Fix issue when a foreign `Err` pollutes scope (#758)

derive/src/derive_enum.rs

@@ -162,7 +162,7 @@ impl DeriveEnum {
         //   if we have fixed value variants
         result.ident_str("variant");
         result.puncts("=>");
-        result.ident_str("Err");
+        result.push_parsed("core::result::Result::Err")?;
         result.group(Delimiter::Parenthesis, |err_inner| {
             err_inner.push_parsed(format!(
                 "{}::error::DecodeError::UnexpectedVariant",

tests/derive.rs

@@ -408,6 +408,10 @@ mod derive_with_polluted_scope {
     #[allow(non_snake_case)]
     fn Ok() {}
 
+    #[allow(dead_code)]
+    #[allow(non_snake_case)]
+    fn Err() {}
+
     #[derive(bincode::Encode, bincode::Decode)]
     struct A {
         a: u32,