Default to packed for repeated primitives in proto3.

protobuf-codegen/src/gen/field/mod.rs

@@ -119,7 +119,7 @@
 
 // Representation of map entry: key type and value type
 #[derive(Clone, Debug)]
 pub struct EntryKeyValue<'a>(FieldElem<'a>, FieldElem<'a>);
 
 #[derive(Clone)]
 pub(crate) struct FieldGen<'a> {
@@ -185,11 +185,43 @@
             }
             RuntimeFieldType::Repeated(..) => {
                 let elem = field_elem(&field, root_scope, &customize);
-
-                FieldKind::Repeated(RepeatedField {
-                    elem,
-                    packed: field.field.proto().options.get_or_default().packed(),
-                })
+                let primitive = match field.field.proto().type_() {
+                    Type::TYPE_DOUBLE
+                    | Type::TYPE_FLOAT
+                    | Type::TYPE_INT64
+                    | Type::TYPE_UINT64
+                    | Type::TYPE_INT32
+                    | Type::TYPE_FIXED64
+                    | Type::TYPE_FIXED32
+                    | Type::TYPE_BOOL
+                    | Type::TYPE_UINT32
+                    | Type::TYPE_SFIXED32
+                    | Type::TYPE_SFIXED64
+                    | Type::TYPE_SINT32
+                    | Type::TYPE_SINT64
+                    | Type::TYPE_ENUM => true,
+                    Type::TYPE_STRING
+                    | Type::TYPE_GROUP
+                    | Type::TYPE_MESSAGE
+                    | Type::TYPE_BYTES => false,
+                };
+                let packed = field
+                    .field
+                    .proto()
+                    .options
+                    .get_or_default()
+                    .packed
+                    .unwrap_or(match field.message.scope.file_scope.syntax() {
+                        Syntax::Proto2 => false,
+                        // in proto3, repeated primitive types are packed by default
+                        Syntax::Proto3 => primitive,
+                    });
+                if packed && !primitive {
+                    anyhow::bail!(
+                        "[packed = true] can only be specified for repeated primitive fields"
+                    );
+                }
+                FieldKind::Repeated(RepeatedField { elem, packed })
             }
             RuntimeFieldType::Singular(..) => {
                 let elem = field_elem(&field, root_scope, &customize);

test-crates/protobuf-codegen-protoc-test/src/common/v2/test_repeated_packed.rs

@@ -1,3 +1,5 @@
+use protobuf::reflect::Syntax;
+use protobuf::MessageFull;
 use protobuf_test_common::*;
 
 use super::test_repeated_packed_pb::*;
@@ -83,3 +85,16 @@ fn test_issue_281() {
     test.values = (0..100).collect();
     test_serialize_deserialize_no_hex(&test);
 }
+
+#[test]
+fn test_write_packed_default() {
+    let mut test = TestPackedDefault::new();
+    test.varints = vec![0, 1, 2, 3, 4, 5];
+
+    // Proto3 packs primitives by default, proto2 does not.
+    let expected_hex = match TestPackedDefault::descriptor().file_descriptor().syntax() {
+        Syntax::Proto2 => "08 00 08 01 08 02 08 03 08 04 08 05",
+        Syntax::Proto3 => "0a 06 00 01 02 03 04 05",
+    };
+    test_serialize_deserialize(expected_hex, &test);
+}

test-crates/protobuf-codegen-protoc-test/src/common/v2/test_repeated_packed_pb.proto

@@ -13,3 +13,7 @@ message TestUnpacked {
 message TestIssue281 {
     repeated fixed32 values = 1 [packed=true];
 }
+
+message TestPackedDefault {
+    repeated uint32 varints = 1;
+}