derive: Replicate proc-macro-error's "dummy implementation

oauth1-request-derive/src/lib.rs

@@ -88,10 +88,6 @@ fn expand_derive_oauth1_authorize(input: DeriveInput) -> TokenStream {
 
  let meta = ContainerMeta::new(input.attrs, &mut cx);
 
- if let Some(tokens) = cx.emit_errors() {
- return tokens;
- }
-
  let use_oauth1_request = if let Some(krate) = meta.krate {
  quote! {
  use #krate as _oauth1_request;
@@ -117,11 +113,31 @@ fn expand_derive_oauth1_authorize(input: DeriveInput) -> TokenStream {
  }
  };
 
- let body = MethodBody::new(&fields);
-
  add_trait_bounds(&mut generics);
  let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
 
+ if let Some(mut tokens) = cx.emit_errors() {
+ tokens.extend(quote! {
+ const _: () = {
+ #use_oauth1_request
+
+ impl #impl_generics _oauth1_request::Request for #name #ty_generics
+ #where_clause
+ {
+ fn serialize<S>(&self, serializer: S) -> S::Output
+ where
+ S: _oauth1_request::serializer::Serializer,
+ {
+ unimplemented!();
+ }
+ }
+ };
+ });
+ return tokens;
+ }
+
+ let body = MethodBody::new(&fields);
+
  quote_spanned! {Span::mixed_site()=>
  const _: () = {
  #use_oauth1_request

oauth1-request-derive/tests/ui/fallback_impl.rs

@@ -0,0 +1,9 @@
+#[derive(oauth1_request::Request)]
+struct Dummy {
+ #[oauth1(unknown)]
+ _field: (),
+}
+
+impl Dummy where Dummy: oauth1_request::Request {} // OK
+
+fn main() {}

oauth1-request-derive/tests/ui/fallback_impl.stderr

@@ -0,0 +1,5 @@
+error: unknown attribute `unknown`
+ --> tests/ui/fallback_impl.rs:3:14
+ |
+3 | #[oauth1(unknown)]
+ | ^^^^^^^