num-derive dependency worth it?

[HeroicKatora]

The crate currently uses num-derive to add some num-traits::FromPrimitive implementations to a couple of enumerations. Since this is done via a procmacro it requires sequencing in the compilation pipeline, compiling proc-macro2/syn prior. All cases are simple enumerations such as :

pub enum Type {
    BYTE = 1,
    ASCII = 2,
    SHORT = 3,
    LONG = 4,
    RATIONAL = 5,
}

The functionality could also be added with a standard macro in such simple cases, see this example in smoltcp which adds very similar converters for std::convert::{From, To}.

[birktj]

It is probably not necessary, for example we already use a macro to create the Tags enum. A new macro that adds From and To impls should be fine.

[Robzz]

Yeah it's totally droppable. image used to depend on it, but only because of the tiff code, and I removed it from image when extracting the code into this crate. It's probably worth doing.

[HeroicKatora]

One remaining question is one of style. Either go with the Unknown(_) variant from Tag or make the conversion return an Option like it does right now in the FromPrimitive trait.