zig cc -MD fails when an explicit filename is given

[marler8997]

Steps to Reproduce

$ echo "int main() { }" > example.c
$ zig cc -Wp,-MD,deps.d example.c
example.c:1:1: error: unable to build C object: FileNotFound

Error goes away if no deps filename is given (i.e. -Wp,-MD without the deps.d).

Tracked the error to this line:

zig/src/Cache.zig

Line 579 in ed5a5e2

const dep_file_contents = try dir.readFileAlloc(self.cache.gpa, dep_file_basename, manifest_file_size_max);

zig tries to open the deps file from the cache but it's not there because it was redirected to another filename on the command-line.

[marler8997]

For more context have a look at this clang source code: https://github.com/llvm/llvm-project/blob/ad48ef1e3142aafcf23ab9b8037e96c9208c2074/clang/lib/Driver/Driver.cpp#L359

    // Rewrite preprocessor options, to replace -Wp,-MD,FOO which is used by
    // some build systems. We don't try to be complete here because we don't
    // care to encourage this usage model.

Here, clang rewrites -Wp,-MD[D],FOO to -Wp,-MD[D],-MF,FOO. Here, -Wp means "pass the following comma-separated values to the proprocessor". -MD enables writing a "depfile" and -MF specifies the depfile filename. However, some build systems (like the linux kernel) depend on omitting -MF and just specifying -Wp,-MD,FILENAME. LLVM decided not to support this shorthand in general (probably because it is ambiguous), but they did add special handling for -Wp,-MD,FIENAME so that clang could be used for a few projects that depend on this (see link above). Note that the other method of passing preprocessor arguments, which would be -Xpreprocessor -MD -Xpreprocessor FILENAME would not work, the workaround is purposely limited to -Wp,-MD,FILENAME.

So, if we want Zig to interpret the command-line arguments the same way as Clang, then it will also need to have special handling for -Wp,-MD[D],FILENAME.

[rsepassi]

busybox also uses this form.

[rsepassi]

Also I think the rewrite is: -Wp,-M[M]D,FOO -> -M[M]D -MF FOO

[rsepassi]

But it seems that zig cc swallows -o if -MD or -MMD is specified...