The staleness check compared an artifact against its own source and its
module imports. Headers were in neither set: the scanner reads `import`
lines, and a .cppm or .cpp keeps its mtime when a header it includes is
edited — so the build reported nothing to do and left objects compiled
against the previous contents. Same silent mixed-layout binary as issue
27, reached through #include instead of import.
Ask the compiler what it actually opened. Every C++ and C compile now
passes -MD -MF <artifact>.d, and Check reads that depfile back through
NewestPrerequisite, comparing every prerequisite's mtime against the
artifact. A missing depfile (an object from a crafter-build that wrote
none) or a prerequisite that no longer exists reads as "rebuild": neither
is evidence of freshness.
The parser unescapes make syntax rather than splitting on whitespace,
since clang wraps depfiles onto continuation lines and escapes spaces in
filenames.
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