[Koi (ck0i)]

Fixes #889

Root cause

With the build_cc and lto features on MSVC, snmalloc is compiled with /GL and cc bundles the static library into the snmalloc-sys rlib. rustc can't read symbols from LTCG objects (rust-lang/rust#26003), so the bundled copy exports nothing and the final link fails with LNK2019 on the sn_rust_* symbols. The CMake build doesn't run into this. It emits a plain rustc-link-lib line, the library isn't bundled, and link.exe gets the original .lib and restarts with /LTCG.

Fix

• When lto is on with MSVC and the library is built with cc, set link_lib_modifier("-bundle") so the .lib goes to link.exe directly.
• Skip the second rustc-link-lib line in main() for build_cc on MSVC, since rustc rejects a duplicate entry for the same library when one of them carries modifiers.
• link_lib_modifier needs cc 1.0.74, so the minimum cc version is bumped to that.
• Add a build_cc-lto entry (--features "build_cc lto") to the Rust CI matrix.

Testing

Windows 11, MSVC 14.51 (Visual Studio 2026), Rust 1.98.1, rebased on current main. cargo test --all in snmalloc-rs, debug and release.

• Without the fix: with --features "build_cc lto", the snmalloc-rs lib tests and memory_stats fail to link with LNK2019 on sn_rust_* symbols such as sn_rust_statistics, in both debug and release.
• With the fix: --features "build_cc lto" passes in debug and release, and so do the other feature sets in the CI matrix (default, debug, check, build_cc usecxx17, native-cpu, local_dynamic_tls, lto).

[Matthew Parkinson (mjp41)]

Lgtm

[Copilot]

🟢 Approval recommended

No active review issues block the MSVC LTO linking changes.

No open issues 🎉

What changed in this PR

Fixes #889 by keeping MSVC link-time-optimized native libraries outside the Rust archive when build_cc and lto are enabled, allowing the final linker to resolve the sn_rust_* symbols.

Changes:

• Disables native-library bundling for this configuration and avoids duplicate MSVC link-library directives.
• Raises the minimum cc version to 1.0.74 for link_lib_modifier support.
• Adds build_cc with lto to the Rust CI matrix.

File	Description
snmalloc-rs/​snmalloc-sys/​Cargo.toml	Raises the minimum supported cc version.
snmalloc-rs/​snmalloc-sys/​build.rs	Adjusts MSVC library bundling and link directives.
.github/​workflows/​rust.yml	Adds CI coverage for combined build_cc and lto features.

_{Review effort: Balanced}