--emit dep-info should support emitting "phony" dependencies for input files

[froydnj]

Running something like:

rustc --emit dep-info -o x.o lib.rs

will produce a depfile containing a rule like:

x.o: /path/to/lib.rs /path/to/a.rs /path/to/b.rs

This output has a problem: if a.rs is deleted from the filesystem and dependencies on it erased from lib.rs, then make will attempt to look for a rule to generate a.rs when we request that things be remade. Not finding one, it will helpfully complain that there is no rule to make a.rs and exit.

This can, of course, be worked around by clearing out your entire build directory and starting over, but this seems undesirable.

GCC has an option (-MP) to produce "phony" targets for all the dependencies of the object file; this prevents issues in cases like the above:

x.o: /path/to/lib.rs /path/to/a.rs /path/to/b.rs

/path/to/lib.rs:

/path/to/a.rs:

/path/to/b.rs:

since make now understands how to "generate" a.rs if it doesn't find it.

[alexcrichton]

cc #28716