Inline assembly: semicolon comments ignored

[itrTie]

or... not ignored?

I tried this code:

use core::arch::asm;

fn main() {
    let mut x: i32 = 0;
    unsafe {
        asm!(
            "mov {0:e}, 1  ; mov {0:e}, 2",
            inout(reg) x
        );
    }
    assert_eq!(x, 1);
}

I expected to see this happen: The program exits quietly as mov {0:e}, 2 is a comment.

Instead, this happened: The assertion fails as the comment is evaluated as assembly. Switching the comment delimiter to #, //, or /* ... */ fixes this issue.

This doesn't really pose an issue when writing assembly in a Rust file, as you would (and should) comment it outside of the asm! macro, but if you were to import an otherwise syntactically correct assembly file with global_asm!(include_str!("some_file.asm"), options(raw)) (which is mentioned as being acceptable in the reference) you would get an error if the assembly file had comments.

Meta

rustc --version --verbose:

rustc 1.87.0-nightly (cbfdf0b01 2025-03-13)
binary: rustc
commit-hash: cbfdf0b014cb04982a9cbeec1578001001167f6e
commit-date: 2025-03-13
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.0

Backtrace

thread 'main' panicked at tst/src/main.rs:11:5:
assertion `left == right` failed
  left: 2
 right: 1
stack backtrace:
   0:     0x55e5cfc3e882 - std::backtrace_rs::backtrace::libunwind::trace::h9174a07d358e23da
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/../../backtrace/src/backtrace/libunwind.rs:117:9
   1:     0x55e5cfc3e882 - std::backtrace_rs::backtrace::trace_unsynchronized::h0238535cda48260e
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/../../backtrace/src/backtrace/mod.rs:66:14
   2:     0x55e5cfc3e882 - std::sys::backtrace::_print_fmt::h7357ca83600008ca
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/sys/backtrace.rs:66:9
   3:     0x55e5cfc3e882 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h2ccbb0687a134d3a
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/sys/backtrace.rs:39:26
   4:     0x55e5cfc5ba93 - core::fmt::rt::Argument::fmt::h490c80f58cf5cd18
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/core/src/fmt/rt.rs:185:76
   5:     0x55e5cfc5ba93 - core::fmt::write::hd28d4ac41c011289
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/core/src/fmt/mod.rs:1449:21
   6:     0x55e5cfc3c463 - std::io::Write::write_fmt::hc802c2c997566d6d
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/io/mod.rs:1890:15
   7:     0x55e5cfc3e6d2 - std::sys::backtrace::BacktraceLock::print::h16905c0409b4bd71
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/sys/backtrace.rs:42:9
   8:     0x55e5cfc3f5f2 - std::panicking::default_hook::{{closure}}::hd73239ef18e9c626
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panicking.rs:298:22
   9:     0x55e5cfc3f3f5 - std::panicking::default_hook::hf79edb81864efc5c
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panicking.rs:325:9
  10:     0x55e5cfc3fec2 - std::panicking::rust_panic_with_hook::h8b4624c1a31cf321
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panicking.rs:831:13
  11:     0x55e5cfc3fd5a - std::panicking::begin_panic_handler::{{closure}}::he1ef97b10af3d992
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panicking.rs:704:13
  12:     0x55e5cfc3ed79 - std::sys::backtrace::__rust_end_short_backtrace::he0d459df86b8ceea
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/sys/backtrace.rs:168:18
  13:     0x55e5cfc3f9ed - rust_begin_unwind
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panicking.rs:695:5
  14:     0x55e5cfc5af30 - core::panicking::panic_fmt::h3463be8214ef2d7a
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/core/src/panicking.rs:75:14
  15:     0x55e5cfc5b1b7 - core::panicking::assert_failed_inner::ha0c4af2aad2e05dc
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/core/src/panicking.rs:425:17
  16:     0x55e5cfc29692 - core::panicking::assert_failed::heb6193dbaff462c9
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/core/src/panicking.rs:380:5
  17:     0x55e5cfc1fb39 - tst::main::h51f7c8543d6ea0ec
                               at /home/i/viridian/tst/src/main.rs:11:5
  18:     0x55e5cfc1fb8b - core::ops::function::FnOnce::call_once::h154b0b87a9cfab3c
                               at /home/i/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
  19:     0x55e5cfc1fbde - std::sys::backtrace::__rust_begin_short_backtrace::hc3d2ecae4d9e94dd
                               at /home/i/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/backtrace.rs:152:18
  20:     0x55e5cfc1fab1 - std::rt::lang_start::{{closure}}::hdbc8ce7d80b361fd
                               at /home/i/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:199:18
  21:     0x55e5cfc3a450 - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::h89b9c257652e56a0
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/core/src/ops/function.rs:284:13
  22:     0x55e5cfc3a450 - std::panicking::try::do_call::h9154a00e6218a7ee
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panicking.rs:587:40
  23:     0x55e5cfc3a450 - std::panicking::try::hc84925c5845b0339
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panicking.rs:550:19
  24:     0x55e5cfc3a450 - std::panic::catch_unwind::h62ee428978f0fff1
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panic.rs:359:14
  25:     0x55e5cfc3a450 - std::rt::lang_start_internal::{{closure}}::h9cc4a7783ce0ae9b
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/rt.rs:168:24
  26:     0x55e5cfc3a450 - std::panicking::try::do_call::hcd34f1f51f4e7728
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panicking.rs:587:40
  27:     0x55e5cfc3a450 - std::panicking::try::hbe9440be03c95ae5
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panicking.rs:550:19
  28:     0x55e5cfc3a450 - std::panic::catch_unwind::h2730f4191fbd1db1
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/panic.rs:359:14
  29:     0x55e5cfc3a450 - std::rt::lang_start_internal::h5544d0e71c42d1c4
                               at /rustc/cbfdf0b014cb04982a9cbeec1578001001167f6e/library/std/src/rt.rs:164:5
  30:     0x55e5cfc1fa97 - std::rt::lang_start::h6412dfef03c3d63c
                               at /home/i/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:198:5
  31:     0x55e5cfc1fb5e - main
  32:     0x7fc6226831ca - <unknown>
  33:     0x7fc62268328b - __libc_start_main
  34:     0x55e5cfc1f995 - _start
  35:                0x0 - <unknown>

[Noratrieb]

The LLVM assembler (just like GCC/GNU as) uses # for comments, not ;.
https://sourceware.org/binutils/docs-2.17/as/Comments.html