darling_core/codegen/
default_expr.rs

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1use proc_macro2::{Span, TokenStream};
2use quote::{quote, quote_spanned, ToTokens, TokenStreamExt};
3use syn::{spanned::Spanned, Ident, Path};
4
5/// This will be in scope during struct initialization after option parsing.
6const DEFAULT_STRUCT_NAME: &str = "__default";
7
8/// The fallback value for a field or container.
9#[derive(Debug, Clone)]
10pub enum DefaultExpression<'a> {
11    /// Only valid on fields, `Inherit` indicates that the value should be taken from a pre-constructed
12    /// fallback object. The value in the variant is the ident of the field.
13    Inherit(&'a Ident),
14    Explicit(&'a Path),
15    Trait {
16        span: Span,
17    },
18}
19
20impl<'a> DefaultExpression<'a> {
21    pub fn as_declaration(&'a self) -> DefaultDeclaration<'a> {
22        DefaultDeclaration(self)
23    }
24}
25
26impl<'a> ToTokens for DefaultExpression<'a> {
27    fn to_tokens(&self, tokens: &mut TokenStream) {
28        tokens.append_all(match *self {
29            DefaultExpression::Inherit(ident) => {
30                let dsn = Ident::new(DEFAULT_STRUCT_NAME, ::proc_macro2::Span::call_site());
31                quote!(#dsn.#ident)
32            }
33            DefaultExpression::Explicit(path) => {
34                // Use quote_spanned to properly set the span of the parentheses
35                quote_spanned!(path.span()=>#path())
36            }
37            DefaultExpression::Trait { span } => {
38                quote_spanned!(span=> ::darling::export::Default::default())
39            }
40        });
41    }
42}
43
44/// Used only by containers, this wrapper type generates code to declare the fallback instance.
45pub struct DefaultDeclaration<'a>(&'a DefaultExpression<'a>);
46
47impl<'a> ToTokens for DefaultDeclaration<'a> {
48    fn to_tokens(&self, tokens: &mut TokenStream) {
49        let name = Ident::new(DEFAULT_STRUCT_NAME, ::proc_macro2::Span::call_site());
50        let expr = self.0;
51        tokens.append_all(quote!(let #name: Self = #expr;));
52    }
53}
darling_core/codegen/default_expr.rs

darling_core/codegen/
default_expr.rs
