Is your feature request related to a problem?

• outPath is an encapsulation leak
• revCount is slow, e.g. on the repl, and in flake compat nix-build fails in shallow clones on Nix 2.35 due to forcing revCount flake-compat#95
• planned isDirty will also be slow (actually the rest speeds up) fetchTree without the git status slowdown #16026
• other sourceInfo attributes are generally unnecessary too

Proposed solution

Do not // sourceInfo, and do not // { outPath; }, perhaps controlled by an opt-in or opt-out.

When the pollution is disabled, a flake can still selectively expose the things it cares to expose.

Alternative solutions

Additional context

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates
• reviewed the contributing guide

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