global_asm! issues LLVM error "undefined temporary symbol" for non-existent numerical labels

[pqcfox]

I've found that if I place undefined local numeric labels in a global_asm! block, that rustc will yield an LLVM (?) error error: Undefined temporary symbol .Ltmp0, presumably as the numeric label is replaced with a temporary symbol .Ltmp0.

Here's a minimum viable example:

use std::arch::global_asm;

fn main () {}

global_asm!(
    "jmp 100f"
);

(Playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ddb9a35af1ac6a0c4d3a145253605252)

Notably, changing the label to a (global) text label as in the following doesn't cause any error to be issued.

use std::arch::global_asm;

fn main () {}

global_asm!(
    "jmp .label"
);

I wasn't sure if this is something that would fall into the purview of Rust to handle with a more direct error message, but wanted to raise it as a potential issue since I'd expect either (a) no error message as in the case of the global label or (b) a typical rustc error message indicating where in the code the issue lies.

Meta

rustc --version --verbose:

rustc 1.85.1 (4eb161250 2025-03-15)
binary: rustc
commit-hash: 4eb161250e340c8f48f66e2b929ef4a5bed7c181
commit-date: 2025-03-15
host: x86_64-unknown-linux-gnu
release: 1.85.1
LLVM version: 19.1.7

Also verified with nightly:

rustc 1.88.0-nightly (d6c1e454a 2025-04-21)
binary: rustc
commit-hash: d6c1e454aa8af5e7e59fbf5c4e7d3128d2f99582
commit-date: 2025-04-21
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

[bjorn3]

Notably, changing the label to a (global) text label as in the following doesn't cause any error to be issued.

That will cause the symbol .label to be imported from another object file. It doesn't cause an error because the entire section that contains the jmp .label gets removed by the linker as we pass --gc-sections. If I pass -Clink-dead-code=yes to disable this, I get:

error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "-m64" "/tmp/rustcXCbmew/symbols.o" "<2 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,libcfg_if-*,liblibc-*,liballoc-*,librustc_std_workspace_core-*,libcore-*,libcompiler_builtins-*}.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-o" "rust_out" "-pie" "-Wl,-z,relro,-z,now" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: /usr/bin/ld: rust_out.rust_out.5f0bdc08ea4efb31-cgu.0.rcgu.o:(.text+0x1): undefined reference to `.label'
          collect2: error: ld returned 1 exit status
          
  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link

error: aborting due to 1 previous error

presumably as the numeric label is replaced with a temporary symbol .Ltmp0.

Indeed. rustc --emit asm shows:

	.text
	.file	"rust_out.5f0bdc08ea4efb31-cgu.0"


	jmp	.Ltmp0



	.section	.text._ZN3std2rt10lang_start17h19f5fd841b73ea7fE,"ax",@progbits
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