From 2fbcbc09b02edf4a0c4c87ff7b2769f4a48638a7 Mon Sep 17 00:00:00 2001
From: Kefu Chai <kefu.chai@scylladb.com>
Date: Mon, 22 May 2023 15:37:19 +0800
Subject: [PATCH] api: specialize fmt::formatter<api::table_info>

this is a part of a series to migrating from `operator<<(ostream&, ..)`
based formatting to fmtlib based formatting. the goal here is to enable
fmtlib to print `api::table_info` without the help of `operator<<`.

but the corresponding `operator<<()` is preserved in this change, as we
still have lots of callers relying on this << operator instorage_service.cc
where std::vector<table_info> is formatted using operator<<(ostream&, const Range&)
defined in to_string.hh. we could have used fmt/ranges.h to print the
std::vector<table_info>. but the combination of operator<<(ostream&, const Range&)
and FMT_DEPRECATED_OSTREAM renders this impossible. because
unlike the builtin range formatter specializations, the fallback formatter
synthesized from the operator<< does not have brackets defined for
the range printer. the brackets are used as the left and right marks
of the range, for instance, the array-alike containers are printed
like [1,2,3], while the tuple-alike containers are printed like
(1,2,3). once we are allowed to remove FMT_DEPRECATED_OSTREAM, we
should be able to use the builtin range formatter, and remove the
operator<< for api::table_info by then.

Refs #13245

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

Closes #13975
---
 api/storage_service.cc | 7 ++++++-
 api/storage_service.hh | 5 +++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/api/storage_service.cc b/api/storage_service.cc
index faa440d380..4d756d4b40 100644
--- a/api/storage_service.cc
+++ b/api/storage_service.cc
@@ -55,12 +55,17 @@ extern logging::logger apilog;
 namespace std {
 
 std::ostream& operator<<(std::ostream& os, const api::table_info& ti) {
-    fmt::print(os, "table{{name={}, id={}}}", ti.name, ti.id);
+    fmt::print(os, "{}", ti);
     return os;
 }
 
 } // namespace std
 
+auto fmt::formatter<api::table_info>::format(const api::table_info& ti,
+                                             fmt::format_context& ctx) const -> decltype(ctx.out()) {
+    return fmt::format_to(ctx.out(), "table{{name={}, id={}}}", ti.name, ti.id);
+}
+
 namespace api {
 
 const locator::token_metadata& http_context::get_token_metadata() {
diff --git a/api/storage_service.hh b/api/storage_service.hh
index 6c7b0142ed..7d655ce263 100644
--- a/api/storage_service.hh
+++ b/api/storage_service.hh
@@ -85,3 +85,8 @@ namespace std {
 std::ostream& operator<<(std::ostream& os, const api::table_info& ti);
 
 } // namespace std
+
+template <>
+struct fmt::formatter<api::table_info> : fmt::formatter<std::string_view> {
+    auto format(const api::table_info&, fmt::format_context& ctx) const -> decltype(ctx.out());
+};