Record in a snapshot what was used to create its hash

I would like to know, for debugging custom snapshots, what information went into particular hash.

The situation goes like this:
1) I build project and make snapshot read-only for next builds of the same stuff.
2) I build another project off the same local snapshot
3) Stack tries to start from scratch and I would like to know why.

There is `stack ls snapshots` that gives hashes, but there's no way to find out what's "inside". Or, in my case, what's changed.