before this change, we use `update_item_suffix` as a format string
fed to `format(...)`, which is resolved to `seastar::format()`.
but with a patch which migrates the `seastar::format()` to the backend
with compile-time format check, the caller sites using `format()` would
fail to build, because `update_item_suffix` is not a `constexpr`:
```
/home/kefu/.local/bin/clang++ -DFMT_SHARED -DSCYLLA_BUILD_MODE=release -DSEASTAR_API_LEVEL=7 -DSEASTAR_LOGGER_COMPILE_TIME_FMT -DSEASTAR_LOGGER_TYPE_STDOUT -DSEASTAR_SCHEDULING_GROUPS_COUNT=16 -DSEASTAR_SSTRING -DXXH_PRIVATE_API -DCMAKE_INTDIR=\"RelWithDebInfo\" -I/home/kefu/dev/scylladb -I/home/kefu/dev/scylladb/build/gen -I/home/kefu/dev/scylladb/seastar/include -I/home/kefu/dev/scylladb/build/seastar/gen/include -I/home/kefu/dev/scylladb/build/seastar/gen/src -isystem /home/kefu/dev/scylladb/abseil -isystem /home/kefu/dev/scylladb/build/rust -ffunction-sections -fdata-sections -O3 -g -gz -std=gnu++20 -fvisibility=hidden -Wall -Werror -Wextra -Wno-error=deprecated-declarations -Wimplicit-fallthrough -Wno-c++11-narrowing -Wno-deprecated-copy -Wno-mismatched-tags -Wno-missing-field-initializers -Wno-overloaded-virtual -Wno-unsupported-friend -Wno-enum-constexpr-conversion -Wno-unused-parameter -ffile-prefix-map=/home/kefu/dev/scylladb=. -march=westmere -mllvm -inline-threshold=2500 -fno-slp-vectorize -U_FORTIFY_SOURCE -Werror=unused-result -MD -MT test/perf/CMakeFiles/test-perf.dir/RelWithDebInfo/perf_alternator.cc.o -MF test/perf/CMakeFiles/test-perf.dir/RelWithDebInfo/perf_alternator.cc.o.d -o test/perf/CMakeFiles/test-perf.dir/RelWithDebInfo/perf_alternator.cc.o -c /home/kefu/dev/scylladb/test/perf/perf_alternator.cc
/home/kefu/dev/scylladb/test/perf/perf_alternator.cc:249:69: error: call to consteval function 'fmt::basic_format_string<char, const char (&)[1]>::basic_format_string<const char *, 0>' is not a constant expression
249 | return make_request(cli, "UpdateItem", prefix + seastar::format(update_item_suffix, ""));
| ^
/usr/include/fmt/core.h:2776:67: note: read of non-constexpr variable 'update_item_suffix' is not allowed in a constant expression
2776 | FMT_CONSTEVAL FMT_INLINE basic_format_string(const S& s) : str_(s) {
| ^
/home/kefu/dev/scylladb/test/perf/perf_alternator.cc:249:69: note: in call to 'basic_format_string<const char *, 0>(update_item_suffix)'
249 | return make_request(cli, "UpdateItem", prefix + seastar::format(update_item_suffix, ""));
| ^~~~~~~~~~~~~~~~~~
/home/kefu/dev/scylladb/test/perf/perf_alternator.cc:198:6: note: declared here
198 | auto update_item_suffix = R"(
| ^
```
so, to prepare the change switching to compile-time format checking,
let's mark this variable `static constexpr`. this is also more correct,
as this variable is
* a compile time constant, and
* is not shared across different compilation units.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#18685
759156b56d