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

2 files changed, 16 insertions, 1 deletions

diff --git a/tools/go_marshal/gomarshal/generator_interfaces.go b/tools/go_marshal/gomarshal/generator_interfaces.go
index 22aae0f6b..3aa299ccd 100644
--- a/tools/go_marshal/gomarshal/generator_interfaces.go
+++ b/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) {
diff --git a/tools/go_marshal/gomarshal/generator_tests.go b/tools/go_marshal/gomarshal/generator_tests.go
index 5ad97af14..8c28b00d0 100644
--- a/tools/go_marshal/gomarshal/generator_tests.go
+++ b/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 {