Cackle is a tool to analyse the transitive dependencies of your crate to see what kinds of APIs each crate uses.
The idea is look for crates that are using APIs that you don't think they should be using. For example a crate that from its description should just be doing some data processing, but is actually using network APIs.
Ah, gotcha. Cackle checks the APIs used by build scripts before it lets them run, so that might help