add some style guide to Contributing.md

[davidhewitt]

Born out of the discussion in #3270 (comment)

[davidhewitt]

Updated and added a commit which applies these two rules to a number of our FFI calls.

[adamreichold]

Wow, this is a lot of generic code to support impl ToPyObject arguments. Certainly not for this PR, but I do wonder whether &dyn ToPyObject might even be a good trade-off for these kinds of API?

[davidhewitt]

Pushed a few commits to apply the suggestions, and also move unsafe { } blocks inside of error_on_minusone calls as reducing scope of the unsafe seemed like a good idea.

Wow, this is a lot of generic code to support impl ToPyObject arguments. Certainly not for this PR, but I do wonder whether &dyn ToPyObject might even be a good trade-off for these kinds of API?

Personally I think this is probably the right level, if users want to have only a single monomorphization I think they can select the &dyn ToPyObject instantiation directly?

[adamreichold]

Personally I think this is probably the right level, if users want to have only a single monomorphization I think they can select the &dyn ToPyObject instantiation directly?

Agreed, disciplined users can opt into this by always calling with &dyn ToPyObject.