[Nix-dev] How do you work on big packages?

[Volth]

For example, you are working on patch for kernel or chromium or
something huge like this.

src= in your xxxxx.nix points to a local working directory, so every
change would result in rebuilding of the deviation and its dependants.
Nice so far.

The problem is any change implies full rebuild, because .o files are
not preserved between the builds, and in case of big projects it takes
hours instead of seconds.

What could be done here?
An nix-build option to forcible preserve tmp directories
(--just-keep-and-reuse-tmp-i-know-what-i-am-doing) ?