ~ renderer[c01e]::shader::{impl#11}::iter), const_param_did: None }) (end of phase transition to Optimized) at bb0[0]:
                                Field projection `(*_1).field[0]` specified type `DSL`, but actual type is <gfx_backend_metal::Backend as gfx_hal::Backend>::DescriptorSetLayout
   --> brainstorm/src/renderer/src/shader.rs:642:25
    |
642 |         std::iter::once(&self.vertex_layout)
    |                         ^^^^^^^^^^^^^^^^^^^
    |
    = note: delayed at compiler/rustc_const_eval/src/transform/validate.rs:129:36

error: internal compiler error: broken MIR in Item(WithOptConstParam { did: DefId(0:3021 ~ renderer[c01e]::shader::{impl#11}::iter), const_param_did: None }) (end of phase transition to Optimized) at bb1[0]:
                                Field projection `(*_1).field[1]` specified type `DSL`, but actual type is <gfx_backend_metal::Backend as gfx_hal::Backend>::DescriptorSetLayout
   --> brainstorm/src/renderer/src/shader.rs:643:36
    |
643 |             .chain(std::iter::once(&self.fragment_layout))
    |                                    ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: delayed at compiler/rustc_const_eval/src/transform/validate.rs:129:36

error: internal compiler error: broken MIR in Item(WithOptConstParam { did: DefId(0:3021 ~ renderer[c01e]::shader::{impl#11}::iter), const_param_did: None }) (end of phase transition to Optimized) at bb3[0]:
                                Field projection `(*_1).field[2]` specified type `DSL`, but actual type is <gfx_backend_metal::Backend as gfx_hal::Backend>::DescriptorSetLayout
   --> brainstorm/src/renderer/src/shader.rs:644:36
    |
644 |             .chain(std::iter::once(&self.fragment_sampler_layout))
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: delayed at compiler/rustc_const_eval/src/transform/validate.rs:129:36

Backtrace

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1426:13
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: std::panic::panic_any::<rustc_errors::ExplicitBug>
   2: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
   3: core::ptr::drop_in_place::<rustc_session::parse::ParseSess>
   4: <alloc::rc::Rc<rustc_session::session::Session> as core::ops::drop::Drop>::drop
   5: core::ptr::drop_in_place::<rustc_interface::interface::Compiler>
   6: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_interface::interface::create_compiler_and_run<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>::{closure#1}>
   7: rustc_interface::interface::create_compiler_and_run::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>
   8: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

This came from a workaround for what seems to be an old compiler bug. removing the workaround gets ride of the ICE:

#[derive(Debug)]
pub struct HalSetLayouts {
    vertex_layout: <B as hal::Backend>::DescriptorSetLayout,
    fragment_layout: <B as hal::Backend>::DescriptorSetLayout,
    fragment_sampler_layout: <B as hal::Backend>::DescriptorSetLayout,
}

impl HalSetLayouts {
    fn iter(
        &self,
    ) -> impl ExactSizeIterator<Item = &<B as hal::Backend>::DescriptorSetLayout> + '_ {
        std::iter::once(&self.vertex_layout)
            .chain(std::iter::once(&self.fragment_layout))
            .chain(std::iter::once(&self.fragment_sampler_layout))
            .exact(3)
    }
}

Can you post what your dependencies are? Or include the use statements? Ideally with the versions too.

Or something that can just be stuck in a main.rs that compiles? I'm not sure where the B is coming from in the code above (in the struct layout)

[ArthurKValladares]

Of course, my bad for not providing this upfront. I've been able to repo it with this setup:

deps/features:

[features]
default = ["metal"]
dx12 = ["gfx-backend-dx12"]
empty = ["gfx-backend-empty"]
gl = ["gfx-backend-gl"]
metal = ["gfx-backend-metal"]
vulkan = ["gfx-backend-vulkan"]

[dependencies]
gfx-backend-dx12 = { version = "0.6.3", optional = true }
gfx-backend-empty = { version = "0.6.0", optional = true }
gfx-backend-gl = { version = "0.5.1", optional = true }
gfx-backend-metal = { version = "0.6.2", optional = true }
gfx-backend-vulkan = { version = "0.6.1", optional = true }
gfx-hal = "0.6.0"

lib.rs

#[cfg(feature = "metal")]
use gfx_backend_metal as back;
pub use gfx_hal as hal;

#[cfg(feature = "vulkan")]
use gfx_backend_vulkan as back;

#[cfg(not(any(feature = "metal", feature = "vulkan")))]
use gfx_backend_empty as back;

pub type B = back::Backend;

pub struct Exacterator<I: Iterator<Item = T>, T> {
    iter: I,
    size: usize,
}

impl<I: Iterator<Item = T>, T> Exacterator<I, T> {
    pub fn new(iter: I, size: usize) -> Self {
        Self { iter, size }
    }
}

impl<I: Iterator<Item = T>, T> Iterator for Exacterator<I, T> {
    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {
        self.iter.next()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.size, Some(self.size))
    }
}

impl<I: Iterator<Item = T>, T> ExactSizeIterator for Exacterator<I, T> {}

pub trait Exact<T>: Iterator<Item = T> + Sized {
    fn exact(self, size: usize) -> Exacterator<Self, T>;
}

impl<I: Iterator<Item = T>, T> Exact<T> for I {
    fn exact(self, size: usize) -> Exacterator<Self, T> {
        Exacterator::new(self, size)
    }
}

#[derive(Debug)]
pub struct HalSetLayouts {
    vertex_layout: <B as hal::Backend>::DescriptorSetLayout,
    fragment_layout: <B as hal::Backend>::DescriptorSetLayout,
    fragment_sampler_layout: <B as hal::Backend>::DescriptorSetLayout,
}

impl HalSetLayouts {
    pub fn iter<DSL>(&self) -> impl ExactSizeIterator<Item = &DSL> + '_
    where
        // rust 1.37 fails to typecheck `iter` without this. This is fixed in nightly.
        B: hal::Backend<DescriptorSetLayout = DSL>,
        DSL: std::fmt::Debug + Send + Sync + 'static,
    {
        std::iter::once(&self.vertex_layout)
            .chain(std::iter::once(&self.fragment_layout))
            .chain(std::iter::once(&self.fragment_sampler_layout))
            .exact(3)
    }
}

Ah yes, thank you very much. I can reproduce it (with the empty feature, so all the other crates can presumably be removed). I'll look into this.

For reference, minimised it down to this:

pub trait Backend {
    type DescriptorSetLayout;
}

pub struct Back;

impl Backend for Back {
    type DescriptorSetLayout = u32;
}

pub struct HalSetLayouts {
    vertex_layout: <Back as Backend>::DescriptorSetLayout,
}

impl HalSetLayouts {
    pub fn iter<DSL>(self) -> DSL
    where
        Back: Backend<DescriptorSetLayout = DSL>,
    {
        self.vertex_layout
    }
}

I also bisected this, and it turns out to be the same rollup as in #99852... So I suspect both these errors were introduced in the same commit. Interesting!

Edit: Adjusted the code (edit2: twice!), check comment history for the original.

[Noratrieb]

Here is the MIR for the iter function when I manually disabled validation (I just noticed, validation doesn't have to be disabled, -Zunpretty=mir works anyways)

fn <impl at lol.rs:17:1: 17:19>::iter(_1: HalSetLayouts) -> DSL {
    debug self => _1;                    // in scope 0 at lol.rs:18:22: 18:26
    let mut _0: DSL;                     // return place in scope 0 at lol.rs:18:31: 18:34

    bb0: {
        _0 = move (_1.0: DSL);           // scope 0 at lol.rs:22:9: 22:27
        return;                          // scope 0 at lol.rs:23:6: 23:6
    }
}

[Noratrieb]

After looking into this a bit, I got the following:

The body is treating the generic type DST and the concrete associated type <Back as Backend>::DescriptorSetLayout to be of the same type. The MIR validator probably thinks that this is very sketchy and bails out (I haven't looked at it closely).

This is actually ok though, since in the where bounds DST is constrained to be equal to <Back as Backend>::DescriptorSetLayout.

To me, this looks like a bug in the MIR validator.

[Noratrieb]

This was probably regressed by #96856. cc @drmeepster

[Noratrieb]

Yes, manually reverting the PR fixed the issue for me locally.

[Noratrieb]

While it regressed in that PR, the PR only called validation that wasn't called before. The validation has been broken for longer.

[Noratrieb]

@rustbot claim