Feature request: add possibility to separate dependency fetching and building steps

[jauhien]

Please, add a possibility to separate these steps:

• dependency fetching
• dependency building
• project building

It is may be not necessary when you are building your project by hand during development, but it is really needed in some automatic builds.

An example is ongoing integration of rust related stuff in Gentoo, where fetching and building phases should be separated even for packages that use git live versions, see https://github.com/Heather/gentoo-rust/issues/25

[sfackler]

This'd also be nice to easily set up the dependency cache for offline development without having to go through a full build.

[SimonSapin]

Is this fixed by #570?

[alexcrichton]

Almost! I think this still needs to be able to separate building dependencies and building the main package itself. @jauhien, does that sound right?