You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: interface.go
    path of file: ./repos/gccrs/libgo/go/go/parser
    
    the code of the file until where you have to start completion: // Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the
    
func readSource(filename string, src any) ([]byte, error) {
	if src != nil {
		switch s := src.(type) {
		case string:
			return []byte(s), nil
		case []byte:
			return s, nil
		case *bytes.Buffer:
			// is io.Reader, but src is already available in []byte form
			if s != nil {
				return s.Bytes(), nil
			}
		case io.Reader:
			return io.ReadAll(s)
		}
		return nil, errors.New("invalid source")
	}
	return os.ReadFile(filename)
}