Emit a depfile for -MMD, not only for -MD

[dkulp]

The bug

func (deps *DepCmdFlags) ShouldGenerateDepFile() bool {
	return deps.flagMD || deps.flagMF != ""
}

An invocation carrying only -MMD produces no .d file at all.

-MMD is not a modifier on -MD; the two are alternatives that differ only in whether system headers are listed, and both emit a dependency file as a side effect of compilation. nocc strips -MMD off cxxArgs along with the rest of the dep flags and generates the file itself on the client, so when this predicate is false nothing generates it — neither nocc nor the compiler on the server.

Why it goes unnoticed

-MMD -MP with no -MF is the usual hand-written-Makefile spelling, as opposed to the -MD -MT ... -MF ... that CMake emits. Such a project built through nocc silently ends up with zero .d files and no header dependency tracking: editing a header rebuilds nothing.

The failure is invisible, because a missing .d is not an error to make — the -include of the dep files just matches nothing.

Why the existing tests did not catch it

Every case in tests/depfiles_test.go also passes -MD, including the two that pass -MMD (Test_MDMTMFMPMMD, Test_MQMQMQMQMMP, Test_MTMTMTMTMMP). So flagMD was always true and the predicate was never exercised with -MMD on its own.

Test

Two cases added in the same style as the rest of the file, comparing contents against real g++ output:

func Test_MMDMP(t *testing.T) {
	var cmdLineStr = "g++ -MMD -MP -o dt/dep1/1.cpp.o -c dt/dep1/1.cpp"
	...
}

func Test_MMDOnly(t *testing.T) {
	var cmdLineStr = "g++ -MMD -o dt/dep1/1.cpp.o -c dt/dep1/1.cpp"
	...
}

Both fail against current master with:

Error parsing dt/dep1/1.cpp.d after nocc: open dt/dep1/1.cpp.d: no such file or directory

and pass with this change. The rest of the suite is unaffected — same results as master on my machine.

The system-header filtering that -MMD asks for was already implemented in calcDepListFromHFiles and keys off flagMMD, so it applies as soon as the file is generated.

[Copilot]

Pull request overview

Fixes a bug in nocc’s depfile generation logic where passing -MMD (without -MD or -MF) would result in no dependency file being produced, because nocc strips dep flags from the remote compile and relies on the client-side generator.

Changes:

• Treat -MMD as a depfile-emitting flag in ShouldGenerateDepFile() (same as -MD).
• Add test coverage for -MMD-only and -MMD -MP command lines, comparing nocc output to real g++ output.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/client/dep-cmd-flags.go	Fixes the depfile-generation predicate to include -MMD.
tests/depfiles_test.go	Adds regression tests ensuring depfiles are emitted for -MMD without -MD / -MF.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.