Threads: add a one time warning to a no-arg nthreads()

[IanButterworth]

It seems helpful to communicate the (soft) deprecation of a no-arg nthreads() a bit more clearly.

re. #48580 (comment)

[vtjnash]

There is not one simple replacement, as many usages must be replaced with task_local_storage instead or dropped entirely. But the replacement is almost never nthreads(:interactive)

Suggested change

	replaced with `nthreads(:default)`, or `nthreads(:interactive)` if interactive threads are requrested.""" maxlog = 1
	replaced with `nthreads(:default)` or `task_local_storage`, as appropriate.

[carstenbauer]

Also, AFAIU, the "number of threads" is - at least in some cases - a dynamic concept now and potentially/likely(?) even more so in the future. That is, the result of nthreads (with or without argument) isn't necessarily constant (per Julia session) anymore. This is something that users would need to be informed/warned about as well.

[IanButterworth]

Updated with both suggestions

[IanButterworth]

This will be quite noisy. Maybe that's a good thing?

from the doctests job, due to https://github.com/JuliaData/Parsers.jl/blob/78699fc450ca8c14ac073ddbec28d57cba31bb3c/src/Parsers.jl#L422

Precompiling project...
--
  | ✓ IOCapture
  | ✓ ANSIColoredPrinters
  | ✓ DocStringExtensions
  | ✓ Parsers
  | ✓ JSON
  | ✓ Documenter
  | 6 dependencies successfully precompiled in 11 seconds
  | 3 dependencies had warnings during precompilation:
  | ┌ Parsers [69de0a69-1ddd-5017-9359-2bf0b02dc9f0]
  | │  WARNING: method definition for dpeekbyte at /cache/build/default-amdci5-3/julialang/julia-master/doc/deps/packages/Parsers/34hDN/src/utils.jl:199 declares type variable T but does not use it.
  | └
  | ┌ JSON [682c06a0-de6a-54ab-a142-c8b1cf79cde6]
  | │  ┌ Warning: In julia 1.9+ which supports threadpools (and changing threadpool sizes during runtime),
  | │  │ use of threads should be in the context of which threadpool is going to be used, so `nthreads()` should be
  | │  │ replaced with `nthreads(:default)`, or `task_local_storage`, as appropriate.
  | │  └ @ Base.Threads deprecated.jl:335
  | └
  | ┌ Documenter [e30172f5-a6a5-5a46-863b-614d45cd2de4]
  | │  ┌ Warning: In julia 1.9+ which supports threadpools (and changing threadpool sizes during runtime),
  | │  │ use of threads should be in the context of which threadpool is going to be used, so `nthreads()` should be
  | │  │ replaced with `nthreads(:default)`, or `task_local_storage`, as appropriate.
  | │  └ @ Base.Threads deprecated.jl:335
  | └
  | ┌ Warning: In julia 1.9+ which supports threadpools (and changing threadpool sizes during runtime),
  | │ use of threads should be in the context of which threadpool is going to be used, so `nthreads()` should be
  | │ replaced with `nthreads(:default)`, or `task_local_storage`, as appropriate.
  | └ @ Base.Threads deprecated.jl:335

Also, it'd be nice if the warning warned about the nthreads() call site, not the @warn call site

@ Base.Threads deprecated.jl:335

[IanButterworth]

What about only warn if an interactive threadpool is available?

@eval Threads function nthreads()
    if Threads.nthreads(:interactive) > 0
        @warn """
        An interactive threadpool is available, use of threads should be in the context of which threadpool is going to
        be used, so `nthreads()` should be replaced with `nthreads(:default)` or `nthreads(:interactive)`""" maxlog = 1
    end
    return threadpoolsize()
end

[JeffBezanson]

This is silly. Just let people call nthreads! There is a default threadpool, so if you don't specify one you get the default, no problem. The right place to explain all this is in the manual.

[IanButterworth]

ok. It's not really being deprecated then, right?

The manual uses nthreads() in various places, and even the Threadpools section does https://docs.julialang.org/en/v1.10-dev/manual/multi-threading/#man-threadpools

So it seems confusing to say it is (which is what prompted opening this PR)

[simsurace]

The original issue hasn't been addressed. I opened a new one: #48644