Quelle  parser.rs

// Copyright 2018 Guillaume Pinot (@TeXitoi) <texitoi@texitoi.eu>,
// Kevin Knapp (@kbknapp) <kbknapp@gmail.com>, and
// Ana Hobden (@hoverbear) <operator@hoverbear.org>
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//
// This work was derived from Structopt (https://github.com/TeXitoi/structopt)
// commit#ea76fa1b1b273e65e3b0b1046643715b49bec51f which is licensed under the
// MIT/Apache 2.0 license.

use proc_macro2::TokenStream;
use quote::quote;
use syn::Ident;
use syn::Variant;
use syn::{
    self, punctuated::Punctuated, token::Comma, Data, DataStruct, DeriveInput, Field, Fields,
    Generics,
};

use crate::derives::args::collect_args_fields;
use crate::derives::{args, into_app, subcommand};
use crate::item::Item;
use crate::item::Name;

pub(crate) fn derive_parser(input: &DeriveInput) -> Result<TokenStream, syn::Error> {
    let ident = &input.ident;
    let pkg_name = std::env::var("CARGO_PKG_NAME").ok().unwrap_or_default();

    match input.data {
        Data::Struct(DataStruct {
            fields: Fields::Named(ref fields),
            ..
        }) => {
            let name = Name::Assigned(quote!(#pkg_name));
            let item = Item::from_args_struct(input, name)?;
            let fields = collect_args_fields(&item, fields)?;
            gen_for_struct(&item, ident, &input.generics, &fields)
        }
        Data::Struct(DataStruct {
            fields: Fields::Unit,
            ..
        }) => {
            let name = Name::Assigned(quote!(#pkg_name));
            let item = Item::from_args_struct(input, name)?;
            let fields = Punctuated::<Field, Comma>::new();
            let fields = fields
                .iter()
                .map(|field| {
                    let item = Item::from_args_field(field, item.casing(), item.env_casing())?;
                    Ok((field, item))
                })
                .collect::<Result<Vec<_>, syn::Error>>()?;
            gen_for_struct(&item, ident, &input.generics, &fields)
        }
        Data::Enum(ref e) => {
            let name = Name::Assigned(quote!(#pkg_name));
            let item = Item::from_subcommand_enum(input, name)?;
            let variants = e
                .variants
                .iter()
                .map(|variant| {
                    let item =
                        Item::from_subcommand_variant(variant, item.casing(), item.env_casing())?;
                    Ok((variant, item))
                })
                .collect::<Result<Vec<_>, syn::Error>>()?;
            gen_for_enum(&item, ident, &input.generics, &variants)
        }
        _ => abort_call_site!("`#[derive(Parser)]` only supports non-tuple structs and enums"),
    }
}

fn gen_for_struct(
    item: &Item,
    item_name: &Ident,
    generics: &Generics,
    fields: &[(&Field, Item)],
) -> Result<TokenStream, syn::Error> {
    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();

    let into_app = into_app::gen_for_struct(item, item_name, generics)?;
    let args = args::gen_for_struct(item, item_name, generics, fields)?;

    Ok(quote! {
        #[automatically_derived]
        #[allow(
            unused_qualifications,
            clippy::redundant_locals,
        )]
        impl #impl_generics clap::Parser for #item_name #ty_generics #where_clause {}

        #into_app
        #args
    })
}

fn gen_for_enum(
    item: &Item,
    item_name: &Ident,
    generics: &Generics,
    variants: &[(&Variant, Item)],
) -> Result<TokenStream, syn::Error> {
    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();

    let into_app = into_app::gen_for_enum(item, item_name, generics)?;
    let subcommand = subcommand::gen_for_enum(item, item_name, generics, variants)?;

    Ok(quote! {
        #[automatically_derived]
        impl #impl_generics clap::Parser for #item_name #ty_generics #where_clause {}

        #into_app
        #subcommand
    })
}