Reuse a single ContentRefDeserializer throughout untagged enum deserialization

Author: dtolnay

serde/src/private/de.rs

@@ -2193,6 +2193,14 @@ mod content {
         }
     }
 
+    impl<'a, 'de: 'a, E> Copy for ContentRefDeserializer<'a, 'de, E> {}
+
+    impl<'a, 'de: 'a, E> Clone for ContentRefDeserializer<'a, 'de, E> {
+        fn clone(&self) -> Self {
+            *self
+        }
+    }
+
     struct EnumRefDeserializer<'a, 'de: 'a, E>
     where
         E: de::Error,

serde_derive/src/de.rs

@@ -1172,13 +1172,7 @@ fn deserialize_enum(
         Some(variant_idx) => {
             let (tagged, untagged) = variants.split_at(variant_idx);
             let tagged_frag = Expr(deserialize_homogeneous_enum(params, tagged, cattrs));
-            let tagged_frag = |deserializer| {
-                Some(Expr(quote_block! {
-                    let __deserializer = #deserializer;
-                    #tagged_frag
-                }))
-            };
-            deserialize_untagged_enum_after(params, untagged, cattrs, tagged_frag)
+            deserialize_untagged_enum_after(params, untagged, cattrs, Some(tagged_frag))
         }
         None => deserialize_homogeneous_enum(params, variants, cattrs),
     }
@@ -1689,17 +1683,16 @@ fn deserialize_untagged_enum(
     variants: &[Variant],
     cattrs: &attr::Container,
 ) -> Fragment {
-    deserialize_untagged_enum_after(params, variants, cattrs, |_| None)
+    let first_attempt = None;
+    deserialize_untagged_enum_after(params, variants, cattrs, first_attempt)
 }
 
 fn deserialize_untagged_enum_after(
     params: &Parameters,
     variants: &[Variant],
     cattrs: &attr::Container,
-    first_attempt: impl FnOnce(TokenStream) -> Option<Expr>,
+    first_attempt: Option<Expr>,
 ) -> Fragment {
-    let deserializer =
-        quote!(_serde::__private::de::ContentRefDeserializer::<__D::Error>::new(&__content));
     let attempts = variants
         .iter()
         .filter(|variant| !variant.attrs.skip_deserializing())
@@ -1708,12 +1701,10 @@ fn deserialize_untagged_enum_after(
                 params,
                 variant,
                 cattrs,
-                deserializer.clone(),
+                quote!(__deserializer),
             ))
         });
-    let attempts = first_attempt(deserializer.clone())
-        .into_iter()
-        .chain(attempts);
+    let attempts = first_attempt.into_iter().chain(attempts);
     // TODO this message could be better by saving the errors from the failed
     // attempts. The heuristic used by TOML was to count the number of fields
     // processed before an error, and use the error that happened after the
@@ -1728,6 +1719,7 @@ fn deserialize_untagged_enum_after(
 
     quote_block! {
         let __content = try!(<_serde::__private::de::Content as _serde::Deserialize>::deserialize(__deserializer));
+        let __deserializer = _serde::__private::de::ContentRefDeserializer::<__D::Error>::new(&__content);
 
         #(
             if let _serde::__private::Ok(__ok) = #attempts {