5 files changed, 60 insertions, 15 deletions
diff --git a/tools/go_marshal/README.md b/tools/go_marshal/README.md
index 75e5c7888..d8045c295 100644
--- a/tools/go_marshal/README.md
+++ b/tools/go_marshal/README.md
@@ -113,3 +113,18 @@ The following are some guidelines for modifying the `go_marshal` tool:
 -   No runtime reflection in the code generated for the marshallable interface.
     The entire point of the tool is to avoid runtime reflection. The generated
     tests may use reflection.
+
+## Debugging
+
+To enable debugging output from the go-marshal tool, use one of the following
+options, depending on how go-marshal is being invoked:
+
+-   Pass `--define gomarshal=verbose` to the bazel command. Note that this can
+    generate a lot of output depending on what's being compiled, as this will
+    enable debugging for all packages built by the command.
+
+-   Set `marshal_debug = True` on the top-level `go_library` BUILD rule.
+
+-   Set `debug = True` on the `go_marshal` BUILD rule.
+
+-   Pass `-debug` to the go-marshal tool invocation.
diff --git a/tools/go_marshal/gomarshal/generator_interfaces.go b/tools/go_marshal/gomarshal/generator_interfaces.go
index cf76b5241..36447b86b 100644
--- a/tools/go_marshal/gomarshal/generator_interfaces.go
+++ b/tools/go_marshal/gomarshal/generator_interfaces.go
@@ -43,8 +43,8 @@ type interfaceGenerator struct {
 	// of t's interfaces.
 	ms map[string]struct{}
 
-	// as records embedded fields in t that are potentially not packed. The key
-	// is the accessor for the field.
+	// as records fields in t that are potentially not packed. The key is the
+	// accessor for the field.
 	as map[string]struct{}
 }
 
diff --git a/tools/go_marshal/gomarshal/generator_interfaces_struct.go b/tools/go_marshal/gomarshal/generator_interfaces_struct.go
index 456662fab..fe76d3785 100644
--- a/tools/go_marshal/gomarshal/generator_interfaces_struct.go
+++ b/tools/go_marshal/gomarshal/generator_interfaces_struct.go
@@ -51,12 +51,6 @@ func (g *interfaceGenerator) areFieldsPackedExpression() (string, bool) {
 // later.
 func (g *interfaceGenerator) validateStruct(ts *ast.TypeSpec, st *ast.StructType) {
 	forEachStructField(st, func(f *ast.Field) {
-		if len(f.Names) == 0 {
-			g.abortAt(f.Pos(), "Cannot marshal structs with embedded fields, give the field a name; use '_' for anonymous fields such as padding fields")
-		}
-	})
-
-	forEachStructField(st, func(f *ast.Field) {
 		fieldDispatcher{
 			primitive: func(_, t *ast.Ident) {
 				g.validatePrimitiveNewtype(t)
@@ -101,7 +95,7 @@ func (g *interfaceGenerator) emitMarshallableForStruct(st *ast.StructType) {
 		var dynamicSizeTerms []string
 
 		forEachStructField(st, fieldDispatcher{
-			primitive: func(n, t *ast.Ident) {
+			primitive: func(_, t *ast.Ident) {
 				if size, dynamic := g.scalarSize(t); !dynamic {
 					primitiveSize += size
 				} else {
@@ -109,13 +103,13 @@ func (g *interfaceGenerator) emitMarshallableForStruct(st *ast.StructType) {
 					dynamicSizeTerms = append(dynamicSizeTerms, fmt.Sprintf("(*%s)(nil).SizeBytes()", t.Name))
 				}
 			},
-			selector: func(n, tX, tSel *ast.Ident) {
+			selector: func(_, tX, tSel *ast.Ident) {
 				tName := fmt.Sprintf("%s.%s", tX.Name, tSel.Name)
 				g.recordUsedImport(tX.Name)
 				g.recordUsedMarshallable(tName)
 				dynamicSizeTerms = append(dynamicSizeTerms, fmt.Sprintf("(*%s)(nil).SizeBytes()", tName))
 			},
-			array: func(n *ast.Ident, a *ast.ArrayType, t *ast.Ident) {
+			array: func(_ *ast.Ident, a *ast.ArrayType, t *ast.Ident) {
 				lenExpr := g.arrayLenExpr(a)
 				if size, dynamic := g.scalarSize(t); !dynamic {
 					dynamicSizeTerms = append(dynamicSizeTerms, fmt.Sprintf("%d*%s", size, lenExpr))
diff --git a/tools/go_marshal/gomarshal/util.go b/tools/go_marshal/gomarshal/util.go
index d94314302..6a42691cd 100644
--- a/tools/go_marshal/gomarshal/util.go
+++ b/tools/go_marshal/gomarshal/util.go
@@ -79,7 +79,7 @@ type fieldDispatcher struct {
 }
 
 // Precondition: All dispatch callbacks that will be invoked must be
-// provided. Embedded fields are not allowed, len(f.Names) >= 1.
+// provided.
 func (fd fieldDispatcher) dispatch(f *ast.Field) {
 	// Each field declaration may actually be multiple declarations of the same
 	// type. For example, consider:
@@ -88,12 +88,24 @@ func (fd fieldDispatcher) dispatch(f *ast.Field) {
 	//     x, y, z int
 	// }
 	//
-	// We invoke the call-backs once per such instance. Embedded fields are not
-	// allowed, and results in a panic.
+	// We invoke the call-backs once per such instance.
+
+	// Handle embedded fields. Embedded fields have no names, but can be
+	// referenced by the type name.
 	if len(f.Names) < 1 {
-		panic("Precondition not met: attempted to dispatch on embedded field")
+		switch v := f.Type.(type) {
+		case *ast.Ident:
+			fd.primitive(v, v)
+		case *ast.SelectorExpr:
+			fd.selector(v.Sel, v.X.(*ast.Ident), v.Sel)
+		default:
+			// Note: Arrays can't be embedded, which is handled here.
+			panic(fmt.Sprintf("Attempted to dispatch on embedded field of unsupported kind: %#v", f.Type))
+		}
+		return
 	}
 
+	// Non-embedded field.
 	for _, name := range f.Names {
 		switch v := f.Type.(type) {
 		case *ast.Ident:
diff --git a/tools/go_marshal/test/test.go b/tools/go_marshal/test/test.go
index f75ca1b7f..d9e9f341b 100644
--- a/tools/go_marshal/test/test.go
+++ b/tools/go_marshal/test/test.go
@@ -174,3 +174,27 @@ type Type9 struct {
 	x int64
 	y [sizeA]int32
 }
+
+// Type10Embed is a test data type which is be embedded into another type.
+//
+// +marshal
+type Type10Embed struct {
+	x int64
+}
+
+// Type10 is a test data type which contains an embedded struct.
+//
+// +marshal
+type Type10 struct {
+	Type10Embed
+	y int64
+}
+
+// Type11 is a test data type which contains an embedded struct from an external
+// package.
+//
+// +marshal
+type Type11 struct {
+	ex.External
+	y int64
+}