sensible 0.1.0

Configurable derive macro for implementing Default
Documentation 
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

#![doc = include_str!(concat!(env!("OUT_DIR"), "/README-rustdocified.md"))]

extern crate proc_macro;

use darling::ast;
use darling::FromDeriveInput;
use darling::FromField;
use proc_macro::TokenStream as TokenStream1;
use proc_macro2::TokenStream;
use quote::quote;
use quote::ToTokens;
use syn::parse_macro_input;
use syn::DeriveInput;

/// Derive a [`std::default::Default`] implementation, using customized values
/// where provided.
///
/// ## Examples
///
/// Literal values may be used:
///
/// ```
/// use sensible::Default;
///
/// #[derive(Default)]
/// struct Foo {
///   a: u64,
///   #[default(42)]
///   b: u64,
/// }
///
/// let foo = Foo::default();
/// assert_eq!(foo.a, 0);
/// assert_eq!(foo.b, 42);
/// ```
///
/// Expressions may also be used where needed:
///
/// ```
/// use sensible::Default;
///
/// #[derive(Default)]
/// struct Foo {
///   a: u64,
///   #[default("bacon".into())]
///   b: String,
/// }
///
/// let foo = Foo::default();
/// assert_eq!(foo.a, 0);
/// assert_eq!(foo.b, "bacon");
/// ```
///
/// Defaults can be nested (regardless of how `Default` is implemented), and
/// `default` attributes persist:
///
/// ```
/// use sensible::Default;
///
/// #[derive(Default)]
/// struct Outer {
///   a: Inner,
///   #[default(Inner{ d: 76 })]
///   b: Inner,
///   #[default(12)]
///   c: u32,
/// }
///
/// #[derive(Default)]
/// struct Inner {
///   #[default(42)]
///   d: u32,
/// }
///
/// let outer = Outer::default();
/// assert_eq!(outer.a.d, 42);
/// assert_eq!(outer.b.d, 76);
/// assert_eq!(outer.c, 12);
/// ```
#[proc_macro_derive(Default, attributes(default))]
pub fn derive_default(in_: TokenStream1) -> TokenStream1 {
  Struct::from_derive_input(&parse_macro_input!(in_ as DeriveInput))
    .map(|receiver| quote!(#receiver))
    .unwrap_or_else(|err| err.write_errors())
    .into()
}

#[derive(FromDeriveInput)]
#[darling(supports(struct_named))]
struct Struct {
  ident: syn::Ident,
  generics: syn::Generics,
  data: ast::Data<(), Field>,
}

impl ToTokens for Struct {
  fn to_tokens(&self, tokens: &mut TokenStream) {
    let (impl_gcs, ty_gcs, where_cl) = self.generics.split_for_impl();
    let ident = &self.ident;
    let fields = &self
      .data
      .as_ref()
      .map_struct_fields(|f| quote! {#f})
      .take_struct()
      .unwrap()
      .fields;
    tokens.extend(quote! {
      impl #impl_gcs ::core::default::Default for #ident #ty_gcs #where_cl {
        /// Return a sensible default object.
        fn default() -> Self {
          Self {
            #(#fields)*
          }
        }
      }
    });
  }
}

#[derive(FromField)]
#[darling(forward_attrs(default))]
struct Field {
  ident: Option<syn::Ident>,
  attrs: Vec<syn::Attribute>,
}

impl ToTokens for Field {
  fn to_tokens(&self, tokens: &mut TokenStream) {
    let ident = &self.ident;
    if self.attrs.is_empty() {
      tokens.extend(quote! { #ident: ::core::default::Default::default(), });
    }
    else {
      let expr: syn::Expr =
        self.attrs[0].parse_args().expect("Could not parse default.");
      tokens.extend(quote! { #ident: #expr, });
    }
  }
}