Option to skip cleaning of some specified src crates in registry/src

[lrubasze]

In theory cache should be enough to recreate sources and reduce the build time.

However, there are some crates, eg. librocksdb-sys that do not benefit from that. Restoring sources from cache is not enough.
(more details ci: enable 'librocksdb-sys' to be able to be properly cached)

How about addressing this problem by checking if some path from cache-directories belongs to registry/src and prevent this path from removal.
Or maybe some additional config param with list of src entries to keep?

[Oppen]

Caching all of target can sometimes lead to errors as well, for example when you run test coverage with the likes of llvm-cov. Granted, you could clean the output before the post steps run, but it would be cool to make it easier to express this has cache significance other than with comments.

[lrubasze]

@Oppen thanks for reply.
Correct me if I am wrong but I suppose the issue you described refers to caching of target in general, not necessarily in relation to the issue I have described. Unless I am not entirely getting your point.

[Oppen]

Oh, you're right. I misinterpreted the current issue as being related to which subfolders were cached.

[Swatinem]

Maybe just a half-solution, but I excluded -sys crates from the .cargo/registry/src cleanup in f6987ea