Currently, following ptr_arg can lead to errors, if

• the function is used as a Fn reference (fn foo(&Vec<u8>) { .. }; let foo : Fn(&Vec<u8>) = foo)
• the function calls another function that expects a &Vec reference.

We could extend the lint to

• check the whole crate if the function is referenced anywhere (we obviously cannot check across crate boundaries, but I guess, that'll be OK.
• look into the function and check the signatures of the contained calls; if those are only functions within the crate, note if those are eligible for ptr_arg, otherwise don't lint. We should also bail on out-of-crate functions.