default_no_bound.rs

// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// 	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use proc_macro2::Span;
use quote::{quote, quote_spanned};
use syn::{spanned::Spanned, Data, DeriveInput, Fields};

/// Derive Default but do not bound any generic.
pub fn derive_default_no_bound(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let input = syn::parse_macro_input!(input as DeriveInput);

    let name = &input.ident;

    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();

    let impl_ = match input.data {
        Data::Struct(struct_) => match struct_.fields {
            Fields::Named(named) => {
                let fields = named.named.iter().map(|field| &field.ident).map(|ident| {
                    quote_spanned! {ident.span() =>
                        #ident: core::default::Default::default()
                    }
                });

                quote!(Self { #( #fields, )* })
            }
            Fields::Unnamed(unnamed) => {
                let fields = unnamed.unnamed.iter().map(|field| {
                    quote_spanned! {field.span()=>
                        core::default::Default::default()
                    }
                });

                quote!(Self( #( #fields, )* ))
            }
            Fields::Unit => {
                quote!(Self)
            }
        },
        Data::Enum(enum_) => {
            if enum_.variants.is_empty() {
                return syn::Error::new_spanned(name, "cannot derive Default for an empty enum")
                    .to_compile_error()
                    .into();
            }

            // all #[default] attrs with the variant they're on; i.e. a var
            let default_variants = enum_
                .variants
                .into_iter()
                .filter(|variant| {
                    variant
                        .attrs
                        .iter()
                        .any(|attr| attr.path().is_ident("default"))
                })
                .collect::<Vec<_>>();

            match &*default_variants {
                [] => {
                    return syn::Error::new(
						name.clone().span(),
						"no default declared, make a variant default by placing `#[default]` above it",
					)
					.into_compile_error()
					.into();
                }
                // only one variant with the #[default] attribute set
                [default_variant] => {
                    let variant_attrs = default_variant
                        .attrs
                        .iter()
                        .filter(|a| a.path().is_ident("default"))
                        .collect::<Vec<_>>();

                    // check that there is only one #[default] attribute on the variant
                    if let [first_attr, second_attr, additional_attrs @ ..] = &*variant_attrs {
                        let mut err =
                            syn::Error::new(Span::call_site(), "multiple `#[default]` attributes");

                        err.combine(syn::Error::new_spanned(
                            first_attr,
                            "`#[default]` used here",
                        ));

                        err.extend([second_attr].into_iter().chain(additional_attrs).map(
                            |variant| {
                                syn::Error::new_spanned(variant, "`#[default]` used again here")
                            },
                        ));

                        return err.into_compile_error().into();
                    }

                    let variant_ident = &default_variant.ident;

                    let fully_qualified_variant_path = quote!(Self::#variant_ident);

                    match &default_variant.fields {
                        Fields::Named(named) => {
                            let fields =
                                named.named.iter().map(|field| &field.ident).map(|ident| {
                                    quote_spanned! {ident.span()=>
                                        #ident: core::default::Default::default()
                                    }
                                });

                            quote!(#fully_qualified_variant_path { #( #fields, )* })
                        }
                        Fields::Unnamed(unnamed) => {
                            let fields = unnamed.unnamed.iter().map(|field| {
                                quote_spanned! {field.span()=>
                                    core::default::Default::default()
                                }
                            });

                            quote!(#fully_qualified_variant_path( #( #fields, )* ))
                        }
                        Fields::Unit => fully_qualified_variant_path,
                    }
                }
                [first, additional @ ..] => {
                    let mut err = syn::Error::new(Span::call_site(), "multiple declared defaults");

                    err.combine(syn::Error::new_spanned(first, "first default"));

                    err.extend(
                        additional
                            .iter()
                            .map(|variant| syn::Error::new_spanned(variant, "additional default")),
                    );

                    return err.into_compile_error().into();
                }
            }
        }
        Data::Union(union_) => {
            return syn::Error::new_spanned(
                union_.union_token,
                "Union type not supported by `derive(DefaultNoBound)`",
            )
            .to_compile_error()
            .into()
        }
    };

    quote!(
        const _: () = {
            impl #impl_generics core::default::Default for #name #ty_generics #where_clause {
                fn default() -> Self {
                    #impl_
                }
            }
        };
    )
    .into()
}