Added range_format::(debug_)string formatter (#3973)

include/fmt/ranges.h

@@ -11,9 +11,13 @@
 #ifndef FMT_IMPORT_STD
 #  include <initializer_list>
 #  include <iterator>
+#  include <string>
 #  include <tuple>
 #  include <type_traits>
 #  include <utility>
+#  ifdef __cpp_lib_ranges
+#    include <ranges>
+#  endif
 #endif
 
 #include "format.h"
@@ -516,9 +520,11 @@ template <typename R, typename Char>
 struct formatter<
     R, Char,
     enable_if_t<conjunction<
-        bool_constant<range_format_kind<R, Char>::value !=
+        bool_constant<
-                          range_format::disabled &&
+            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 &&
+            range_format_kind<R, Char>::value != range_format::debug_string>
 // Workaround a bug in MSVC 2015 and earlier.
 #if !FMT_MSC_VERSION || FMT_MSC_VERSION >= 1910
         ,
@@ -607,6 +613,49 @@ struct formatter<
   }
 };
 
+// A (debug_)string formatter.
+template <typename R, typename Char>
+struct formatter<
+    R, Char,
+    enable_if_t<range_format_kind<R, Char>::value == range_format::string ||
+                range_format_kind<R, Char>::value ==
+                    range_format::debug_string>> {
+ private:
+  using range_type = detail::maybe_const_range<R>;
+#ifdef __cpp_lib_ranges
+  using string_type = conditional_t<
+      std::is_constructible_v<std::basic_string_view<Char>,
+                              std::ranges::iterator_t<range_type>,
+                              std::ranges::sentinel_t<range_type>>,
+      std::basic_string_view<Char>, std::basic_string<Char>>;
+#else
+  using string_type = std::basic_string<Char>;
+#endif
+
+  formatter<string_type, Char> underlying_;
+
+ public:
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return underlying_.parse(ctx);
+  }
+
+  template <typename FormatContext>
+  auto format(range_type& range, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    auto out = ctx.out();
+    if (detail::const_check(range_format_kind<R, Char>::value ==
+                            range_format::debug_string))
+      *out++ = '"';
+    out = underlying_.format(
+        string_type{detail::range_begin(range), detail::range_end(range)}, ctx);
+    if (detail::const_check(range_format_kind<R, Char>::value ==
+                            range_format::debug_string))
+      *out++ = '"';
+    return out;
+  }
+};
+
 template <typename It, typename Sentinel, typename Char = char>
 struct join_view : detail::view {
   It begin;

test/ranges-test.cc

@@ -268,6 +268,33 @@ TEST(ranges_test, disabled_range_formatting_of_path) {
             fmt::range_format::disabled);
 }
 
+struct vector_string : std::vector<char> {
+  using base = std::vector<char>;
+  using base::base;
+};
+struct vector_debug_string : std::vector<char> {
+  using base = std::vector<char>;
+  using base::base;
+};
+FMT_BEGIN_NAMESPACE
+template <>
+struct range_format_kind<vector_string, char>
+    : std::integral_constant<range_format, range_format::string> {};
+template <>
+struct range_format_kind<vector_debug_string, char>
+    : std::integral_constant<range_format, range_format::debug_string> {};
+FMT_END_NAMESPACE
+
+TEST(ranges_test, range_format_string) {
+  const vector_string v{'f', 'o', 'o'};
+  EXPECT_EQ(fmt::format("{}", v), "foo");
+}
+
+TEST(ranges_test, range_format_debug_string) {
+  const vector_debug_string v{'f', 'o', 'o'};
+  EXPECT_EQ(fmt::format("{}", v), "\"foo\"");
+}
+
 // A range that provides non-const only begin()/end() to test fmt::join
 // handles that.
 //