Linking an `std` program with the `*-unknown-uefi` targets fails with "undefined symbol: wcslen"

[nicholasbishop]

With a simple main.rs like this:

fn main() {}

Build with cargo +nightly build --target x86_64-unknown-uefi. It fails like this:

error: linking with `rust-lld` failed: exit status: 1
  |
  = note:  "rust-lld" "-flavor" "link" "/NOLOGO" "/entry:efi_main" "/subsystem:efi_application" "/var/home/nbishop/tmp/uefi-std-test/target/x86_64-unknown-uefi/debug/build/uefi-std-test/e1b313fb8e551b59/out/rustcAdIZq5/symbols.o" "<6 object files omitted>" "<sysroot>/lib/rustlib/x86_64-unknown-uefi/lib/{libpanic_abort-*,libstd-*,libr_efi_alloc-*,libr_efi-*,libcfg_if-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,liblibc-*,librustc_std_workspace_core-*,liballoc-*,libcore-*,libcompiler_builtins-*}.rlib" "/NXCOMPAT" "/OUT:/var/home/nbishop/tmp/uefi-std-test/target/x86_64-unknown-uefi/debug/build/uefi-std-test/e1b313fb8e551b59/out/uefi_std_test.efi" "/OPT:REF,NOICF" "/DEBUG" "/PDBALTPATH:%_PDB%" "/NODEFAULTLIB"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: rust-lld: error: undefined symbol: wcslen
          >>> referenced by /rustc/771916f9028e7fe56d2685f2c4f698de5d7d6a45/library/std/src/sys/process/uefi.rs:744
          >>>               libstd-95535557a15d2937.rlib(std-95535557a15d2937.std.61c682eb00e56aa5-cgu.0.rcgu.o):(<std::sys::process::uefi::uefi_command_internal::PipeProtocol>::output_string)
          >>> referenced by /rustc/771916f9028e7fe56d2685f2c4f698de5d7d6a45/library/std/src/process.rs:1140
          >>>               libstd-95535557a15d2937.rlib(std-95535557a15d2937.std.61c682eb00e56aa5-cgu.0.rcgu.o):(<std::process::Command>::output)
          >>> referenced by /rustc/771916f9028e7fe56d2685f2c4f698de5d7d6a45/library/std/src/env.rs:54
          >>>               libstd-95535557a15d2937.rlib(std-95535557a15d2937.std.61c682eb00e56aa5-cgu.0.rcgu.o):(std::env::current_dir)
          >>> referenced 8 more times

Similar errors occur for the i686 and aarch64 UEFI targets.

Note that this only fails if std is enabled; a no_std+no_main program still works fine.

$ rustc +nightly --version --verbose
rustc 1.99.0-nightly (771916f90 2026-08-08)
binary: rustc
commit-hash: 771916f9028e7fe56d2685f2c4f698de5d7d6a45
commit-date: 2026-08-08
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 23.1.0

I bisected this to #158734, which is the LLVM 23 uprev.

[nicholasbishop]

CC @dvdhrm @Ayush1325 @phip1611

[Ayush1325]

Maybe cased by this: llvm/llvm-project@9694844?
The Linux kernel uses -fno-builtin-wcslen now: https://patchew.org/linux/20250407-fno-builtin-wcslen-v1-1-6775ce759b15@kernel.org/
Or maybe it should be added to compiler-builtins.

[dvdhrm]

Thanks for tracking this down @Ayush1325! I don't think this is specific to std. I think you can trigger this with a simple loop with no-std as well.

-fno-builtin-<fn> is a clang frontend option, isn't it? But I think I agree that we should pass this to LLVM, if we can.

To be honest, though, this does not look specific to UEFI, does it? Any of the raw targets will hit this, right? I assume it is just that no-one uses wide-strings, except for UEFI and windows targets. And strlen() seems to be defined somewhere. Hm, so maybe compiler_builtins is the right choice.

[Ryan1202]

I can reproduce this on the freestanding aarch64-unknown-none-softfloat target, while trying to compile AsahiLinux/m1n1.

# nightly-2026-08-05
# rustc 1.99.0-nightly (1ed2df61a 2026-08-04)
# LLVM 22.1.8
$ nm -u test.o
# nothing

# nightly-2026-08-06
# rustc 1.99.0-nightly (7608eb7b0 2026-08-05)
# LLVM 23.1.0
$ nm -u test.o
                 U wcslen

Minimal reproducer:

#![no_std]

#[no_mangle]
pub unsafe extern "C" fn scan_i32(p: *const i32) -> usize {
    let mut n = 0;

    while unsafe { *p.add(n) } != 0 {
        n += 1;
    }

    n
}

compiled with:

rustc +nightly-2026-08-06 \
    --crate-type=lib \
    --target=aarch64-unknown-none-softfloat \
    -O \
    --emit=obj \
    repro.rs \
    -o test.o

nm -u test.o

disabling the LLVM wcslen loop idiom transformation helps:

rustc +nightly-2026-08-06 \
    --crate-type=lib \
    --target=aarch64-unknown-none-softfloat \
    -O \
    -C llvm-args=-disable-loop-idiom-wcslen \
    --emit=obj \
    repro.rs \
    -o test.o