util/codegen: add AssertStructUnchanged

Refactored out from cmd/cloner.

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

util/codegen/codegen.go

@@ -6,8 +6,10 @@
 package codegen
 
 import (
+	"bytes"
 	"fmt"
 	"go/format"
+	"go/types"
 	"os"
 )
 
@@ -38,3 +40,42 @@ func WriteFormatted(code []byte, path string) error {
 	}
 	return nil
 }
+
+// AssertStructUnchanged generates code that asserts at compile time that type t is unchanged.
+// tname is the named type corresponding to t.
+// ctx is a single-word context for this assertion, such as "Clone".
+// thisPkg is the package containing t.
+// If non-nil, AssertStructUnchanged will add elements to imports
+// for each package path that the caller must import for the returned code to compile.
+func AssertStructUnchanged(t *types.Struct, tname, ctx string, thisPkg *types.Package, imports map[string]struct{}) []byte {
+	buf := new(bytes.Buffer)
+	w := func(format string, args ...interface{}) {
+		fmt.Fprintf(buf, format+"\n", args...)
+	}
+	w("// A compilation failure here means this code must be regenerated, with the command at the top of this file.")
+	w("var _%s%sNeedsRegeneration = %s(struct {", tname, ctx, tname)
+
+	for i := 0; i < t.NumFields(); i++ {
+		fname := t.Field(i).Name()
+		ft := t.Field(i).Type()
+		qname, imppath := importedName(ft, thisPkg)
+		if imppath != "" && imports != nil {
+			imports[imppath] = struct{}{}
+		}
+		w("\t%s %s", fname, qname)
+	}
+
+	w("}{})\n")
+	return buf.Bytes()
+}
+
+func importedName(t types.Type, thisPkg *types.Package) (qualifiedName, importPkg string) {
+	qual := func(pkg *types.Package) string {
+		if thisPkg == pkg {
+			return ""
+		}
+		importPkg = pkg.Path()
+		return pkg.Name()
+	}
+	return types.TypeString(t, qual), importPkg
+}