More attribute refactoring

[jplatte]

This has two effects that may not be obvious from the commit messages / on first sight:

• strum(default = "true") and strum(disabled = "true") no longer produce a dedicated "deprecation" compiler error
• the strum_discriminants "passthrough" of other attributes now supports attributes that don't parse as syn::Meta, e.g. foo(key = some::Path), since syn only supports literals in the value position of MetaLists but the compiler allows arbitrary token streams in attributes

I'll probably work on converting all of the panics to syn::Errors next :)

[Peternator7]

Looks really great! Thanks for putting all this work in. Couple of minor points, and the TryFrom back-compat issue, but otherwise not much.

[Peternator7]

Could you help me understand the difference here?

[jplatte]

#() interpolations can contain arbitrary extra tokens in addition to an iterable of impl ToTokens. So instead of

let values = vec![1, 2, 3];
let push_values = values.into_iter().map(|v| quote! { vec.push(#v); });
quote! {
    let vec = Vec::new();
    #(#push_values)
}

you can just write

let values = vec![1, 2, 3];
quote! {
    let vec = Vec::new();
    #( vec.push(#values); )
}

I did the same kind of simplification here (context).

[Peternator7]

I don't think Self alias for enums was stablized until 1.37. Otherwise, still seems good.

[jplatte]

Sorry, not getting notifications about Travis CI failure. Should be fixed now :)

[Peternator7]

Everything looks good. Thanks for all the changes! Do you have more coming?

[jplatte]

Yeah, still planning to replace panic!s by syn::Error and implement most or all of the features I have recently requested. Can't make any promises as for when that'll happen, but I hope to have another PR up in the coming days / weeks.