darling_core 0.3.1

Helper crate for proc-macro library for reading attributes into structs when implementing custom derives. Use https://crates.io/crates/darling in your code.
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

use quote::{Tokens, ToTokens};
use syn::{Ident, Path};

/// This will be in scope during struct initialization after option parsing.
const DEFAULT_STRUCT_NAME: &str = "__default";

/// The fallback value for a field or container.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum DefaultExpression<'a> {
    /// Only valid on fields, `Inherit` indicates that the value should be taken from a pre-constructed
    /// fallback object. The value in the variant is the ident of the field.
    Inherit(&'a Ident),
    Explicit(&'a Path),
    Trait,
}

impl<'a> DefaultExpression<'a> {
    pub fn as_declaration(&'a self) -> DefaultDeclaration<'a> {
        DefaultDeclaration(self)
    }
}

impl<'a> ToTokens for DefaultExpression<'a> {
    fn to_tokens(&self, tokens: &mut Tokens) {
        tokens.append_all(match *self {
            DefaultExpression::Inherit(ident) => {
                let dsn = Ident::new(DEFAULT_STRUCT_NAME, ::proc_macro2::Span::call_site());
                quote!(#dsn.#ident)
            },
            DefaultExpression::Explicit(path) => quote!(#path()),
            DefaultExpression::Trait => quote!(::darling::export::Default::default()),
        });
    }
}

/// Used only by containers, this wrapper type generates code to declare the fallback instance.
pub struct DefaultDeclaration<'a>(&'a DefaultExpression<'a>);

impl<'a> ToTokens for DefaultDeclaration<'a> {
    fn to_tokens(&self, tokens: &mut Tokens) {
        let name = Ident::new(DEFAULT_STRUCT_NAME, ::proc_macro2::Span::call_site());
        let expr = self.0;
        tokens.append_all(quote!(let #name: Self = #expr;));
    }
}