[dev.typeparams] cmd/compile/internal/syntax: add utility functions for testing

Preparation step for types2 package.

Change-Id: I8f9557b1a48ad570ba38aac7b720e639218dc6a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/263623
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/testing.go

@@ -0,0 +1,72 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements testing support.
+
+package syntax
+
+import (
+	"io"
+	"regexp"
+	"strings"
+)
+
+// CommentsDo parses the given source and calls the provided handler for each
+// comment or error. If the text provided to handler starts with a '/' it is
+// the comment text; otherwise it is the error message.
+func CommentsDo(src io.Reader, handler func(line, col uint, text string)) {
+	var s scanner
+	s.init(src, handler, comments)
+	for s.tok != _EOF {
+		s.next()
+	}
+}
+
+// ERROR comments must start with text `ERROR "msg"` or `ERROR msg`.
+// Space around "msg" or msg is ignored.
+var errRx = regexp.MustCompile(`^ *ERROR *"?([^"]*)"?`)
+
+// ErrorMap collects all comments with comment text of the form
+// `ERROR "msg"` or `ERROR msg` from the given src and returns them
+// as []Error lists in a map indexed by line number. The position
+// for each Error is the position of the token immediately preceding
+// the comment, the Error message is the message msg extracted from
+// the comment, with all errors that are on the same line collected
+// in a slice. If there is no preceding token (the `ERROR` comment
+// appears in the beginning of the file), then the recorded position
+// is unknown (line, col = 0, 0). If there are no ERROR comments, the
+// result is nil.
+func ErrorMap(src io.Reader) (errmap map[uint][]Error) {
+	// position of previous token
+	var base *PosBase
+	var prev struct{ line, col uint }
+
+	var s scanner
+	s.init(src, func(_, _ uint, text string) {
+		if text[0] != '/' {
+			return // error, ignore
+		}
+		if text[1] == '*' {
+			text = text[:len(text)-2] // strip trailing */
+		}
+		if s := errRx.FindStringSubmatch(text[2:]); len(s) == 2 {
+			pos := MakePos(base, prev.line, prev.col)
+			err := Error{pos, strings.TrimSpace(s[1])}
+			if errmap == nil {
+				errmap = make(map[uint][]Error)
+			}
+			errmap[prev.line] = append(errmap[prev.line], err)
+		}
+	}, comments)
+
+	for s.tok != _EOF {
+		s.next()
+		if s.tok == _Semi && s.lit != "semicolon" {
+			continue // ignore automatically inserted semicolons
+		}
+		prev.line, prev.col = s.line, s.col
+	}
+
+	return
+}