put std/threads under the umbrella of nimPreviewSlimSystem (#20711)

* put `std/threads` under the umbrella of `nimPreviewSlimSystem`

* add changelog

* fixes tests

* fixes tests again

* fixes tests

lib/pure/concurrency/threadpool.nim

@@ -24,7 +24,7 @@ when not compileOption("threads"):
 import cpuinfo, cpuload, locks, os
 
 when defined(nimPreviewSlimSystem):
-  import std/assertions
+  import std/[assertions, threads]
 
 {.push stackTrace:off.}
 

lib/pure/random.nim

@@ -76,7 +76,7 @@ import algorithm, math
 import std/private/since
 
 when defined(nimPreviewSlimSystem):
-  import std/assertions
+  import std/[assertions]
 
 include system/inclrtl
 {.push debugger: off.}

lib/std/threads.nim

@@ -95,16 +95,6 @@ when defined(zephyr):
 
 
 
-proc onThreadDestruction*(handler: proc () {.closure, gcsafe, raises: [].}) =
-  ## Registers a *thread local* handler that is called at the thread's
-  ## destruction.
-  ##
-  ## A thread is destructed when the `.thread` proc returns
-  ## normally or when it raises an exception. Note that unhandled exceptions
-  ## in a thread nevertheless cause the whole process to die.
-  nimThreadDestructionHandlers.add handler
-
-
 {.push stack_trace:off.}
 when defined(windows):
   proc threadProcWrapper[TArg](closure: pointer): int32 {.stdcall.} =

lib/system.nim

@@ -2093,9 +2093,10 @@ when not defined(js):
   when hasThreadSupport:
     when hostOS != "standalone":
       include system/threadimpl
-
-      import std/threads
-      export threads
+      when not defined(nimPreviewSlimSystem):
+        {.deprecated: "threads is about to move out of system; use `-d:nimPreviewSlimSystem` and import `std/threads`".}
+        import std/threads
+        export threads
 
   elif not defined(nogc) and not defined(nimscript):
     when not defined(useNimRtl) and not defined(createNimRtl): initStackBottom()

lib/system/threadimpl.nim

@@ -33,6 +33,15 @@ template afterThreadRuns() =
   for i in countdown(nimThreadDestructionHandlers.len-1, 0):
     nimThreadDestructionHandlers[i]()
 
+proc onThreadDestruction*(handler: proc () {.closure, gcsafe, raises: [].}) =
+  ## Registers a *thread local* handler that is called at the thread's
+  ## destruction.
+  ##
+  ## A thread is destructed when the `.thread` proc returns
+  ## normally or when it raises an exception. Note that unhandled exceptions
+  ## in a thread nevertheless cause the whole process to die.
+  nimThreadDestructionHandlers.add handler
+
 when defined(boehmgc):
   type GCStackBaseProc = proc(sb: pointer, t: pointer) {.noconv.}
   proc boehmGC_call_with_stack_base(sbp: GCStackBaseProc, p: pointer)