feat: fast-path fingerprint coherence, diagnostic warning, cxx_scan restat#35
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P1: .build_cache now stores fingerprint hex (4th line). try_fast_build validates that the cached fingerprint matches the outputDir basename; if inconsistent (e.g. switched mcpp installation), the cache is invalidated immediately instead of silently falling through to a different fingerprint directory. P1.5: run_build_plan prints a warning when the outputDir fingerprint differs from the previous .build_cache entry, so users immediately see why a full rebuild is happening instead of getting a silent 26s. P2: cxx_scan rule now writes .ddi to $out.tmp first, then compares with existing $out via cmp -s. If content is identical, old $out is kept (preserving mtime). Combined with restat = 1, this prevents downstream dyndep/compile edges from being marked dirty when a source file's mtime changed but its module dependencies didn't.
The $out.tmp + cmp -s approach for content-stable .ddi writes caused linker failures in the CI self-host smoke step. GCC's -fdeps-file= interacts with ninja's depfile tracking in ways that make the temp-file approach fragile. Revert to direct -fdeps-file=$out for now. P1 (fingerprint coherence) and P1.5 (diagnostic warning) are retained. P2 needs a different approach — possibly ninja's built-in restat alone without temp-file wrappers.
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Summary
Three incremental build experience improvements:
.build_cachenow stores fingerprint hex (4th line).try_fast_buildvalidates consistency; if fingerprint doesn't match outputDir, cache is invalidated immediately instead of silently falling through to a stale directoryrun_build_planprintswarning: fingerprint changed (old → new), full rebuildwhen outputDir differs from cached, so users see WHY a full rebuild happened.ddito$out.tmpfirst, compares withcmp -s, keeps old file if identical. Withrestat = 1, downstream edges are not dirtied when source mtime changed but module deps didn'tTest plan
mcpp buildcompiles successfullymcpp test— 12/12 pass.build_cachenow has 4 lines (fingerprint hex on line 4).build_cachefiles handled gracefully (empty fingerprint = skip check)