--emit dep-info is a little difficult to use

[alexcrichton]

The compiler's dep-info output doesn't work quite as one might expect when
coming from the land of gcc/clang, for example:

1. If only a dep-info file is generated, the output filename is pretty
   unlikely to what's going to be compiled later on:

   $ rustc foo.rs --emit dep-info
   $ ls
   foo.d  foo.rs
   

2. If a dep-info and link file are generated, you can't customize the output
   filename of the dep-info file:

   $ rustc foo.rs --emit dep-info,link -o foo.pp
   warning: ignoring specified output filename because multiple outputs were requested
   $ ls
   foo.d       foo.rs      libfoo.rlib
   

The compiler probably wants the ability to specify a dedicated location for the dep-info file during the same compilation of a staticlib or executable. For example, something like:

    --dep-file FILE     Configure output location of the dep-info output

This would allow customizing the output location as well as enabling dep-info to know what the actual output artifact is (e.g. of the link output type).

Some more info can be found on this bug.

[apasel422]

Session's write_dependency_info field actually has an Option<PathBuf> that represents a specified output filename, but it doesn't appear to be set anywhere.

[apasel422]

The comments in librustc_driver/driver.rs refer to some obsolete flags:

// Write out dependency rules to the dep-info file if requested with
// --dep-info
let deps_filename = match sess.opts.write_dependency_info {
    // Use filename from --dep-file argument if given

[alexcrichton]

Yeah these flags have gone through some various iterations over time, and it's probably high time to refactor all this support in the compiler!