Add support for StringRef in BasicFormatter.

format.h

@@ -862,6 +862,11 @@ class BasicFormatter {
       string.size = value.size();
     }
 
+    Arg(StringRef value) : type(STRING), formatter(0) {
+      string.value = value.c_str();
+      string.size = value.size();
+    }
+
     template <typename T>
     Arg(const T &value) : type(CUSTOM), formatter(0) {
       custom.value = &value;

format_test.cc

@@ -1091,6 +1091,10 @@ TEST(FormatterTest, FormatString) {
   EXPECT_EQ("test", str(Format("{0}") << std::string("test")));
 }
 
+TEST(FormatterTest, FormatStringRef) {
+  EXPECT_EQ("test", str(Format("{0}") << StringRef("test")));
+}
+
 TEST(FormatterTest, FormatUsingIOStreams) {
   EXPECT_EQ("a string", str(Format("{0}") << TestString("a string")));
   std::string s = str(fmt::Format("The date is {0}") << Date(2012, 12, 9));