Introduce a dedicated function for generating Field enum

(the enum that represents all fields of a struct)

serde_derive/src/de.rs

@@ -965,12 +965,7 @@ fn deserialize_struct(
             )
         })
         .collect();
-    let field_visitor = Stmts(deserialize_generated_identifier(
+    let field_visitor = deserialize_field_identifier(&field_names_idents, cattrs);
-        &field_names_idents,
-        cattrs,
-        false,
-        None,
-    ));
 
     // untagged struct variants do not get a visit_seq method. The same applies to
     // structs that only have a map representation.
@@ -1128,12 +1123,7 @@ fn deserialize_struct_in_place(
         })
         .collect();
 
-    let field_visitor = Stmts(deserialize_generated_identifier(
+    let field_visitor = deserialize_field_identifier(&field_names_idents, cattrs);
-        &field_names_idents,
-        cattrs,
-        false,
-        None,
-    ));
 
     let mut_seq = if field_names_idents.is_empty() {
         quote!(_)
@@ -2052,6 +2042,20 @@ fn deserialize_generated_identifier(
     }
 }
 
+/// Generates enum and its `Deserialize` implementation that represents each
+/// non-skipped field of the struct
+fn deserialize_field_identifier(
+    fields: &[(&str, Ident, &BTreeSet<String>)],
+    cattrs: &attr::Container,
+) -> Stmts {
+    Stmts(deserialize_generated_identifier(
+        fields,
+        cattrs,
+        false,
+        None,
+    ))
+}
+
 // Generates `Deserialize::deserialize` body for an enum with
 // `serde(field_identifier)` or `serde(variant_identifier)` attribute.
 fn deserialize_custom_identifier(