Write unused crate detection tool

[brson]

There doesn't seem to be a way to automatically detect when a crate listed in the manifest is unused. Removing unused crates from the build could help compile times, and is just good housekeeping.

After some investigation it looks like the new -Z binary-dep-depinfo flag, which adds binary dependencies to the makefile rules it outputs as part of the build, can be parsed and compared to the manifest to accomplish this.

It might also be worth adding such logging at the info! level to rustc itself, using the existing bookkeeping for binary-dep-depinfo - this would probably be an easier approach.

[brson]

This has since been solved by est31 https://github.com/est31/cargo-udeps

[overvenus]

How about changing the title to "Remove unused crates" and label help wanted?