Support deserialization of struct keys from integers

serde-cbor supports a "packed" serialization flag which causes keys to
be serialized as their indices, but the deserializer currently has to
hardcode support for this format. We can simply support deserialization
of struct keys from integers as we already do for enum variants.

serde_derive/src/de.rs

@@ -1386,26 +1386,21 @@ fn deserialize_identifier(
         "field identifier"
     };
 
-    let visit_index = if is_variant {
-        let variant_indices = 0u32..;
-        let fallthrough_msg = format!("variant index 0 <= i < {}", fields.len());
-        let visit_index = quote! {
-            fn visit_u32<__E>(self, __value: u32) -> _serde::export::Result<Self::Value, __E>
-                where __E: _serde::de::Error
-            {
-                match __value {
-                    #(
-                        #variant_indices => _serde::export::Ok(#constructors),
-                    )*
-                    _ => _serde::export::Err(_serde::de::Error::invalid_value(
-                                _serde::de::Unexpected::Unsigned(__value as u64),
-                                &#fallthrough_msg))
-                }
+    let variant_indices = 0u32..;
+    let fallthrough_msg = format!("variant index 0 <= i < {}", fields.len());
+    let visit_index = quote! {
+        fn visit_u32<__E>(self, __value: u32) -> _serde::export::Result<Self::Value, __E>
+            where __E: _serde::de::Error
+        {
+            match __value {
+                #(
+                    #variant_indices => _serde::export::Ok(#constructors),
+                )*
+                _ => _serde::export::Err(_serde::de::Error::invalid_value(
+                            _serde::de::Unexpected::Unsigned(__value as u64),
+                            &#fallthrough_msg))
             }
-        };
-        Some(visit_index)
-    } else {
-        None
+        }
     };
 
     let bytes_to_str = if fallthrough.is_some() {

test_suite/tests/test_de.rs

@@ -605,6 +605,17 @@ declare_tests! {
             Token::StructEnd,
         ],
     }
+    test_struct_integer_keys {
+        Struct { a: 1, b: 2, c: 0 } => &[
+            Token::Struct { name: "Struct", len: 2 },
+                Token::U32(0),
+                Token::I32(1),
+
+                Token::U32(1),
+                Token::I32(2),
+            Token::StructEnd,
+        ],
+    }
     test_enum_unit {
         Enum::Unit => &[
             Token::UnitVariant { name: "Enum", variant: "Unit" },