Add a UDL suffix to define a StringPiece

Summary:
Use it like this:
```
using namespace folly::string_piece_literals;
StringPiece p = "A literal string"_sp;
```

In some compilation environments it can be more efficient than the implicit
conversion from `char const *` to `StringPiece`.

Reviewed By: yfeldblum

Differential Revision: D5562782

fbshipit-source-id: ce715edc65b1510761e127bf89a6936370253a68

folly/Range.h

@@ -1322,6 +1322,40 @@ struct IsSomeString {
   };
 };
 
+/**
+ * _sp is a user-defined literal suffix to make an appropriate Range
+ * specialization from a literal string.
+ *
+ * Modeled after C++17's `sv` suffix.
+ */
+inline namespace literals {
+inline namespace string_piece_literals {
+constexpr Range<char const*> operator"" _sp(
+    char const* str,
+    size_t len) noexcept {
+  return Range<char const*>(str, len);
+}
+
+constexpr Range<char16_t const*> operator"" _sp(
+    char16_t const* str,
+    size_t len) noexcept {
+  return Range<char16_t const*>(str, len);
+}
+
+constexpr Range<char32_t const*> operator"" _sp(
+    char32_t const* str,
+    size_t len) noexcept {
+  return Range<char32_t const*>(str, len);
+}
+
+constexpr Range<wchar_t const*> operator"" _sp(
+    wchar_t const* str,
+    size_t len) noexcept {
+  return Range<wchar_t const*>(str, len);
+}
+} // inline namespace string_piece_literals
+} // inline namespace literals
+
 } // namespace folly
 
 FOLLY_POP_WARNING

folly/test/RangeTest.cpp

@@ -1357,3 +1357,21 @@ TEST(Range, ConstexprAccessors) {
   static_assert(*piece.begin() == 'h', "");
   static_assert(*(piece.end() - 1) == '\0', "");
 }
+
+TEST(Range, LiteralSuffix) {
+  constexpr auto literalPiece = "hello"_sp;
+  constexpr StringPiece piece = "hello";
+  EXPECT_EQ(literalPiece, piece);
+  constexpr auto literalPiece8 = u8"hello"_sp;
+  constexpr Range<char const*> piece8 = u8"hello";
+  EXPECT_EQ(literalPiece8, piece8);
+  constexpr auto literalPiece16 = u"hello"_sp;
+  constexpr Range<char16_t const*> piece16{u"hello", 5};
+  EXPECT_EQ(literalPiece16, piece16);
+  constexpr auto literalPiece32 = U"hello"_sp;
+  constexpr Range<char32_t const*> piece32{U"hello", 5};
+  EXPECT_EQ(literalPiece32, piece32);
+  constexpr auto literalPieceW = L"hello"_sp;
+  constexpr Range<wchar_t const*> pieceW{L"hello", 5};
+  EXPECT_EQ(literalPieceW, pieceW);
+}