As in the title. Reason: currently I have an asynchronous logger. If one call the macro ``` LOG("{a} {b}", "a"_a=x, "b"_a=y); ``` I'd like to be able to 1. compute `x1 = TransformOnCallingThread(x); y1 = TransformOnCallingThread(y);` 2. send the format string, `x1` and `y1` to the logging thread with an inter-thread queue 3. call `fmt::format("{a} {b}", "a"_a=x1, "b"_a=y1)` Currently, as far as I can see, (1) requires reaching into the `fmt::detail` namespace.