const_default_derive/
lib.rs

1#![doc(html_root_url = "http://docs.rs/const-default-derive/0.2.0")]
2
3extern crate proc_macro;
4
5use proc_macro::TokenStream;
6use proc_macro2::{Ident, Literal, Span, TokenStream as TokenStream2};
7use proc_macro_crate::{crate_name, FoundCrate};
8use quote::{quote, quote_spanned};
9use syn::{spanned::Spanned, Error};
10
11/// Derives an implementation for the [`ConstDefault`] trait.
12///
13/// # Note
14///
15/// Currently only works with `struct` inputs.
16///
17/// # Example
18///
19/// ## Struct
20///
21/// ```
22/// # use const_default::ConstDefault;
23/// #[derive(ConstDefault)]
24/// # #[derive(Debug, PartialEq)]
25/// pub struct Color {
26///     r: u8,
27///     g: u8,
28///     b: u8,
29/// }
30///
31/// assert_eq!(
32///     <Color as ConstDefault>::DEFAULT,
33///     Color { r: 0, g: 0, b: 0 },
34/// )
35/// ```
36///
37/// ## Tuple Struct
38///
39/// ```
40/// # use const_default::ConstDefault;
41/// #[derive(ConstDefault)]
42/// # #[derive(Debug, PartialEq)]
43/// pub struct Vec3(f32, f32, f32);
44///
45/// assert_eq!(
46///     <Vec3 as ConstDefault>::DEFAULT,
47///     Vec3(0.0, 0.0, 0.0),
48/// )
49/// ```
50#[proc_macro_derive(ConstDefault, attributes(const_default))]
51pub fn derive(input: TokenStream) -> TokenStream {
52	match derive_default(input.into()) {
53		Ok(output) => output.into(),
54		Err(error) => error.to_compile_error().into(),
55	}
56}
57
58/// Implements the derive of `#[derive(ConstDefault)]` for struct types.
59fn derive_default(input: TokenStream2) -> Result<TokenStream2, syn::Error> {
60	let crate_ident = query_crate_ident()?;
61	let input = syn::parse2::<syn::DeriveInput>(input)?;
62	let ident = input.ident;
63	let data_struct = match input.data {
64		syn::Data::Struct(data_struct) => data_struct,
65		_ => {
66			return Err(Error::new(
67				Span::call_site(),
68				"ConstDefault derive only works on struct types",
69			))
70		}
71	};
72	let default_impl =
73		generate_default_impl_struct(&crate_ident, &data_struct)?;
74	let mut generics = input.generics;
75	generate_default_impl_where_bounds(
76		&crate_ident,
77		&data_struct,
78		&mut generics,
79	)?;
80	let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
81	Ok(quote! {
82		impl #impl_generics #crate_ident::ConstDefault for #ident #ty_generics #where_clause {
83			const DEFAULT: Self = #default_impl;
84		}
85	})
86}
87
88/// Queries the dependencies for the derive root crate name and returns the identifier.
89///
90/// # Note
91///
92/// This allows to use crate aliases in `Cargo.toml` files of dependencies.
93fn query_crate_ident() -> Result<TokenStream2, syn::Error> {
94	let query = crate_name("const-default").map_err(|error| {
95		Error::new(
96			Span::call_site(),
97			format!(
98				"could not find root crate for ConstDefault derive: {}",
99				error
100			),
101		)
102	})?;
103	match query {
104		FoundCrate::Itself => Ok(quote! { crate }),
105		FoundCrate::Name(name) => {
106			let ident = Ident::new(&name, Span::call_site());
107			Ok(quote! { ::#ident })
108		}
109	}
110}
111
112/// Generates the `ConstDefault` implementation for `struct` input types.
113///
114/// # Note
115///
116/// The generated code abuses the fact that in Rust struct types can always
117/// be represented with braces and either using identifiers for fields or
118/// raw number literals in case of tuple-structs.
119///
120/// For example `struct Foo(u32)` can be represented as `Foo { 0: 42 }`.
121fn generate_default_impl_struct(
122	crate_ident: &TokenStream2,
123	data_struct: &syn::DataStruct,
124) -> Result<TokenStream2, syn::Error> {
125	let fields_impl =
126		data_struct.fields.iter().enumerate().map(|(n, field)| {
127			let field_span = field.span();
128			let field_type = &field.ty;
129			let field_pos = Literal::usize_unsuffixed(n);
130			let field_ident = field
131				.ident
132				.as_ref()
133				.map(|ident| quote_spanned!(field_span=> #ident))
134				.unwrap_or_else(|| quote_spanned!(field_span=> #field_pos));
135			quote_spanned!(field_span=>
136				#field_ident: <#field_type as #crate_ident::ConstDefault>::DEFAULT
137			)
138		});
139	Ok(quote! {
140		Self {
141			#( #fields_impl ),*
142		}
143	})
144}
145
146/// Generates `ConstDefault` where bounds for all fields of the input.
147fn generate_default_impl_where_bounds(
148	crate_ident: &TokenStream2,
149	data_struct: &syn::DataStruct,
150	generics: &mut syn::Generics,
151) -> Result<(), syn::Error> {
152	let where_clause = generics.make_where_clause();
153	for field in &data_struct.fields {
154		let field_type = &field.ty;
155		where_clause.predicates.push(syn::parse_quote!(
156			#field_type: #crate_ident::ConstDefault
157		))
158	}
159	Ok(())
160}
const_default_derive/lib.rs

const_default_derive/
lib.rs
