diff --git a/modfile/ext.go b/modfile/ext.go
index cd94a93..dccd2db 100644
--- a/modfile/ext.go
+++ b/modfile/ext.go
@@ -17,6 +17,7 @@
 package modfile
 
 import (
+	"fmt"
 	"path"
 	"strings"
 
@@ -44,6 +45,17 @@ failed:
 	}
 }
 
+type InvalidExtError struct {
+	Ext string
+	Err error
+}
+
+func (e *InvalidExtError) Error() string {
+	return fmt.Sprintf("ext %s invalid: %s", e.Ext, e.Err)
+}
+
+func (e *InvalidExtError) Unwrap() error { return e.Err }
+
 // SplitFname splits fname into (hasGoxExt, className, classExt).
 func SplitFname(fname string) (hasGoxExt bool, className, classExt string) {
 	classExt = path.Ext(fname)
diff --git a/modfile/rule.go b/modfile/rule.go
index 83b4c17..0e103a3 100644
--- a/modfile/rule.go
+++ b/modfile/rule.go
@@ -340,17 +340,6 @@ func isPkgPath(s string) bool {
 	return s != "" && (s[0] != '.' && s[0] != '_')
 }
 
-type InvalidExtError struct {
-	Ext string
-	Err error
-}
-
-func (e *InvalidExtError) Error() string {
-	return fmt.Sprintf("ext %s invalid: %s", e.Ext, e.Err)
-}
-
-func (e *InvalidExtError) Unwrap() error { return e.Err }
-
 type InvalidSymbolError struct {
 	Sym string
 	Err error