Enable FMT_STRING() use with types other than string literals

include/fmt/core.h

@@ -371,6 +371,9 @@ template <typename Char> class basic_string_view {
     the size with ``std::char_traits<Char>::length``.
     \endrst
    */
+#if __cplusplus >= 201703L  // C++17's char_traits::length() is constexpr.
+  FMT_CONSTEXPR
+#endif
   basic_string_view(const Char* s)
       : data_(s), size_(std::char_traits<Char>::length(s)) {}
 

include/fmt/format.h

@@ -2238,6 +2238,7 @@ enum class arg_id_kind { none, index, name };
 // An argument reference.
 template <typename Char> struct arg_ref {
   FMT_CONSTEXPR arg_ref() : kind(arg_id_kind::none), val() {}
+
   FMT_CONSTEXPR explicit arg_ref(int index)
       : kind(arg_id_kind::index), val(index) {}
   FMT_CONSTEXPR explicit arg_ref(basic_string_view<Char> name)
@@ -3525,9 +3526,21 @@ template <typename Char> struct udl_arg {
   }
 };
 
+// Converts string literals to basic_string_view.
 template <typename Char, size_t N>
-FMT_CONSTEXPR basic_string_view<Char> literal_to_view(const Char (&s)[N]) {
-  return {s, N - 1};
+FMT_CONSTEXPR basic_string_view<Char> compile_string_to_view(
+    const Char (&s)[N]) {
+  // Remove trailing null character if needed. Won't be present if this is used
+  // with raw character array (i.e. not defined as a string).
+  return {s,
+          N - ((std::char_traits<Char>::to_int_type(s[N - 1]) == 0) ? 1 : 0)};
+}
+
+// Converts string_view to basic_string_view.
+template <typename Char>
+FMT_CONSTEXPR basic_string_view<Char> compile_string_to_view(
+    const std_string_view<Char>& s) {
+  return {s.data(), s.size()};
 }
 }  // namespace internal
 
@@ -3585,18 +3598,17 @@ FMT_CONSTEXPR internal::udl_arg<wchar_t> operator"" _a(const wchar_t* s,
 #endif  // FMT_USE_USER_DEFINED_LITERALS
 FMT_END_NAMESPACE
 
-#define FMT_STRING_IMPL(s, ...)                              \
-  [] {                                                       \
-    /* Use a macro-like name to avoid shadowing warnings. */ \
-    struct FMT_COMPILE_STRING : fmt::compile_string {        \
-      using char_type = fmt::remove_cvref_t<decltype(*s)>;   \
-      FMT_MAYBE_UNUSED __VA_ARGS__ FMT_CONSTEXPR             \
-      operator fmt::basic_string_view<char_type>() const {   \
-        /* FMT_STRING only accepts string literals. */       \
-        return fmt::internal::literal_to_view(s);            \
-      }                                                      \
-    };                                                       \
-    return FMT_COMPILE_STRING();                             \
+#define FMT_STRING_IMPL(s, ...)                                     \
+  [] {                                                              \
+    /* Use a macro-like name to avoid shadowing warnings. */        \
+    struct FMT_COMPILE_STRING : fmt::compile_string {               \
+      using char_type = fmt::remove_cvref_t<decltype(s[0])>;        \
+      FMT_MAYBE_UNUSED __VA_ARGS__ FMT_CONSTEXPR                    \
+      operator fmt::basic_string_view<char_type>() const {          \
+        return fmt::internal::compile_string_to_view<char_type>(s); \
+      }                                                             \
+    };                                                              \
+    return FMT_COMPILE_STRING();                                    \
   }()
 
 /**