Add conversion from fbstring to std::string_view

Summary:
Add an implicit conversion from `folly::fbstring` to `std::string_view` for
compatibility with `std::string`. Among other things this enables formatting of
`fbstring` with fmt.

Reviewed By: yfeldblum

Differential Revision: D15230438

fbshipit-source-id: 8f5a606c5e2a761cb332421130de638abfab4065

folly/FBString.h

@@ -26,6 +26,10 @@
 #include <stdexcept>
 #include <type_traits>
 
+#if FOLLY_HAS_STRING_VIEW
+#include <string_view>
+#endif
+
 // This file appears in two locations: inside fbcode and in the
 // libstdc++ source code (when embedding fbstring as std::string).
 // To aid in this schizophrenic use, _LIBSTDCXX_FBSTRING is defined in
@@ -1221,6 +1225,12 @@ class basic_fbstring {
     return assign(il.begin(), il.end());
   }
 
+#if FOLLY_HAS_STRING_VIEW
+  operator std::basic_string_view<value_type, traits_type>() const noexcept {
+    return {data(), size()};
+  }
+#endif
+
   // C++11 21.4.3 iterators:
   iterator begin() {
     return store_.mutableData();

folly/test/FBStringTest.cpp

@@ -1739,3 +1739,16 @@ TEST(WFBString, compareToStdWStringLong) {
   EXPECT_TRUE(fbB >= stdB);
 }
 #endif
+
+#if FOLLY_HAS_STRING_VIEW
+struct custom_traits : public std::char_traits<char> {};
+
+TEST(FBString, convertToStringView) {
+  folly::fbstring s("foo");
+  std::string_view sv = s;
+  EXPECT_EQ(sv, "foo");
+  folly::basic_fbstring<char, custom_traits> s2("bar");
+  std::basic_string_view<char, custom_traits> sv2 = s2;
+  EXPECT_EQ(sv2, "bar");
+}
+#endif