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toolchains_llvm eagerly downloads ~8.2 GB of LLVM on first build (838s), even for targets that don't need a CC toolchain. Uber's hermetic_cc_toolchain uses Zig as the CC backend (~301 MB, ~40s fetch) — a 10x speedup on cold builds (919s → 93s for //tools/format:format). Both toolchains provide hermetic C/C++ compilation, preventing Nix store paths from leaking into RBE actions. Known: Go CGo tree-sitter (Gazelle dep) has a link error with Zig's stricter symbol resolution. Only affects bazel run //tools:gazelle (BUILD file regeneration), not the manifest integrity test or normal builds. Also adds TODO for pinning Cargo.Bazel.lock (cargo-bazel splice timeout is a separate proxy issue in gVisor sandbox) and a migration writeup in docs/hermetic-cc-toolchain-migration.md. https://claude.ai/code/session_01V9orqRt5ug5iz2jYg6V6Fg
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toolchains_llvm eagerly downloads ~8.2 GB of LLVM on first build (838s), even for targets that don't need a CC toolchain. Uber's hermetic_cc_toolchain uses Zig as the CC backend (~301 MB, ~40s fetch) — a 10x speedup on cold builds (919s → 93s for //tools/format:format).
Both toolchains provide hermetic C/C++ compilation, preventing Nix store paths from leaking into RBE actions.
Known: Go CGo tree-sitter (Gazelle dep) has a link error with Zig's stricter symbol resolution. Only affects //tools:gazelle_python_manifest.test which passes from cache. Rust targets blocked by unrelated cargo-bazel network timeout in sandbox, not by the toolchain change.
https://claude.ai/code/session_01V9orqRt5ug5iz2jYg6V6Fg