[derive] use known crate name aliases

[ascjones]

Brought up in this comment by @Robbepop #53 (comment)

When I read originally, I overlooked the fact that ::scale is of course referring to parity-scale-codec, so this means that our derive only works now that crate is imported and renamed to scale.

This PR introduces @bkchr's https://github.com/bkchr/proc-macro-crate to use a common known alias for both scale_info and parity-scale-codec dependencies.

[Robbepop]

I'd personally prefer a solution where we'd still keep having absolute paths everything.
This means that include_crate needed to return a TokenStream in the form of quote! { codec } or quote { scale } or whatever is the actual crate alias for it and users would then use it as:

let parity_scale_codec_alias = include_crate("parity-scale-codec");
quote! { :: #parity_scale_codec_alias ::rest::of::my::path }

This solution would also not need yet another alias, e.g. _scale_info.
The extern crate solution was required in ancient times of Rust edition 2015.

[bkchr]

Why not directly accessing the crate with its ident, instead of renaming it?

[Robbepop]

I agree: #61 (comment)

[bkchr]

This is in general a bad style, to require some other crate than the macro providing crate in the Cargo.toml.

Why aren't you re-exporting the required stuff from scale-info?

[Robbepop]

I am not entirely sold on this.
While I agree that your proposal solves certain problems the scale-info crate works with multiple parity-scale-codec versions. So re-exporting one of the supported versions would case a lock-in and potentially a duplication of dependencies in case the scale-info user does not also use the same version.

The reality is that scale-info and parity-scale-codec are somehow tightly coupled by design (one does encoding, the other does encoding of the encoding) and could actually even live under the same crate.

[ascjones]

Indeed that is a much better idea

[Robbepop]

LGTM

[dvdplm]

Having to thread the crate names all over is a bit unfortunate. Do you think something like this would work?

use once_cell::sync::Lazy;

struct IdentHelper(Lazy<String>);

impl quote::ToTokens for IdentHelper {
    fn to_tokens(&self, tokens: &mut TokenStream2) {
        Ident::new(&self.0, Span::call_site()).to_tokens(tokens)
    }
}
/// Get the name of the scale-info crate, to be robust against renamed dependencies.
static SCALE_INFO: IdentHelper = IdentHelper(Lazy::new(|| {
    proc_macro_crate::crate_name("scale-info")
        .expect("Missing scale-info dependency. Please add it to your Cargo.toml")
}));
/// Get the name of the parity-scale-codec crate, to be robust against renamed dependencies.
static CODEC: IdentHelper = IdentHelper(Lazy::new(|| {
    proc_macro_crate::crate_name("parity-scale-codec")
        .expect("Missing parity-scale-codec dependency. Please add it to your Cargo.toml")
}));

…and then use it e.g. like so:

        quote! {
            :: #SCALE_INFO ::meta_type::<#ty_ident>()
        }

It seems to work fine, all tests pass etc, but maybe it will mess up the span?