scanner: Fail if U+E123 is found in input.

This (invalid) Unicode codepoint is used by the printer package to fix up
the indentation of generated files. If this codepoint is present in the
input, the package gets confused and removes more than it should,
producing unparsable output.

hcl/scanner/scanner.go

@@ -100,6 +100,11 @@ func (s *Scanner) next() rune {
 		return eof
 	}
 
+	if ch == '\uE123' {
+		s.err("unicode code point U+E123 reserved for internal use")
+		return utf8.RuneError
+	}
+
 	// debug
 	// fmt.Printf("ch: %q, offset:column: %d:%d\n", ch, s.srcPos.Offset, s.srcPos.Column)
 	return ch

hcl/scanner/scanner_test.go

@@ -509,6 +509,7 @@ func TestScan_crlf(t *testing.T) {
 func TestError(t *testing.T) {
 	testError(t, "\x80", "1:1", "illegal UTF-8 encoding", token.ILLEGAL)
 	testError(t, "\xff", "1:1", "illegal UTF-8 encoding", token.ILLEGAL)
+	testError(t, "\uE123", "1:1", "unicode code point U+E123 reserved for internal use", token.ILLEGAL)
 
 	testError(t, "ab\x80", "1:3", "illegal UTF-8 encoding", token.IDENT)
 	testError(t, "abc\xff", "1:4", "illegal UTF-8 encoding", token.IDENT)