Lint pub interfaces that contain non-std and/or non-core types

I propose kind of an information leak lint. Sometimes it is entirely unnecessary to e.g. return a value of a type that is only required within a crate. E.g., instead of returning a regex::Matches type a pub fn could return a newly defined type expressing the right amount of information e.g. in combination with impl Trait. The advantage would be that your crate is coupled less tightly to its dependencies, with all various advantageous implications of that.

The issue proposes a Clippy lint for public functions whose interfaces expose non-standard or non-core types, using regex::Matches and impl Trait as examples. No files, tests, or entry points are identified; first determine the intended lint scope and decide what cases count as an information leak, then add coverage showing that the intended public interfaces are reported.