Behold, a unit test for Scanner::BookmarkScope (& scanner bookmarking).

test/cctest/parsing/test-scanner.cc

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Tests v8::internal::Scanner. Note that presently most unit tests for the
+// Scanner are in cctest/test-parsing.cc, rather than here.
+
+#include "src/handles-inl.h"
+#include "src/parsing/scanner-character-streams.h"
+#include "src/parsing/scanner.h"
+#include "src/unicode-cache.h"
+#include "test/cctest/cctest.h"
+
+using namespace v8::internal;
+
+namespace {
+
+const char src_simple[] = "function foo() { var x = 2 * a() + b; }";
+
+static UnicodeCache* unicode_cache = new UnicodeCache();
+
+std::unique_ptr<Scanner> make_scanner(const char* src) {
+  std::unique_ptr<Scanner> scanner(new Scanner(new UnicodeCache()));
+  scanner->Initialize(ScannerStream::ForTesting(src).release());
+  return scanner;
+}
+
+}  // anonymous namespace
+
+TEST(Bookmarks) {
+  // Scan through the given source and record the tokens for use as reference
+  // below.
+  std::vector<Token::Value> tokens;
+  {
+    auto scanner = make_scanner(src_simple);
+    do {
+      tokens.push_back(scanner->Next());
+    } while (scanner->current_token() != Token::EOS);
+  }
+
+  // For each position (between tokens, i.e. not first or last):
+  // - Scan through file,
+  // - set a bookmark once the position is reached,
+  // - scan a bit more,
+  // - reset to the bookmark, and
+  // - scan until the end.
+  // At each step, compare to the reference token sequence generated above.
+  for (size_t bookmark_pos = 1; bookmark_pos < tokens.size() - 1;

[marja, 2016/09/19 11:50:39]

You could also set the bookmark to pos 0 without scanning anything, no?

[marja, 2016/09/19 11:50:39]

Can you also set it to the end? Or does that not make sense?

[vogelheim, 2016/09/19 16:04:45]

Done.

[My thinking was that this doesn't test anything useful, since that equates a
normal pass through the stream, without any actual 'reset' going on. I guess it
does test one more edge case, though.]

[vogelheim, 2016/09/19 16:04:45]

Done.

+       bookmark_pos++) {
+    auto scanner = make_scanner(src_simple);
+    Scanner::BookmarkScope bookmark(scanner.get());
+
+    for (size_t i = 0; i < std::min(bookmark_pos + 10, tokens.size()); i++) {
+      if (i == bookmark_pos) {
+        bookmark.Set();
+      }
+      DCHECK_EQ(tokens[i], scanner->Next());
+    }
+
+    bookmark.Reset();
+    for (size_t i = bookmark_pos; i < tokens.size(); i++) {
+      DCHECK_EQ(tokens[i], scanner->Next());
+    }
+  }
+}