hints: host_filter: add formatter for hints::host_filter

before this change, we rely on the default-generated fmt::formatter
created from operator<<, but fmt v10 dropped the default-generated
formatter.

in this change, we define formatters for `hints::host_filter`. its
operator<< is preserved as it's still used by the homebrew generic
formatter for vector<>, which is in turn used by db/config.cc.

Refs scylladb#13245

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes scylladb#17347

db/hints/host_filter.cc

@@ -68,6 +68,11 @@ host_filter host_filter::parse_from_dc_list(sstring opt) {
     return host_filter(std::unordered_set<sstring>(dcs.begin(), dcs.end()));
 }
 
+std::ostream& operator<<(std::ostream& os, const host_filter& f) {
+    fmt::print(os, "{}", f);
+    return os;
+}
+
 std::istream& operator>>(std::istream& is, host_filter& f) {
     sstring tmp;
     is >> tmp;
@@ -100,16 +105,17 @@ std::string_view host_filter::enabled_kind_to_string(host_filter::enabled_kind e
     throw std::logic_error("Uncovered variant of enabled_kind");
 }
 
-std::ostream& operator<<(std::ostream& os, const host_filter& f) {
-    fmt::print(os, "host_filter{{enabled_kind={}",
+}
+}
+
+auto fmt::formatter<db::hints::host_filter>::format(const db::hints::host_filter& f, fmt::format_context& ctx) const
+        -> decltype(ctx.out()) {
+    using db::hints::host_filter;
+    auto out = ctx.out();
+    out = fmt::format_to(out, "host_filter{{enabled_kind={}",
                host_filter::enabled_kind_to_string(f._enabled_kind));
     if (f._enabled_kind == host_filter::enabled_kind::enabled_selectively) {
-        fmt::print(os, ", dcs={{{}}}", fmt::join(f._dcs, ","));
+        out = fmt::format_to(out, ", dcs={{{}}}", fmt::join(f._dcs, ","));
     }
-    fmt::print(os, "}}");
-    return os;
-}
-
-}
+    return fmt::format_to(out, "}}");
 }
-

db/hints/host_filter.hh

@@ -79,9 +79,10 @@ public:
 
     sstring to_configuration_string() const;
 
-    friend std::ostream& operator<<(std::ostream& os, const host_filter& f);
+    friend fmt::formatter<host_filter>;
 };
 
+std::ostream& operator<<(std::ostream& os, const host_filter& f);
 std::istream& operator>>(std::istream& is, host_filter& f);
 
 class hints_configuration_parse_error : public std::runtime_error {
@@ -91,3 +92,8 @@ public:
 
 }
 }
+
+template <>
+struct fmt::formatter<db::hints::host_filter> : fmt::formatter<std::string_view> {
+    auto format(const db::hints::host_filter&, fmt::format_context& ctx) const -> decltype(ctx.out());
+};