Returning from inline asm! is unsound (always UB, and made worse by MIR inlining).

[oisyn]

EDIT(@eddyb): updated the title to reflect better understanding of the underlying problem.
The reason the crashes make no sense is because the MIR we get is already broken, having inlined calls to functions that contain asm!("OpReturnValue"), which changes meaning to return from the caller, not the callee.

If you run into a similar crash, you could try RUSTGPU_RUSTFLAGS=-Zmir-opt-level=0 as a workaround for now

---

When compiling this shader (with capability Int64 enabled)

#![cfg_attr(
    target_arch = "spirv",
    no_std
)]
#![allow(unused_variables)]

use spirv_std::{spirv, ray_tracing::AccelerationStructure};
use spirv_std::glam::*;

#[derive(Clone, Copy)]
#[repr(C)]
pub struct PushConstants {
    pub acceleration_structure_address: u64,
}


#[spirv(fragment)]
pub fn main_fs(
    #[spirv(push_constant)] push_constants: &PushConstants,
    output: &mut Vec4,
) {
    let acceleration_structure =
    unsafe { AccelerationStructure::from_u64(push_constants.acceleration_structure_address) };
}

The compiler panics with the following callstack:

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', d:\.cargo\registry\src\github.com-1ecc6299db9ec823\rustc_codegen_spirv-0.5.0\src\linker\inline.rs:523:44
stack backtrace:
   0:     0x7fffc5999de2 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hea3121a92230cef2
   1:     0x7fffc59d5d0b - core::fmt::write::h0bc3f53e8dca163a
   2:     0x7fffc598c8fa - <std::io::IoSlice as core::fmt::Debug>::fmt::h7890ee78f4472cf6
   3:     0x7fffc5999b2b - std::sys::common::alloc::realloc_fallback::h2905b67a6dea1c71
   4:     0x7fffc599d459 - std::panicking::default_hook::h111ce7f1b363c3ab
   5:     0x7fffc599d0db - std::panicking::default_hook::h111ce7f1b363c3ab
   6:     0x7fffc3d0a7f6 - _rustc_codegen_backend
   7:     0x7fffc599ddc0 - std::panicking::rust_panic_with_hook::hfc6ef76eb93085aa
   8:     0x7fffc599dadb - <std::panicking::begin_panic_handler::StrPanicPayload as core::panic::BoxMeUp>::get::ha20f814ea23fbc14
   9:     0x7fffc599aadf - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hea3121a92230cef2
  10:     0x7fffc599d7d0 - rust_begin_unwind
  11:     0x7fffc5a0bb85 - core::panicking::panic_fmt::h0dc3dee6c5b102b4
  12:     0x7fffc5a0bc7c - core::panicking::panic::hfcd8358d78f3c4e7
  13:     0x7fffc3c6458a - rustc_codegen_spirv::linker::inline::inline::h48d5fd4c7cffb474
  14:     0x7fffc3cfc526 - rustc_codegen_spirv::linker::link::h2839c3b263de87e8
  15:     0x7fffc3bb93c1 - rustc_codegen_spirv::link::link::h6961ee106edcc752
  16:     0x7fffc3d0766d - <rustc_codegen_spirv::SpirvCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::link::hb518a1efad20a829
  17:     0x7fff643f10e2 - <rustc_interface[e09bab31b5025fb2]::queries::Linker>::link
  18:     0x7fff641e4633 - rustc_driver[5e4fd5aa1121bb7c]::args::arg_expand_all
  19:     0x7fff641d4983 - <rustc_span[25fa311e2ce9dc22]::DebuggerVisualizerFile>::new
  20:     0x7fff641ca24d - <rustc_span[25fa311e2ce9dc22]::DebuggerVisualizerFile>::new
  21:     0x7fffc59afe9c - std::sys::windows::thread::Thread::new::h4025a5964e95cbad
  22:     0x7ff844597614 - BaseThreadInitThunk
  23:     0x7ff8456426a1 - RtlUserThreadStart
error: internal compiler error: unexpected panic

Culprit was found to be a the AccelerationStructure::from_u64() call, using a push_constant input.

Interestingly, if the input doesn't come from a push constant (but from, say, a constant 0_u64), you'll get this:

  error: OpReturnValue Value <id> '17[%17]'s type does not match OpFunction's return type.
           OpReturnValue %17
    |
    = note: module `D:\work\private\rust-gpu-empty\target\spirv-builder\spirv-unknown-spv1.3\release\deps\shaders.spv`

  warning: an unknown error occurred
    |
    = note: spirv-opt failed, leaving as unoptimized
    = note: module `D:\work\private\rust-gpu-empty\target\spirv-builder\spirv-unknown-spv1.3\release\deps\shaders.spv`

  error: error:0:0 - OpReturnValue Value <id> '17[%17]'s type does not match OpFunction's return type.
           OpReturnValue %17
    |
    = note: spirv-val failed
    = note: module `D:\work\private\rust-gpu-empty\target\spirv-builder\spirv-unknown-spv1.3\release\deps\shaders.spv`

[eddyb]

It's..... like that we get it. It's pre-inlined. This is MIR inlining, wow.
Explains why it started happening all of a sudden: we didn't do this, MIR inlining was just turned on by default.

EDIT: temporary workaround for testing is RUSTGPU_RUSTFLAGS=-Zmir-opt-level=0.