`<chrono>`: Can we avoid including `<format>`?

On reddit, u/tgolyi reports:

std::format is very cool, but why did you include <format> to <chrono>? This change alone slowed down compilation for our project by 20%. Is it required by the standard now to include <format> (to support std::formatter specialization)? It's just that <chrono> is included by so many other headers like <mutex> and <thread> and Qt too.

Separately from any throughput improvements for <format>, I believe it should be possible to remove this dependency:

https://github.com/microsoft/STL/blob/4c862ee11ce956556b810a813e77b0f8f97fb642/stl/inc/chrono#L37-L40

The fix would presumably involve some combination of:

• Forward declarations (of formatter etc.)
• Extracting common machinery to a centralized header somewhere (used sparingly, since this costs throughput)
• Possibly templatizing stuff so that it isn't instantiated until needed (need to follow the Standard's lookup rules)

Related to DevCom-10046069 "Including chrono and compiling with -std:c++20 is ten time slower than before" and internal VSO-1543202 / AB#1543202 .

Start at stl/inc/chrono lines 37-40 and trace why includes , including the formatter declarations and standard lookup requirements. Evaluate the proposed forward declarations or shared machinery, with completion requiring removal of the unnecessary dependency without violating the standard or regressing compilation throughput.