util/codegen: add AssertStructUnchanged

Refactored out from cmd/cloner.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

cmd/cloner/cloner.go

@@ -164,18 +164,6 @@ func gen(buf *bytes.Buffer, imports map[string]struct{}, name string, typ *types
 
 	switch t := typ.Underlying().(type) {
 	case *types.Struct:
-		// We generate two bits of code simultaneously while we walk the struct.
-		// One is the Clone method itself, which we write directly to buf.
-		// The other is a variable assignment that will fail if the struct
-		// changes without the Clone method getting regenerated.
-		// We write that to regenBuf, and then append it to buf at the end.
-		regenBuf := new(bytes.Buffer)
-		writeRegen := func(format string, args ...interface{}) {
-			fmt.Fprintf(regenBuf, format+"\n", args...)
-		}
-		writeRegen("// A compilation failure here means this code must be regenerated, with the command at the top of this file.")
-		writeRegen("var _%sNeedsRegeneration = %s(struct {", name, name)
-
 		name := typ.Obj().Name()
 		fmt.Fprintf(buf, "// Clone makes a deep copy of %s.\n", name)
 		fmt.Fprintf(buf, "// The result aliases no memory with the original.\n")
@@ -191,9 +179,6 @@ func gen(buf *bytes.Buffer, imports map[string]struct{}, name string, typ *types
 		for i := 0; i < t.NumFields(); i++ {
 			fname := t.Field(i).Name()
 			ft := t.Field(i).Type()
-
-			writeRegen("\t%s %s", fname, importedName(ft))
-
 			if !containsPointers(ft) {
 				continue
 			}
@@ -258,9 +243,7 @@ func gen(buf *bytes.Buffer, imports map[string]struct{}, name string, typ *types
 		writef("return dst")
 		fmt.Fprintf(buf, "}\n\n")
 
-		writeRegen("}{})\n")
-
-		buf.Write(regenBuf.Bytes())
+		buf.Write(codegen.AssertStructUnchanged(t, name, "", thisPkg, imports))
 	}
 }