Ensure Marshallable.SizeBytes() always works on a typed nil pointer.

This lets go-marshal replace various calls to binary.Size() throughout
the sentry without requiring concrete objects.

PiperOrigin-RevId: 295299965

tools/go_marshal/gomarshal/generator_interfaces.go

@@ -301,7 +301,7 @@ func (g *interfaceGenerator) emitMarshallable() {
 					primitiveSize += size
 				} else {
 					g.recordUsedMarshallable(t.Name)
-					dynamicSizeTerms = append(dynamicSizeTerms, fmt.Sprintf("%s.SizeBytes()", g.fieldAccessor(n)))
+					dynamicSizeTerms = append(dynamicSizeTerms, fmt.Sprintf("(*%s)(nil).SizeBytes()", t.Name))
 				}
 			},
 			selector: func(n, tX, tSel *ast.Ident) {

tools/go_marshal/gomarshal/generator_tests.go

@@ -167,11 +167,26 @@ func (g *testGenerator) emitTestWriteToUnmarshalPreservesData() {
 	})
 }
 
+func (g *testGenerator) emitTestSizeBytesOnTypedNilPtr() {
+	g.inTestFunction("TestSizeBytesOnTypedNilPtr", func() {
+		g.emit("var x %s\n", g.typeName())
+		g.emit("sizeFromConcrete := x.SizeBytes()\n")
+		g.emit("sizeFromTypedNilPtr := (*%s)(nil).SizeBytes()\n\n", g.typeName())
+
+		g.emit("if sizeFromTypedNilPtr != sizeFromConcrete {\n")
+		g.inIndent(func() {
+			g.emit("t.Fatalf(\"SizeBytes() on typed nil pointer (%v) doesn't match size returned by a concrete object (%v).\\n\", sizeFromTypedNilPtr, sizeFromConcrete)")
+		})
+		g.emit("}\n")
+	})
+}
+
 func (g *testGenerator) emitTests() {
 	g.emitTestNonZeroSize()
 	g.emitTestSuspectAlignment()
 	g.emitTestMarshalUnmarshalPreservesData()
 	g.emitTestWriteToUnmarshalPreservesData()
+	g.emitTestSizeBytesOnTypedNilPtr()
 }
 
 func (g *testGenerator) write(out io.Writer) error {

tools/go_marshal/marshal/marshal.go

@@ -48,6 +48,10 @@ type Marshallable interface {
 
 	// SizeBytes is the size of the memory representation of a type in
 	// marshalled form.
+	//
+	// SizeBytes must handle a nil receiver. Practically, this means SizeBytes
+	// cannot deference any fields on the object implementing it (but will
+	// likely make use of the type of these fields).
 	SizeBytes() int
 
 	// MarshalBytes serializes a copy of a type to dst. dst must be at least