ci: put ccache in front of the Hermes build

[kraenhansen]

Hermes is the iOS build. Everything else is rounding error.

Measured from the job logs of run 31691647982:

Phase	Duration	Share of Build test app
[RN] [1] Build Hermesc	3m40s	19%
[RN] [2] Build Hermes	13m35s	72%
Hermes total	17m15s	91%
Build test app step	18m58s	100%

It is not hidden concurrency: no CompileC or SwiftCompile task appears anywhere in the 13m35s [2] Build Hermes window — just ninja progress — and all 54 CompileC tasks in the entire build land in the final 66 seconds. Hermes serialises the build.

And ccache never saw any of it. C_COMPILER_LAUNCHER is an Xcode build setting, whereas Hermes is built by CMake from a script phase, so the two never meet. That job logged 549 ccache calls in total, of which only 68 were cacheable — against 2190 calls / 1082 cacheable on a build that compiles React Native itself.

Approach

build-hermes-xcode.sh hard-codes its CMake flags and offers no hook for extra arguments, but it does not sanitise the environment — and CMake reads CMAKE_TOOLCHAIN_FILE from there. So a generated toolchain file setting CMAKE_C_COMPILER_LAUNCHER / CMAKE_CXX_COMPILER_LAUNCHER is enough to put ccache in front of Hermes' compiler without patching React Native.

Verified locally before pushing: with CMAKE_TOOLCHAIN_FILE present only in the environment, CMake applies the toolchain and the launcher reaches the compile rule.

The ccache ceiling also goes from 3G to 6G, since Hermes is far larger than anything cached here so far.

Known limit

[RN] [1] Build Hermesc is out of reach. It runs its CMake under env -i with only PATH and SDKROOT, so no environment-based injection can reach it and it keeps costing its 3m40s. Reaching that one would need a change in React Native itself.

What this does not do

The bigger prize is not compiling Hermes at all. React Native supports it — HERMES_ENGINE_TARBALL_PATH selects LOCAL_PREBUILT_TARBALL and the script phases then do not run — but hermes_source_type() checks override_hermes_dir_envvar_defined() first, and that override comes from this repo's Podfile integration rather than from CI. So consuming a prebuilt Hermes is a change to how the library vendors Hermes, affecting consumers and not just this workflow. Deliberately left for a separate PR.

Test plan

• Test app (iOS) passes (label-gated on Apple 🍎, added)
• First run seeds the cache — expect no speed-up, and confirm the Hermes compiles now register as cacheable in the ccache stats
• A second run on the same commit should show the [2] Build Hermes phase drop sharply

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Generated by Claude Code