Added range_format::string formatter

Either the basic_string_view<Char> range constructor or a user-defined conversion operator will be used.

Author: matt77hias

Diff for: include/fmt/ranges.h

@@ -515,7 +515,8 @@ struct formatter<
     enable_if_t<conjunction<
         bool_constant<range_format_kind<R, Char>::value !=
                           range_format::disabled &&
-                      range_format_kind<R, Char>::value != range_format::map>
+                      range_format_kind<R, Char>::value != range_format::map &&
+                      range_format_kind<R, Char>::value != range_format::string>
 // Workaround a bug in MSVC 2015 and earlier.
 #if !FMT_MSC_VERSION || FMT_MSC_VERSION >= 1910
         ,
@@ -604,6 +605,30 @@ struct formatter<
   }
 };
 
+// A string formatter.
+template <typename R, typename Char>
+struct formatter<
+    R, Char,
+    enable_if_t<range_format_kind<R, Char>::value == range_format::string>> {
+ private:
+  using range_type = detail::maybe_const_range<R>;
+  formatter<std::basic_string_view<Char>, Char> range_formatter_;
+
+ public:
+  FMT_CONSTEXPR formatter() {}
+
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return range_formatter_.parse(ctx);
+  }
+
+  template <typename FormatContext>
+  auto format(range_type& range, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    return range_formatter_.format(std::basic_string_view<Char>(range), ctx);
+  }
+};
+
 template <typename It, typename Sentinel, typename Char = char>
 struct join_view : detail::view {
   It begin;