implement implicit conversion from FixedString to std::string_view

Summary:
This makes it possible to pass `FixedString` objects to APIs that accept
`std::string_view`.  `FixedString` already had implicit conversion to
`std::string` and `folly::StringPiece`, this simply implements parity for
`string_view`.

Reviewed By: yfeldblum

Differential Revision: D26940391

fbshipit-source-id: 5f411ef9a901e0855593b090f1cbe3345971a140

folly/FixedString.h

@@ -36,6 +36,10 @@
 #include <folly/lang/Ordering.h>
 #include <folly/portability/Constexpr.h>
 
+#if FOLLY_HAS_STRING_VIEW
+#include <string_view>
+#endif
+
 namespace folly {
 
 template <class Char, std::size_t N>
@@ -774,6 +778,16 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
     return std::basic_string<Char>{begin(), end()};
   }
 
+#if FOLLY_HAS_STRING_VIEW
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Conversion to std::basic_string_view<Char>
+   * \return `std::basic_string_view<Char>{begin(), end()}`
+   */
+  /* implicit */ constexpr operator std::basic_string_view<Char>() const {
+    return std::basic_string_view<Char>{begin(), size()};
+  }
+#endif
+
   // Think hard about whether this is a good idea. It's certainly better than
   // an implicit conversion to `const Char*` since `delete "hi"_fs` will fail
   // to compile. But it creates ambiguities when passing a FixedString to an

folly/test/FixedStringTest.cpp

@@ -653,7 +653,7 @@ TEST(FixedStringReverseIteratorTest, ConstexprReverseIteration) {
 #include <folly/Range.h>
 
 TEST(FixedStringConversionTest, ConversionToFollyRange) {
-  // The following declaraction is static for compilers that haven't implemented
+  // The following declaration is static for compilers that haven't implemented
   // the resolution of:
   // http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1454
   static constexpr folly::FixedString<16> tmp{"This is a string"};
@@ -661,3 +661,12 @@ TEST(FixedStringConversionTest, ConversionToFollyRange) {
   static_assert(tmp.begin() == piece.begin(), "");
   static_assert(tmp.end() == piece.end(), "");
 }
+
+#if FOLLY_HAS_STRING_VIEW
+TEST(FixedStringConversionTest, ConversionToStringView) {
+  static constexpr folly::FixedString<16> tmp{"This is a string"};
+  constexpr std::string_view view = tmp;
+  static_assert(tmp.data() == view.data(), "");
+  static_assert(tmp.size() == view.size(), "");
+}
+#endif // FOLLY_HAS_STRING_VIEW