Rust 1.99.1 fixes an FFI bool miscompilation, a missing wcslen symbol and rust-lld on macOS

Victoria Shinder16 October 2026, 14:022 min read

The Rust release team announced Rust 1.99.1 on 15 October 2026. It is a point release that fixes three regressions in 1.99.0; users who installed Rust through rustup get it with rustup update stable.

FFI bool return values on x86_64. The most serious fix is a miscompilation. According to the announcement, Rust functions returning bool through FFI could leave bits 1–7 of the return register unspecified, so callers saw raw values other than 0 or 1. The return type is now marked for zero-extension again, "as required by the ABI". The bug report (rust-lang/rust#163911, labelled P-critical and I-miscompile) shows a minimal case: an extern "C" function computing (flags & 0x1) == 0x1 returned 16 to a C caller for the input 0x10. In the disassembly attached to the issue, 1.98.0 emitted movl %edi, %eax followed by andb $0x1, %al; 1.99.0 emitted only the move. The reporter reproduced it with both gcc and clang as the C compiler.

Missing wcslen on UEFI and embedded targets. Some targets hit link errors because wcslen was missing. The announcement explains that LLVM's loop-idiom-wcslen pass may insert calls to wcslen as an optimisation, and some Rust targets had no such function in libc or elsewhere. The UEFI targets were broken for any project using std, because some of these calls were introduced in the precompiled standard library, but similar user code could trigger it too. The function is now present in Rust's compiler-builtins as a fallback. The report (#163614) shows rust-lld failing with "undefined symbol: wcslen" for a no_std UEFI binary built with --release on 1.99.0, while 1.98.1 built it.

rust-lld on macOS. The bundled linker could fail to start when the compiler was invoked without the rustup proxy, because there was no direct setting for the library search path it needs. rust-lld is now built with an rpath entry for the directory that contains libLLVM.dylib (#163947).

Who should update. Anyone exposing Rust functions that return bool to C or other languages on x86_64, and anyone building for UEFI or embedded targets, is directly affected by the first two fixes. The release team also asks developers to test the beta and nightly channels locally and in CI and to report bugs.

Source: The Rust Release Team, "Announcing Rust 1.99.1" — https://blog.rust-lang.org/2026/10/15/Rust-1.99.1/