debate-derive 1.0.0

derive macro for the `debate` CLI arguments parser
Documentation 
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use itertools::Itertools;
use proc_macro2::{Literal, TokenStream as TokenStream2};
use quote::quote;
use syn::{
    Data, DeriveInput, Ident, Lifetime, Token, Variant, punctuated::Punctuated, spanned::Spanned,
};

use crate::{
    common::value::AnalyzedEnum,
    generics::{AngleBracedLifetime, compute_generics},
};

fn derive_value_enum(
    ident: &Ident,
    variants: &Punctuated<Variant, Token![,]>,
    lifetime: &Lifetime,
    type_lifetime: Option<&AngleBracedLifetime>,
) -> syn::Result<TokenStream2> {
    let analyzed = AnalyzedEnum::from_variants(variants)?;

    // TODO: case insensitivity
    let unit_byte_arms = analyzed.variants.iter().map(|(name, variant)| {
        let scrutinee = Literal::byte_string(name.as_bytes());
        quote! {
            #scrutinee => ::core::result::Result::Ok(Self::#variant),
        }
    });

    let names = analyzed.variants.iter().map(|(name, _)| name.as_str());

    let byte_fallback_arm = match analyzed.fallback {
        Some(fallback) => {
            let variant = fallback.variant;

            let body = match fallback.field {
                Some(field) => quote! { { #field: value, } },
                None => quote! { ( value ) },
            };

            quote! {
                _ => match ::debate::parameter::Value::from_arg(argument) {
                    ::core::result::Result::Err(err) => ::core::result::Result::Err(err),
                    ::core::result::Result::Ok(value) => ::core::result::Result::Ok(
                        Self::#variant #body
                    ),
                },
            }
        }
        None => quote! {
            _ => ::core::result::Result::Err(
                ::debate::parameter::Error::should_be(
                    argument,
                    &[ #(#names,)* ]
                )
            ),
        },
    };

    Ok(quote! {
        impl<#lifetime> ::debate::parameter::Value<#lifetime> for #ident #type_lifetime {
            fn from_arg<E: ::debate::parameter::Error<#lifetime>>(
                argument: & #lifetime ::debate::Arg,
            ) -> Result<Self, E> {
                match argument.bytes() {
                    #(#unit_byte_arms)*
                    #byte_fallback_arm
                }
            }
        }
    })
}

fn derive_value_newtype(
    ident: &Ident,
    field: Option<&Ident>,
    lifetime: &Lifetime,
    type_lifetime: Option<&AngleBracedLifetime>,
) -> TokenStream2 {
    let struct_body = match field {
        Some(field) => quote! { { #field: value} },
        None => quote! { ( value ) },
    };

    quote! {
        impl<#lifetime> ::debate::parameter::Value<#lifetime> for #ident #type_lifetime {
            fn from_arg<E: ::debate::parameter::Error<#lifetime>>(
                argument: & #lifetime ::debate::Arg,
            ) -> Result<Self, E> {
                match ::debate::parameter::Value::from_arg(argument) {
                    ::core::result::Result::Ok(value) => ::core::result::Result::Ok(
                        Self #struct_body
                    ),
                    ::core::result::Result::Err(err) => ::core::result::Result::Err(err),
                }
            }
        }
    }
}

pub fn derive_value_result(item: TokenStream2) -> syn::Result<TokenStream2> {
    let input: DeriveInput = syn::parse2(item)?;
    let (lifetime, type_lifetime) = compute_generics(&input.generics)?;

    match input.data {
        Data::Struct(ref data) => {
            let field = data.fields.iter().exactly_one().map_err(|_| {
                syn::Error::new(
                    input.span(),
                    "can only derive `Value` on structs with exactly one field",
                )
            })?;

            Ok(derive_value_newtype(
                &input.ident,
                field.ident.as_ref(),
                &lifetime,
                type_lifetime.as_ref(),
            ))
        }
        Data::Enum(ref data) => derive_value_enum(
            &input.ident,
            &data.variants,
            &lifetime,
            type_lifetime.as_ref(),
        ),
        Data::Union(_) => Err(syn::Error::new(
            input.span(),
            "can't derive `Value` on a union",
        )),
    }
}