Function merge_patch 

pub fn merge_patch<C: Conventions>(input: TokenStream) -> TokenStream

#[derive(MergePatch)]: the functions merge patching a model with its merge patch model counterpart.

Every field not marked #[merge_patch(skip)] must exist on the merge patch model as an Option<P> where P: Into<FieldType>; Some replaces the field, None leaves it untouched.

Attributes: merge_patch (patch = path on the struct, skip on a field).