Rust 1.99.1 fixes a bool return miscompilation and two linker regressions
The point release addresses an FFI return-value bug on x86_64, missing wcslen symbols on some targets, and a macOS rust-lld loading problem.
The Rust team published point release 1.99.1 on October 15. It fixes three regressions introduced in earlier releases, and users who installed Rust through rustup can pick it up with the usual stable update command.
One fix concerns Rust functions that return a bool across an FFI boundary on x86_64. The upper bits of the return register could be left unspecified, so a caller might read a value other than 0 or 1. The return type is again marked for zero-extension, as the ABI requires.
The other two fixes are about linking. Some targets, including UEFI targets that use std, hit errors for a missing wcslen symbol because an LLVM optimization can introduce calls to it; a fallback now ships in compiler-builtins. On macOS, rust-lld could fail to find libLLVM when run without the rustup proxy, and it is now built with an rpath entry for that library. Projects that expose bool-returning functions over FFI, target UEFI, or invoke rust-lld directly should update.
Sources
Verified Oct 22, 2026 (official source). rustdeveloper-toolscompilerrelease