Is your feature request related to a problem? Please describe.

The dumping large paths message has been slightly helpful by providing a hint that something big is getting added to the store each time.
This message is disappearing with #4030 (unless you use an older daemon), so you'll be filling up your store with slight variations of large paths if you aren't careful. Virtually nobody checks the size of their derivation closures.

Describe the solution you'd like

Provide a warning when adding a large local path to the store, when it is

• not the output of an evaluation time fetcher with a content hash, because these tend to be unavoidable and tend to change less often
• larger than a threshold that defaults to, say, 250 MB
• is not a single file (which would seem rather intentional)

Example warning

You're copying to the Nix store from path `/home/user/workspace` which seems to be rather large (> 100MB). In most cases this happens by accident because the path wasn't filtered correctly. Alternatively, you may want to split your build into smaller derivations if possible, or set `warn-large-path-threshold = 250 #MB` to a larger value.

Describe alternatives you've considered

Nix flakes do alleviate the problem by only including (potentially unstaged!) files that are listed in git's staging area.
However, user can still reference paths that aren't in a repo.

Additional context