Closes #28 #29 parse ItemStruct and ItemEnum

Using combination of `syn` parse definitons and use `quote` to assemble
it back into `proc_macro2::TokenStream`.

alloy-macros/src/cst.rs

@@ -0,0 +1,56 @@
+use proc_macro2::{Ident, TokenStream};
+use quote::quote;
+use syn::{ItemEnum, ItemStruct};
+
+// Strip the `CST` suffix from the given identifier if it exists, otherwise
+// add `AST` suffix.
+fn get_ast_ident(ident: &Ident) -> Ident {
+    let cst = ident.to_string();
+    let ast = if let Some(s) = cst.strip_suffix("CST") {
+        s.to_string()
+    } else {
+        format!("{cst}AST")
+    };
+    Ident::new(&ast, ident.span())
+}
+
+pub(super) fn struct_ast(s: ItemStruct) -> TokenStream {
+    let ItemStruct {
+        attrs,
+        vis,
+        struct_token,
+        ident,
+        generics,
+        fields: _,
+        semi_token,
+    } = s;
+    let ast_ident = get_ast_ident(&ident);
+    let fields = if semi_token.is_some() {
+        quote! {();}
+    } else {
+        quote! {{}}
+    };
+    quote! {
+        #(#attrs)*
+        #vis #struct_token #ast_ident #generics
+        #fields
+    }
+}
+
+pub(super) fn enum_ast(e: ItemEnum) -> TokenStream {
+    let ItemEnum {
+        attrs,
+        vis,
+        enum_token,
+        ident,
+        generics,
+        brace_token: _,
+        variants: _,
+    } = e;
+    let ast_ident = get_ast_ident(&ident);
+    quote! {
+        #(#attrs)*
+        #vis #enum_token #ast_ident #generics {
+        }
+    }
+}

alloy-macros/src/lib.rs

@@ -2,6 +2,7 @@ use proc_macro::TokenStream;
 use quote::quote;
 use syn::{parse_macro_input, spanned::Spanned, Expr, Item, LitStr};
 
+mod cst;
 mod expand;
 
 #[proc_macro]
@@ -32,8 +33,8 @@ pub fn assert_expr(input: TokenStream) -> TokenStream {
 pub fn cst_to_ast(input: TokenStream) -> TokenStream {
     let s = parse_macro_input!(input as Item);
     let tokens = match s {
-        Item::Enum(_e) => quote! {},
-        Item::Struct(_s) => quote! {},
+        Item::Enum(e) => cst::enum_ast(e),
+        Item::Struct(s) => cst::struct_ast(s),
         _ => panic!("only enums and structs can derive AST"),
     };
     tokens.into()