Generate --dep-info earlier in the compillation

[SiegeLord]

The --dep-info command line option allows a nice way to generate make-style dependencies, but it currently does so alongside building of the output binary. This isn't a problem for make, as it mixes dependency graph generation and actual building, but it is problematic for other tools (e.g. CMake) which keep them separate.

To play more nicely with those tools, I've moved the --dep-info output from phase 6 (linking) up to after phase 2 (expansion of macros). Also, since there was no prior option to do so, I added a command line switch (--no-analysis) to stop compilation just before phase 3 (type-checking) which speeds this up even further.

Here's the beginning of a CMake function which is enabled by this change:

function(get_rust_deps root_file out_var)
    execute_process(COMMAND rustc ${RUSTC_FLAGS} --no-analysis --dep-info "${CMAKE_BINARY_DIR}/.deps" "${root_file}")

    # Read and parse the dependency information
    file(READ "${CMAKE_BINARY_DIR}/.deps" crate_deps)
    file(REMOVE "${CMAKE_BINARY_DIR}/.deps")
    # parsing follows...

[emberian]

Ohh this is a nice PR. <3 me some cmake.

[metajack]

Just a small note, CMake must do this fine because it uses the same method that GCC does right? Or does it pass GCC arguments that skip all the compilation phases too?

[SiegeLord]

I believe CMake uses a custom dependency scanner for C/C++.

[metajack]

Ah. Ninja uses -MMD I think, so I guess I assumed everyone did. The CMake stuff I wrote computed the crate hash itself since I did it before --crate-file-name existed.

[SiegeLord]

I played around with this change some more in the context of CMake and I managed to create a demonstration project here: https://github.com/SiegeLord/RustCMake. Another use of this change that I highlight there is to grab dependencies for rustdoc... although that could be an argument for adding --dep-info to rustdoc.