Refine Slice read error test

test/readerat.go

@@ -1,7 +1,11 @@
 // Package test contains helpers for testing parsers.
 package test
 
-import "io"
+import (
+	"io"
+
+	"git.codemonkeysoftware.net/b/gigaparsec/cursor"
+)
 
 type errReaderAt struct {
 	err error
@@ -15,3 +19,26 @@ func (r errReaderAt) ReadAt([]byte, int64) (int, error) {
 func ErrReaderAt(err error) io.ReaderAt {
 	return errReaderAt{err: err}
 }
+
+type errCursor[T any] struct {
+	err error
+	pos uint64
+}
+
+func (c errCursor[T]) Read([]T) (uint64, cursor.Cursor[T], error) {
+	return 0, c, c.err
+}
+
+func (c errCursor[T]) At(pos uint64) cursor.Cursor[T] {
+	c.pos = pos
+	return c
+}
+
+func (c errCursor[T]) Pos() uint64 {
+	return c.pos
+}
+
+// ErrCursor return a [cursor.Cursor] with a Read method that always returns err.
+func ErrCursor[T any](err error) cursor.Cursor[T] {
+	return errCursor[T]{err: err}
+}