1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161

// 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
							.into_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 _: () = {
			#[automatically_derived]
			impl #impl_generics ::core::default::Default for #name #ty_generics #where_clause {
				fn default() -> Self {
					#impl_
				}
			}
		};
	)
	.into()
}