Derive 宏如何生成代码

  derive 宏的实现(derive_fields/src/lib.rs):
    use proc_macro::TokenStream;
    use quote::quote;
    use syn::{parse_macro_input, Data, DeriveInput, Fields};
    
    #[proc_macro_derive(FieldNames)]
    pub fn field_names(input: TokenStream) -> TokenStream {
        let ast = parse_macro_input!(input as DeriveInput);
        let name = &ast.ident;
        let (impl_g, ty_g, where_c) = ast.generics.split_for_impl();
        let fields = match &ast.data {
            Data::Struct(s) => match &s.fields {
                Fields::Named(f) => f.named.iter().map(|f| f.ident.as_ref().unwrap().to_string()).collect::<Vec<_>>(),
                _ => return syn::Error::new_spanned(&ast.ident, "FieldNames 只支持有命名字段的 struct").to_compile_error().into(),
            },
            _ => return syn::Error::new_spanned(&ast.ident, "FieldNames 只能用于 struct").to_compile_error().into(),
        };
        let n = fields.len();
        quote! {
            impl #impl_g #name #ty_g #where_c {
                pub const FIELD_COUNT: usize = #n;
                pub fn field_names() -> [&'static str; #n] { [#(#fields),*] }
            }
        }.into()
    }
  cargo build(含下载编译 syn/quote/proc-macro2)用时 1.6 s,退出码 0
  运行 app:
    User: 3 个字段 ["id", "name", "active"]
    Wrapper<i32>: 2 个字段 ["inner", "tag"]

  impl User {
      pub const FIELD_COUNT: usize = 3usize;
      pub fn field_names() -> [&'static str; 3usize] {
          ["id", "name", "active"]
      }
  }
  impl<T: Clone> Wrapper<T> where T: Default {
      pub const FIELD_COUNT: usize = 2usize;
      pub fn field_names() -> [&'static str; 2usize] { ["inner", "tag"] }
  }

  error: FieldNames 只能用于 struct
   --> src/main.rs:3:28
    |
  3 | #[derive(FieldNames)] enum Color { Red, Green }
    |                            ^^^^^
  error: FieldNames 只支持有命名字段的 struct
   --> src/main.rs:4:30
    |
  4 | #[derive(FieldNames)] struct Pair(i32, i32);
    |                              ^^^^
  error: could not compile `app` (bin "app") due to 2 previous errors

  无 derive:2000 个 struct,cargo build(只重编 app)最快 0.28 s
  有 derive:2000 个 struct,cargo build(只重编 app)最快 0.53 s