ICE when creating ! instance via asm macro

[kangalio]

Code

#![feature(asm)]

fn hmm() -> ! {
    let x;
    unsafe {
        asm!("/* {0} */", out(reg) x);
    }
    x
}

fn main() {
    hmm();
}

Meta

rustc --version --verbose:

rustc 1.56.0-nightly (25b764849 2021-08-04)
binary: rustc
commit-hash: 25b764849625cb090e8b81d12d2bb2295d073788
commit-date: 2021-08-04
host: x86_64-unknown-linux-gnu
release: 1.56.0-nightly
LLVM version: 12.0.1

Error output

error: internal compiler error: compiler/rustc_codegen_llvm/src/asm.rs:347:28: LLVM asm constraint validation failed
 --> src/main.rs:6:15
  |
6 |         asm!("/* {0} */", out(reg) x);
  |               ^^^^^^^^^

thread 'rustc' panicked at 'Box<dyn Any>', /rustc/25b764849625cb090e8b81d12d2bb2295d073788/compiler/rustc_errors/src/lib.rs:980:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.56.0-nightly (25b764849 2021-08-04) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack

Backtrace

   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::span_bug
   3: rustc_errors::Handler::span_bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::span_bug_fmt
   7: rustc_codegen_llvm::asm::<impl rustc_codegen_ssa::traits::asm::AsmBuilderMethods for rustc_codegen_llvm::builder::Builder>::codegen_inline_asm
   8: rustc_codegen_ssa::mir::block::<impl rustc_codegen_ssa::mir::FunctionCx<Bx>>::codegen_terminator
   9: rustc_codegen_ssa::mir::codegen_mir
  10: rustc_codegen_ssa::base::codegen_instance
  11: <rustc_middle::mir::mono::MonoItem as rustc_codegen_ssa::mono_item::MonoItemExt>::define
  12: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
  13: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  14: rustc_codegen_llvm::base::compile_codegen_unit
  15: rustc_codegen_ssa::base::codegen_crate
  16: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  17: rustc_interface::passes::QueryContext::enter
  18: rustc_interface::queries::Queries::ongoing_codegen
  19: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  20: rustc_span::with_source_map
  21: rustc_interface::interface::create_compiler_and_run
  22: scoped_tls::ScopedKey<T>::set

[kangalio]

Somewhere during compilation of this code, LLVM's InlineAsm::Verify function returns false when checking "if the specified constraint string is legal for the type" (relevant rustc line).

Because of that, rustc_codegen_llvm::asm::inline_asm_call returns None, and the parent function then reports the bug.

[kangalio]

I added a dbg! statement to log the function type and constraints string passed to LLVM:

[compiler/rustc_codegen_llvm/src/asm.rs:453] fty = { [0 x i8] } ()
[compiler/rustc_codegen_llvm/src/asm.rs:453] cons = "=&r,~{dirflag},~{fpsr},~{flags},~{memory}"

[nbdd0121]

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