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
162
extern crate proc_macro;
extern crate syn;
#[macro_use]
extern crate quote;
use proc_macro::TokenStream;
#[doc(hidden)]
#[proc_macro_derive(SmartDefault, attributes(default))]
pub fn derive_smart_default(input: TokenStream) -> TokenStream {
let ast = syn::parse_derive_input(&input.to_string()).unwrap();
impl_my_derive(&ast).parse().unwrap()
}
fn impl_my_derive(ast: &syn::DeriveInput) -> quote::Tokens {
let name = &ast.ident;
match ast.body {
syn::Body::Struct(ref body) => {
let body_assignment = default_body_tt(body);
quote! {
impl Default for #name {
fn default() -> Self {
#name #body_assignment
}
}
}
}
syn::Body::Enum(ref variants) => {
let default_variant = find_only(variants, |variant| {
if let Some(default_attr) =
find_only(&variant.attrs, |attr| attr.name() == "default") {
if let syn::MetaItem::Word(_) = default_attr.value {
true
} else {
panic!("Attribute #[default] on variants should have no value");
}
} else {
false
}
});
let default_variant = default_variant.expect("No default variant");
let default_variant_name = &default_variant.ident;
let body_assignment = default_body_tt(&default_variant.data);
quote! {
impl Default for #name {
fn default() -> Self {
#name :: #default_variant_name #body_assignment
}
}
}
}
}
}
fn default_body_tt(body: &syn::VariantData) -> quote::Tokens {
match body {
&syn::VariantData::Struct(ref fields) => {
let field_assignments = fields.iter().map(|field| {
let field_name = field.ident.as_ref();
let default = field_default_expr(field);
quote! { #field_name : #default }
});
quote!{
{
#( #field_assignments ),*
}
}
}
&syn::VariantData::Tuple(ref fields) => {
let field_assignments = fields.iter().map(|field| field_default_expr(field));
quote! {
(
#( #field_assignments ),*
)
}
}
&syn::VariantData::Unit => quote!{},
}
}
fn field_default_expr(field: &syn::Field) -> quote::Tokens {
if let Some(default_attr) = find_only(&field.attrs, |attr| attr.name() == "default") {
if let syn::MetaItem::NameValue(_, syn::Lit::Str(ref lit, _)) = default_attr.value {
let field_value = syn::parse_token_trees(lit).unwrap();
return quote! {
#( #field_value )*
};
} else {
panic!("Attribute #[default] on fields must have a value");
}
}
quote! {
Default::default()
}
}
fn find_only<T, F>(iter: &[T], pred: F) -> Option<&T>
where
F: Fn(&T) -> bool,
{
let mut result = None;
for item in iter {
if pred(item) {
assert!(result.is_none(), "Multiple defaults");
result = Some(item);
}
}
result
}