Add a StringPiece formatter

Summary: Add an {fmt} formatter for `StringPiece`. This is much faster than going through ostream `operator<<` which we fallback to otherwise.

Reviewed By: yfeldblum

Differential Revision: D24337424

fbshipit-source-id: c6f1775cf83cfa6bcaa25cb4f6b665a76672e7c3

folly/Range.h

@@ -41,6 +41,8 @@
 #include <string_view> // @manual
 #endif
 
+#include <fmt/format.h>
+
 #include <folly/CpuId.h>
 #include <folly/Likely.h>
 #include <folly/Traits.h>
@@ -1524,6 +1526,19 @@ constexpr Range<wchar_t const*> operator"" _sp(
 
 } // namespace folly
 
+// Avoid ambiguity in older fmt versions due to StringPiece's conversions.
+#if FMT_VERSION >= 70000
+template <>
+struct fmt::formatter<folly::StringPiece> : private formatter<string_view> {
+  using formatter<string_view>::parse;
+
+  template <typename Context>
+  auto format(folly::StringPiece s, Context& ctx) {
+    return formatter<string_view>::format({s.data(), s.size()}, ctx);
+  }
+};
+#endif
+
 FOLLY_POP_WARNING
 
 FOLLY_ASSUME_FBVECTOR_COMPATIBLE_1(folly::Range)

folly/test/RangeTest.cpp

@@ -1638,6 +1638,10 @@ TEST(StringPiece, StringViewConversion) {
   EXPECT_EQ(tt3.which, 1);
 }
 
+TEST(StringPiece, Format) {
+  EXPECT_EQ("  foo", fmt::format("{:>5}", folly::StringPiece("foo")));
+}
+
 namespace {
 
 // Range with non-pod value type should not cause compile errors.