Add ArgInserter::str().

format.h

@@ -380,6 +380,8 @@ class ArgInserter {
     return Proxy(f);
   }
 
+  std::string str() const { return Format()->str(); }
+
   // Performs formatting and returns a C string with the output.
   friend const char *c_str(Proxy p) {
     return Format(p)->c_str();

format_test.cc

@@ -789,6 +789,17 @@ TEST(TempFormatterTest, Example) {
   ReportError("File not found: {0}") << path;
 }
 
+template <typename T>
+std::string str(const T &value) {
+  return (fmt::Format("{0}") << value).str();
+}
+
+TEST(StrTest, Convert) {
+  EXPECT_EQ("42", str(42));
+  std::string s = str(Date(2012, 12, 9));
+  EXPECT_EQ("2012-12-9", s);
+}
+
 int main(int argc, char **argv) {
 #ifdef _WIN32
   // Disable message boxes on assertion failures.