Implement derive for unit struct errors

impl/src/expand.rs

@@ -1,7 +1,47 @@
 use proc_macro2::TokenStream;
-use syn::{DeriveInput, Result};
+use quote::quote;
+use syn::{Data, DataEnum, DataStruct, DeriveInput, Error, Result, FieldsNamed, FieldsUnnamed, Fields};
 
-pub fn derive(input: DeriveInput) -> Result<TokenStream> {
+pub fn derive(input: &DeriveInput) -> Result<TokenStream> {
+    match &input.data {
+        Data::Struct(data) => struct_error(input, data),
+        Data::Enum(data) => enum_error(input, data),
+        Data::Union(_) => Err(Error::new_spanned(
+            input,
+            "union as errors are not supported",
+        )),
+    }
+}
+
+fn struct_error(input: &DeriveInput, data: &DataStruct) -> Result<TokenStream> {
+    match &data.fields {
+        Fields::Named(fields) => braced_struct_error(input, fields),
+        Fields::Unnamed(fields) => tuple_struct_error(input, fields),
+        Fields::Unit => unit_struct_error(input),
+    }
+}
+
+fn braced_struct_error(input: &DeriveInput, fields: &FieldsNamed) -> Result<TokenStream> {
     let _ = input;
+    let _ = fields;
+    unimplemented!()
+}
+
+fn tuple_struct_error(input: &DeriveInput, fields: &FieldsUnnamed) -> Result<TokenStream> {
+    let _ = input;
+    let _ = fields;
+    unimplemented!()
+}
+
+fn unit_struct_error(input: &DeriveInput) -> Result<TokenStream> {
+    let ident = &input.ident;
+    Ok(quote! {
+        impl std::error::Error for #ident {}
+    })
+}
+
+fn enum_error(input: &DeriveInput, data: &DataEnum) -> Result<TokenStream> {
+    let _ = input;
+    let _ = data;
     unimplemented!()
 }

impl/src/lib.rs

@@ -8,7 +8,7 @@ use syn::{parse_macro_input, DeriveInput};
 #[proc_macro_derive(Error)]
 pub fn derive_error(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
-    expand::derive(input)
+    expand::derive(&input)
         .unwrap_or_else(|err| err.to_compile_error())
         .into()
 }