Set dst slice to nil when src slice is nil.

copy.go

@@ -169,6 +169,12 @@ func structToStruct(filter FieldFilter, src, dst *reflect.Value, userOptions *op
 		}
 
 	case reflect.Slice:
+		if src.IsNil() {
+			// If the source slice is nil the dst slice is set to nil too.
+			dst.Set(*src)
+			break
+		}
+
 		dstLen := dst.Len()
 		srcLen := userOptions.CopyListSize(src)
 

copy_test.go

@@ -530,6 +530,46 @@ func TestStructToStruct_EntireSlice_NonEmptyDst(t *testing.T) {
 	}, dst)
 }
 
+func TestStructToStruct_NilSrcSlice_NonEmptyDst(t *testing.T) {
+	type A struct {
+		Field1 string
+		Field2 int
+	}
+	type B struct {
+		Field1 string
+		A      []A
+	}
+	src := &B{
+		Field1: "src StringerB field1",
+		A:      nil,
+	}
+	dst := &B{
+		Field1: "dst StringerB field1",
+		A: []A{
+			{
+				Field1: "dst StringerA field1 0",
+				Field2: 10,
+			},
+			{
+				Field1: "dst StringerA field1 1",
+				Field2: 20,
+			},
+			{
+				Field1: "dst StringerA field1 2",
+				Field2: 30,
+			},
+		},
+	}
+
+	mask := fieldmask_utils.MaskFromString("Field1,A")
+	err := fieldmask_utils.StructToStruct(mask, src, dst)
+	require.NoError(t, err)
+	assert.Equal(t, &B{
+		Field1: src.Field1,
+		A:      src.A,
+	}, dst)
+}
+
 func TestStructToStruct_SliceOfPtrsToStruct_EmptyDst(t *testing.T) {
 	type A struct {
 		Field1 string
@@ -1404,6 +1444,36 @@ func TestStructToMap_EntireSlice_NonEmptyDst(t *testing.T) {
 	}, dst)
 }
 
+func TestStructToMap_NilSrcSlice_NonEmptyDst(t *testing.T) {
+	type A struct {
+		Field1 string
+		Field2 string
+	}
+	type B struct {
+		FieldA []A
+	}
+	src := &B{FieldA: nil}
+	dst := map[string]interface{}{
+		"FieldA": []map[string]interface{}{
+			{
+				"Field1": "dst ele1 field1",
+			},
+			{
+				"Field2": "dst ele2 field 2",
+			},
+			{
+				"Field1": "dst ele3 field 3",
+			},
+		},
+	}
+	mask := fieldmask_utils.MaskFromString("FieldA")
+	err := fieldmask_utils.StructToMap(mask, src, dst)
+	require.NoError(t, err)
+	assert.Equal(t, map[string]interface{}{
+		"FieldA": []map[string]interface{}{},
+	}, dst)
+}
+
 func TestStructToMap_EntireSlice_DstSliceLenIsLessThanSource(t *testing.T) {
 	type A struct {
 		Field1 string