[Nix-dev] Incremental recompilation

[stewart mackenzie]

Isn't the simplest solution this:

declare an output

`outputs = ["out" "cache"];`

now the word cache is hardcoded to create a directory
/tmp/${derivation-name}-cache/ this name doesn't change.
Now, in your derivation you can set the build tool (via env var if it
supports that) to use this cache folder as a place to store the build
artifacts.
You may then copy the final build product from `$cache` to `$out` at
some in the `installPhase`.

The last thing about `cache` is that it is _not_ deleted after the
derivation builds.

kr
/sjm