Strip absolute path to compiled rust package to share crates cache when possible.

[versusvoid]

Fix #196

[mmun]

I ran into this as well.

[luser]

Hey, sorry for letting this sit for so long, I would really like to fix this! I took a look at your patch, and I think we could do things in a slightly simpler way (just remove -L, --extern, --out-dir args from the hash calculation entirely). However, from local testing unfortunately the build directory winds up in the generated rlib in a number of places, so I don't think we can actually do this without figuring something out there. ☹️ There is a rustc issue that tracks adding a compiler option to remap paths, but it hasn't stabilized yet. It's available in nightly Rust as -Zremap-path-prefix-from=... -Zremap-path-prefix-to=..., but even with that I don't seem to get bit-identical rlibs out for dependent crates when building in different directories.

[mitchmindtree]

@luser just thought I'd mention that it looks like the blocking issue you linked has since been stabilised in rustc v1.26.

@freesig and I are currently trying to reduce some massive build times nannou-org/nannou#178 and it looks like this might be a nice win, at least until cargo gets some sort of built-in caching. That said, I haven't actually tested this PR to know if it contributes significantly - today's the first day I've taken a look at sccache and I'm still getting my head around what it is and how it works!

[luser]

I've got an alternate patch for this that seems to work reasonably well. I'm going to close this out and open a separate PR for that. Thanks for the patch, I'm sorry it didn't get merged!