darling_core/codegen/
attr_extractor.rs

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1use proc_macro2::TokenStream;
2use quote::{quote, ToTokens};
3
4use crate::util::PathList;
5
6use super::ForwardAttrs;
7
8/// Infrastructure for generating an attribute extractor.
9pub trait ExtractAttribute {
10    /// A set of mutable declarations for all members of the implementing type.
11    fn local_declarations(&self) -> TokenStream;
12
13    /// Gets the list of attribute names that should be parsed by the extractor.
14    fn attr_names(&self) -> &PathList;
15
16    fn forward_attrs(&self) -> &ForwardAttrs<'_>;
17
18    /// Gets the name used by the generated impl to return to the `syn` item passed as input.
19    fn param_name(&self) -> TokenStream;
20
21    /// Get the tokens to access a borrowed list of attributes where extraction will take place.
22    ///
23    /// By default, this will be `&#input.attrs` where `#input` is `self.param_name()`.
24    fn attrs_accessor(&self) -> TokenStream {
25        let input = self.param_name();
26        quote!(&#input.attrs)
27    }
28
29    /// Gets the core from-meta-item loop that should be used on matching attributes.
30    fn core_loop(&self) -> TokenStream;
31
32    /// Generates the main extraction loop.
33    fn extractor(&self) -> TokenStream {
34        let mut declarations = self.local_declarations();
35        self.forward_attrs()
36            .as_declaration()
37            .to_tokens(&mut declarations);
38
39        let will_parse_any = !self.attr_names().is_empty();
40
41        // Forwarding requires both that there be some items we would forward,
42        // and a place that will keep the forwarded items.
43        let will_fwd_any = self.forward_attrs().will_forward_any();
44
45        if !(will_parse_any || will_fwd_any) {
46            return quote! {
47                #declarations
48            };
49        }
50
51        let attrs_accessor = self.attrs_accessor();
52
53        // The block for parsing attributes whose names have been claimed by the target
54        // struct. If no attributes were claimed, this is a pass-through.
55        let parse_handled = if will_parse_any {
56            let attr_names = self.attr_names().to_strings();
57            let core_loop = self.core_loop();
58            quote!(
59                #(#attr_names)|* => {
60                    match ::darling::util::parse_attribute_to_meta_list(__attr) {
61                        ::darling::export::Ok(__data) => {
62                            match ::darling::export::NestedMeta::parse_meta_list(__data.tokens) {
63                                ::darling::export::Ok(ref __items) => {
64                                    if __items.is_empty() {
65                                        continue;
66                                    }
67
68                                    #core_loop
69                                }
70                                ::darling::export::Err(__err) => {
71                                    __errors.push(__err.into());
72                                }
73                            }
74                        }
75                        // darling was asked to handle this attribute name, but the actual attribute
76                        // isn't one that darling can work with. This either indicates a typing error
77                        // or some misunderstanding of the meta attribute syntax; in either case, the
78                        // caller should get a useful error.
79                        ::darling::export::Err(__err) => {
80                            __errors.push(__err);
81                        }
82                    }
83                }
84            )
85        } else {
86            quote!()
87        };
88
89        let fwd_population = self.forward_attrs().as_value_populator();
90
91        // Specifies the behavior for unhandled attributes. They will either be silently ignored or
92        // forwarded to the inner struct for later analysis.
93        let forward_unhandled = self.forward_attrs().as_match_arms();
94
95        quote!(
96            #declarations
97            use ::darling::ToTokens;
98
99            for __attr in #attrs_accessor {
100                // Filter attributes based on name
101                match ::darling::export::ToString::to_string(&__attr.path().clone().into_token_stream()).as_str() {
102                    #parse_handled
103                    #forward_unhandled
104                }
105            }
106
107            #fwd_population
108        )
109    }
110}
darling_core/codegen/attr_extractor.rs

darling_core/codegen/
attr_extractor.rs
