1use proc_macro::TokenStream;
 2use quote::quote;
 3use syn::{Data, DeriveInput, Fields, parse_macro_input};
 4
 5/// Derives the `MergeFrom` trait for a struct.
 6///
 7/// This macro automatically implements `MergeFrom` by calling `merge_from`
 8/// on all fields in the struct.
 9///
10/// # Example
11///
12/// ```ignore
13/// #[derive(Clone, MergeFrom)]
14/// struct MySettings {
15///     field1: Option<String>,
16///     field2: SomeOtherSettings,
17/// }
18/// ```
19#[proc_macro_derive(MergeFrom)]
20pub fn derive_merge_from(input: TokenStream) -> TokenStream {
21    let input = parse_macro_input!(input as DeriveInput);
22
23    let name = &input.ident;
24    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
25
26    let merge_body = match &input.data {
27        Data::Struct(data_struct) => match &data_struct.fields {
28            Fields::Named(fields) => {
29                let field_merges = fields.named.iter().map(|field| {
30                    let field_name = &field.ident;
31                    quote! {
32                        self.#field_name.merge_from(&other.#field_name);
33                    }
34                });
35
36                quote! {
37                    #(#field_merges)*
38                }
39            }
40            Fields::Unnamed(fields) => {
41                let field_merges = fields.unnamed.iter().enumerate().map(|(i, _)| {
42                    let field_index = syn::Index::from(i);
43                    quote! {
44                        self.#field_index.merge_from(&other.#field_index);
45                    }
46                });
47
48                quote! {
49                    #(#field_merges)*
50                }
51            }
52            Fields::Unit => {
53                quote! {
54                    // No fields to merge for unit structs
55                }
56            }
57        },
58        Data::Enum(_) => {
59            quote! {
60                *self = other.clone();
61            }
62        }
63        Data::Union(_) => {
64            panic!("MergeFrom cannot be derived for unions");
65        }
66    };
67
68    let expanded = quote! {
69        impl #impl_generics crate::merge_from::MergeFrom for #name #ty_generics #where_clause {
70            fn merge_from(&mut self, other: &Self) {
71                use crate::merge_from::MergeFrom as _;
72                #merge_body
73            }
74        }
75    };
76
77    TokenStream::from(expanded)
78}