[wpiutil] StringExtras: Add UnescapeCString (#4707)

Based on implementation in glass but enhanced for generic use.

wpiutil/src/test/native/cpp/UnescapeCStringTest.cpp

@@ -0,0 +1,55 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "gtest/gtest.h"
+#include "wpi/SmallString.h"
+#include "wpi/StringExtras.h"
+
+using namespace wpi;
+
+namespace {
+
+TEST(UnescapeCStringTest, Basic) {
+  SmallString<64> buf;
+  auto [out, rem] = UnescapeCString("abc\\\\\\a\\b\\f\\n\\r\\t\\v\\", buf);
+  EXPECT_EQ(out, "abc\\\a\b\f\n\r\t\v\\");
+  EXPECT_TRUE(rem.empty());
+}
+
+TEST(UnescapeCStringTest, QuoteEnd) {
+  SmallString<64> buf;
+  auto [out, rem] = UnescapeCString("abc\\\"\"123", buf);
+  EXPECT_EQ(out, "abc\"");
+  EXPECT_EQ(rem, "\"123");
+}
+
+TEST(UnescapeCStringTest, Hex) {
+  SmallString<64> buf;
+  auto [out, rem] = UnescapeCString("\\xfe\\xFE\\x01", buf);
+  EXPECT_EQ(out, "\xfe\xFE\x01");
+  EXPECT_TRUE(rem.empty());
+}
+
+TEST(UnescapeCStringTest, HexPartial) {
+  SmallString<64> buf;
+  auto [out, rem] = UnescapeCString("\\xz\\x5z\\x2", buf);
+  EXPECT_EQ(out, "xz\x05z\x02");
+  EXPECT_TRUE(rem.empty());
+}
+
+TEST(UnescapeCStringTest, HexPartial2) {
+  SmallString<64> buf;
+  auto [out, rem] = UnescapeCString("\\x", buf);
+  EXPECT_EQ(out, "x");
+  EXPECT_TRUE(rem.empty());
+}
+
+TEST(UnescapeCStringTest, Octal) {
+  SmallString<64> buf;
+  auto [out, rem] = UnescapeCString("\\3\\11\\222\\4", buf);
+  EXPECT_EQ(out, "\3\11\222\4");
+  EXPECT_TRUE(rem.empty());
+}
+
+}  // namespace