Add a formatter for fbstring

Summary: {fmt} is moving away from relying on implicit conversions because they may introduce ODR violations and not supported consistently. For example, `fbstring` can be formatted via an implicit conversion or via `operator<<`. To make sure this doesn't happen and make the code compatible with future versions of {fmt} add a `formatter` specialization for `fbstring`. This is consistent with `folly::StringPiece` which already has such specialization.

Reviewed By: ot

Differential Revision: D33994222

fbshipit-source-id: 8150415ef45bd97b24aa12dd209b56c977c148c6

folly/FBString.h

@@ -31,6 +31,7 @@
 #include <type_traits>
 #include <utility>
 
+#include <fmt/format.h>
 #include <folly/CPortability.h>
 #include <folly/CppAttributes.h>
 #include <folly/Likely.h>
@@ -2730,3 +2731,12 @@ struct IsSomeString;
 template <>
 struct IsSomeString<fbstring> : std::true_type {};
 } // namespace folly
+
+template <>
+struct fmt::formatter<folly::fbstring> : formatter<fmt::string_view> {
+  template <typename Context>
+  typename Context::iterator format(
+      const folly::fbstring& s, Context& ctx) const {
+    return formatter<fmt::string_view>::format({s.data(), s.size()}, ctx);
+  }
+};

folly/test/FBStringTest.cpp

@@ -1738,3 +1738,7 @@ TEST(FBString, convertToStringView) {
   EXPECT_EQ(sv2, "bar");
 }
 #endif
+
+TEST(FBString, Format) {
+  EXPECT_EQ("  foo", fmt::format("{:>5}", folly::fbstring("foo")));
+}