Add an install.rs that lets a file perform installation setup, as well as a permanent installation outdir

[Manishearth]

Currently, cargo install builds a binary using the regular build script and copies it to .cargo/bin.

In this process we lose the outdir. Which makes sense, since usually that's only needed for compilation.

However, some binaries may need to do extra work on installation, as well as store some data in an output dir. It would be nice if cargo install would run a specified install.rs which has access to an $INSTALL_OUT_DIR variable which is a directory under .cargo/bin which is deleted if empty after installation.

This, for example, makes it possible to run cargo install rust-clippy (clippy needs a place to stash the compiled plugin).

[mbrubeck]

See also #545.

[Luthaf]

I think this could be a solution for both #545 and #2729.

Do we need a full RFC for this? If so, should the rfc go to the main rust-lang/rfcs repository?

[joshtriplett]

Please see https://github.com/rust-lang/rfcs/2376 for an RFC proposal on this.

[zslayton]

Please see https://github.com/rust-lang/rfcs/2376 for an RFC proposal on this.

Updated link: rust-lang/rfcs#2376

[epage]

As this is a specific solution to the problem in #2729, I'm closing in favor of #2729. If I missed something about that and we should keep this open, feel free to speak up!