Internal `stdexec` instantiation dominating compilation time -- any room for optimization?

[vittorioromeo]

Hello! We heavily use stdexec in one of our projects, which unfortunately is taking longer and longer to compile as we add more functionality.

I've run a trace using ClangBuildAnalyzer on our project, and the compilation time is dominated by internal instantiations of stdexec templates:

**** Template sets that took longest to instantiate:
696601 ms: stdexec::__sigs::get_completion_signatures_t::__impl<stdexec::(anony... (11136 times, avg 62 ms)
627460 ms: stdexec::__sigs::get_completion_signatures_t::__impl<stdexec::(anony... (12135 times, avg 51 ms)
589798 ms: stdexec::dependent_domain::transform_sender<stdexec::(anonymous name... (4923 times, avg 119 ms)
585881 ms: stdexec::dependent_domain::__is_nothrow_transform_sender<stdexec::(a... (4923 times, avg 119 ms)
456104 ms: stdexec::__sigs::get_completion_signatures_t::__impl<stdexec::(anony... (8388 times, avg 54 ms)
427458 ms: stdexec::__detail::__transform_sender::operator()<stdexec::__detail:... (7188 times, avg 59 ms)
421909 ms: stdexec::__detail::__transform_sender_1::operator()<stdexec::depende... (7032 times, avg 59 ms)
421252 ms: stdexec::__detail::__transform_sender_1::__is_nothrow<stdexec::depen... (6930 times, avg 60 ms)
414298 ms: stdexec::__sigs::get_completion_signatures_t::__impl<stdexec::(anony... (9252 times, avg 44 ms)
406173 ms: stdexec::dependent_domain::transform_sender<stdexec::(anonymous name... (3942 times, avg 103 ms)
403291 ms: stdexec::dependent_domain::__is_nothrow_transform_sender<stdexec::(a... (3942 times, avg 102 ms)
360123 ms: stdexec::dependent_domain::transform_sender<stdexec::(anonymous name... (4923 times, avg 73 ms)
356306 ms: stdexec::dependent_domain::__is_nothrow_transform_sender<stdexec::(a... (4923 times, avg 72 ms)
294444 ms: stdexec::__detail::__transform_sender::operator()<stdexec::__detail:... (5754 times, avg 51 ms)
290450 ms: stdexec::__detail::__transform_sender_1::operator()<stdexec::depende... (5631 times, avg 51 ms)
289950 ms: stdexec::__detail::__transform_sender_1::__is_nothrow<stdexec::depen... (5544 times, avg 52 ms)
258438 ms: stdexec::__detail::__transform_sender::operator()<stdexec::__detail:... (3960 times, avg 65 ms)
256370 ms: stdexec::__detail::__transform_sender_1::operator()<stdexec::depende... (3543 times, avg 72 ms)
256090 ms: stdexec::__detail::__transform_sender_1::__is_nothrow<stdexec::depen... (3522 times, avg 72 ms)
235384 ms: stdexec::dependent_domain::transform_sender<stdexec::(anonymous name... (3942 times, avg 59 ms)
232346 ms: stdexec::dependent_domain::__is_nothrow_transform_sender<stdexec::(a... (3942 times, avg 58 ms)
212618 ms: stdexec::__sigs::get_completion_signatures_t::__impl<stdexec::(anony... (5829 times, avg 36 ms)
184241 ms: stdexec::__sigs::get_completion_signatures_t::__impl<exec::__scope::... (942 times, avg 195 ms)
172626 ms: stdexec::__detail::__transform_dependent_sender::operator()<stdexec:... (1413 times, avg 122 ms)
171259 ms: stdexec::dependent_domain::__is_nothrow_transform_sender()::(anonymo... (1404 times, avg 121 ms)
168674 ms: stdexec::__detail::__transform_sender::operator()<stdexec::__detail:... (3105 times, avg 54 ms)
167214 ms: stdexec::__detail::__transform_sender_1::operator()<stdexec::depende... (2823 times, avg 59 ms)

I've also ran ClangBuildAnalyzer on stdexec's test suite. Here are the results:

**** Template sets that took longest to instantiate:
 52891 ms: stdexec::__connect::connect_t::__get_declfn<$> (1002 times, avg 52 ms)
 45473 ms: stdexec::__sigs::get_completion_signatures_t::__get_declfn<$> (2660 times, avg 17 ms)
 28993 ms: std::vformat_to<$> (206 times, avg 140 ms)
 28956 ms: std::__format::__do_vformat_to<$> (205 times, avg 141 ms)
 20063 ms: std::formatter<$>::format<$> (2530 times, avg 7 ms)
 17154 ms: stdexec::default_domain::apply_sender<$> (290 times, avg 59 ms)
 13637 ms: stdexec::__queries::get_completion_domain_t<$>::__get_domain<$> (2998 times, avg 4 ms)
 13535 ms: stdexec::__get_submit_result_kind<$> (467 times, avg 28 ms)
 12269 ms: stdexec::__detail::__transform_sender_t<$>::__get_declfn<$> (1537 times, avg 7 ms)
 10940 ms: stdexec::__sync_wait::sync_wait_t::operator()<stdexec::(anonymous na... (126 times, avg 86 ms)
 10507 ms: stdexec::__let::__let_state<$> (504 times, avg 20 ms)  9639 ms: stdexec::__start_detached::start_detached_t::apply_sender<$> (32 times, avg 301 ms)
  8330 ms: stdexec::__op_state<$>::__complete<$> (1254 times, avg 6 ms)
  7977 ms: stdexec::__op_state<$>::start (701 times, avg 11 ms)
  7940 ms: stdexec::(anonymous namespace)::__sexpr<$>::static_connect<$> (826 times, avg 9 ms)
  7867 ms: std::__format::__formatter_int<$>::format<$> (1428 times, avg 5 ms)
  7802 ms: stdexec::__queries::get_completion_scheduler_t<$>::__get_declfn<$> (3840 times, avg 2 ms)
  7566 ms: stdexec::__op_state<$>::__op_state (821 times, avg 9 ms)
  7515 ms: exec::__sequence_sndr::subscribe_t::__get_declfn<$> (43 times, avg 174 ms)
  7429 ms: stdexec::__rcvr<$>::__t::set_value<$> (956 times, avg 7 ms)
  7375 ms: stdexec::__sync_wait::sync_wait_t::apply_sender<$> (254 times, avg 29 ms)
  7251 ms: stdexec::(anonymous namespace)::__sexpr<$>::apply<$> (734 times, avg 9 ms)  6843 ms: exec::__ignore_all_values::__operation<$>::__t (35 times, avg 195 ms)
  6579 ms: exec::__transform_each::__operation<$>::__t (24 times, avg 274 ms)  6378 ms: (anonymous namespace)::wait_for_value<$> (48 times, avg 132 ms)  5766 ms: stdexec::__sync_wait::sync_wait_t::operator()<stdexec::(anonymous na... (7 times, avg 823 ms)
  5574 ms: std::__format::__formatter_str<$>::format<$> (204 times, avg 27 ms)
  5343 ms: stdexec::__op_state<$> (5073 times, avg 1 ms)
  5282 ms: stdexec::__trnsfr::__state<$>::__state (756 times, avg 6 ms)  5182 ms: std::__and_<$> (5371 times, avg 0 ms)

The full report is available here on a gist.

Have these function been already heavily optimized for compilation times? It would be great to liberally use stdexec without having to worry about harming our developer velocity/productivity.