bon_macros/builder/builder_gen/
start_fn.rs

1use super::member::PosFnMember;
2use crate::util::prelude::*;
3
4impl super::BuilderGenCtx {
5    pub(super) fn start_fn(&self) -> syn::ItemFn {
6        let builder_ident = &self.builder_type.ident;
7        let docs = &self.start_fn.docs;
8        let vis = &self.start_fn.vis;
9
10        let start_fn_ident = &self.start_fn.ident;
11
12        // TODO: we can use a shorter syntax with anonymous lifetimes to make
13        // the generated code and function signature displayed by rust-analyzer
14        // a bit shorter and easier to read. However, the caveat is that we can
15        // do this only for lifetimes that have no bounds and if they don't appear
16        // in the where clause. Research `darling`'s lifetime tracking API and
17        // maybe implement this in the future
18
19        let generics = self.start_fn.generics.as_ref().unwrap_or(&self.generics);
20
21        let generics_decl = &generics.decl_without_defaults;
22        let where_clause = &generics.where_clause;
23        let generic_args = &self.generics.args;
24
25        let receiver = self.receiver();
26
27        let receiver_field_init = receiver.map(|receiver| {
28            let ident = &receiver.field_ident;
29            let self_token = &receiver.with_self_keyword.self_token;
30            quote! {
31                #ident: #self_token,
32            }
33        });
34
35        let receiver = receiver.map(|receiver| {
36            let mut receiver = receiver.with_self_keyword.clone();
37
38            if receiver.reference.is_none() {
39                receiver.mutability = None;
40            }
41
42            quote! { #receiver, }
43        });
44
45        let start_fn_params = self.start_fn_args().map(PosFnMember::fn_input_param);
46
47        // Assign `start_fn_args` to intermediate variables, which may be used
48        // by custom fields init expressions. This is needed only if there is
49        // a conversion configured for the `start_fn` members, otherwise these
50        // are already available in scope as function arguments directly.
51        let start_fn_vars = self.start_fn_args().filter_map(|member| {
52            let ident = &member.ident;
53            let ty = &member.ty.orig;
54            let conversion = member.conversion()?;
55
56            Some(quote! {
57                let #ident: #ty = #conversion;
58            })
59        });
60
61        let start_fn_args_fields_idents = self.start_fn_args().map(|member| &member.ident);
62
63        // Create custom fields in separate variables. This way custom fields
64        // declared lower in the struct definition can reference custom fields
65        // declared higher in their init expressions.
66        let custom_fields_vars = self.custom_fields().map(|field| {
67            let ident = &field.ident;
68            let ty = &field.norm_ty;
69            let init = field
70                .init
71                .as_ref()
72                .map(|init| quote! { (|| #init)() })
73                .unwrap_or_else(|| quote! { ::core::default::Default::default() });
74
75            quote! {
76                let #ident: #ty = #init;
77            }
78        });
79
80        let custom_fields_idents = self.custom_fields().map(|field| &field.ident);
81
82        let ide_hints = self.ide_hints();
83
84        // `Default` trait implementation is provided only for tuples up to 12
85        // elements in the standard library 😳:
86        // https://github.com/rust-lang/rust/blob/67bb749c2e1cf503fee64842963dd3e72a417a3f/library/core/src/tuple.rs#L213
87        let named_members_field_init = if self.named_members().take(13).count() <= 12 {
88            quote!(::core::default::Default::default())
89        } else {
90            let none = format_ident!("None");
91            let nones = self.named_members().map(|_| &none);
92            quote! {
93                (#(#nones,)*)
94            }
95        };
96
97        syn::parse_quote! {
98            #(#docs)*
99            #[inline(always)]
100            #[allow(
101                // This is intentional. We want the builder syntax to compile away
102                clippy::inline_always,
103                // We normalize `Self` references intentionally to simplify code generation
104                clippy::use_self,
105                // Let's keep it as non-const for now to avoid restricting ourselfves to only
106                // const operations.
107                clippy::missing_const_for_fn,
108            )]
109            #vis fn #start_fn_ident< #(#generics_decl),* >(
110                #receiver
111                #(#start_fn_params,)*
112            ) -> #builder_ident< #(#generic_args,)* >
113            #where_clause
114            {
115                #ide_hints
116                #( #start_fn_vars )*
117                #( #custom_fields_vars )*
118
119                #builder_ident {
120                    __unsafe_private_phantom: ::core::marker::PhantomData,
121                    #( #custom_fields_idents, )*
122                    #receiver_field_init
123                    #( #start_fn_args_fields_idents, )*
124                    __unsafe_private_named: #named_members_field_init,
125                }
126            }
127        }
128    }
129}
bon_macros/builder/builder_gen/start_fn.rs

bon_macros/builder/builder_gen/
start_fn.rs
