Add string parsing and a print builtin (#721)

It was in my mind to add String in order to support libraries in `package`.  `print` is added in order to have a String go to stdout. I've tried to do `print` in a way that won't be too hard to add other printable types, but it's probably also somewhat optional here -- that is, if desired, I could remove it. But it was a lot easier to doublecheck `\n` behavior with it, and I suspect it'll be helpful in other tests if it supports more value types.

On the side, this also fixes dereferencing in Pattern/Expression Print() calls, which I was noticing printing pointers instead of values. This may be another argument for moving away from passing pointers, since this seems to be a difficult-to-catch error.

Co-authored-by: Geoff Romer <gromer@google.com>

common/string_helpers_test.cpp

@@ -0,0 +1,54 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "common/string_helpers.h"
+
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+using ::testing::Eq;
+using ::testing::Optional;
+
+namespace Carbon {
+namespace {
+
+TEST(UnescapeStringLiteral, Valid) {
+  EXPECT_THAT(UnescapeStringLiteral("test"), Optional(Eq("test")));
+  EXPECT_THAT(UnescapeStringLiteral("test\n"), Optional(Eq("test\n")));
+  EXPECT_THAT(UnescapeStringLiteral("test\\n"), Optional(Eq("test\n")));
+  EXPECT_THAT(UnescapeStringLiteral("abc\\ndef"), Optional(Eq("abc\ndef")));
+  EXPECT_THAT(UnescapeStringLiteral("test\\\\n"), Optional(Eq("test\\n")));
+  EXPECT_THAT(UnescapeStringLiteral("\\xAA"), Optional(Eq("\xAA")));
+  EXPECT_THAT(UnescapeStringLiteral("\\x12"), Optional(Eq("\x12")));
+}
+
+TEST(UnescapeStringLiteral, Invalid) {
+  // Missing char after `\`.
+  EXPECT_THAT(UnescapeStringLiteral("a\\"), Eq(std::nullopt));
+  // Not a supported escape.
+  EXPECT_THAT(UnescapeStringLiteral("\\e"), Eq(std::nullopt));
+  // Needs 2 hex chars.
+  EXPECT_THAT(UnescapeStringLiteral("\\x"), Eq(std::nullopt));
+  // Needs 2 hex chars.
+  EXPECT_THAT(UnescapeStringLiteral("\\xA"), Eq(std::nullopt));
+  // Needs uppercase hex.
+  EXPECT_THAT(UnescapeStringLiteral("\\xaa"), Eq(std::nullopt));
+  // Reserved.
+  EXPECT_THAT(UnescapeStringLiteral("\\00"), Eq(std::nullopt));
+}
+
+TEST(UnescapeStringLiteral, Nul) {
+  std::optional<std::string> str = UnescapeStringLiteral("a\\0b");
+  ASSERT_NE(str, std::nullopt);
+  EXPECT_THAT(str->size(), Eq(3));
+  EXPECT_THAT(strlen(str->c_str()), Eq(1));
+  EXPECT_THAT((*str)[0], Eq('a'));
+  EXPECT_THAT((*str)[1], Eq('\0'));
+  EXPECT_THAT((*str)[2], Eq('b'));
+}
+
+}  // namespace
+}  // namespace Carbon