Make format work with C++17 std::string_view (#571)

test/format-test.cc

@@ -137,11 +137,6 @@ TEST(StringViewTest, Ctor) {
   EXPECT_EQ(4u, string_view(std::string("defg")).size());
 }
 
-TEST(StringViewTest, ConvertToString) {
-  std::string s = string_view("abc").to_string();
-  EXPECT_EQ("abc", s);
-}
-
 TEST(WriterTest, NotCopyConstructible) {
   EXPECT_FALSE(std::is_copy_constructible<fmt::writer>::value);
 }

test/printf-test.cc

@@ -41,7 +41,7 @@ const unsigned BIG_NUM = INT_MAX + 1u;
 
 // Makes format string argument positional.
 std::string make_positional(fmt::string_view format) {
-  std::string s(format.to_string());
+  std::string s(format.data(), format.size());
   s.replace(s.find('%'), 1, "%1$");
   return s;
 }