fix #16752: threadvar now works with importcpp types; osx now uses native TLS (--tlsEmulation:off), which can be orders of magnitude faster (#16750)

* osx now uses native TLS, which can be orders of magnitude faster

* add {.cppNonPod.}

* improve test

* changelog, docs, disable part of windows test

tests/misc/mtlsemulation.h

@@ -0,0 +1,37 @@
+#include <stdio.h>
+
+struct Foo1 {
+  /*
+  uncommenting would give:
+  error: initializer for thread-local variable must be a constant expression
+  N_LIB_PRIVATE NIM_THREADVAR Foo1 g1__9brEZhPEldbVrNpdRGmWESA;
+  */
+  // Foo1() noexcept { }
+
+  /*
+  uncommenting would give:
+  error: type of thread-local variable has non-trivial destruction
+  */
+  // ~Foo1() { }
+  int x;
+};
+
+struct Foo2 {
+  Foo2() noexcept { }
+  ~Foo2() { }
+  int x;
+};
+
+static int ctorCalls = 0;
+static int dtorCalls = 0;
+
+struct Foo3 {
+  Foo3() noexcept {
+    ctorCalls = ctorCalls + 1;
+    x = 10;
+  }
+  ~Foo3() {
+    dtorCalls = dtorCalls + 1;
+  }
+  int x;
+};

tests/misc/ttlsemulation.nim

@@ -0,0 +1,75 @@
+discard """
+  matrix: "-d:nimTtlsemulationCase1 --threads --tlsEmulation:on; -d:nimTtlsemulationCase2 --threads --tlsEmulation:off; -d:nimTtlsemulationCase3 --threads"
+  targets: "c cpp"
+"""
+
+#[
+tests for: `.cppNonPod`, `--tlsEmulation`
+]#
+
+import std/sugar
+
+block:
+  # makes sure the logic in config/nim.cfg or testament doesn't interfere with `--tlsEmulation` so we test the right thing.
+  when defined(nimTtlsemulationCase1):
+    doAssert compileOption("tlsEmulation")
+  elif defined(nimTtlsemulationCase2):
+    doAssert not compileOption("tlsEmulation")
+  elif defined(nimTtlsemulationCase3):
+    when defined(osx):
+      doAssert not compileOption("tlsEmulation")
+  else:
+    doAssert false
+
+block:
+  proc main1(): int =
+    var g0 {.threadvar.}: int
+    g0.inc
+    g0
+  let s = collect:
+    for i in 0..<3: main1()
+  doAssert s == @[1,2,3]
+
+when defined(cpp): # bug #16752
+  when defined(windows) and defined(nimTtlsemulationCase2):
+    discard # xxx this failed with exitCode 1
+  else:
+    type Foo1 {.importcpp: "Foo1", header: "mtlsemulation.h".} = object
+      x: cint
+    type Foo2 {.cppNonPod, importcpp: "Foo2", header: "mtlsemulation.h".} = object
+      x: cint
+
+    var ctorCalls {.importcpp.}: cint
+    var dtorCalls {.importcpp.}: cint
+    type Foo3 {.cppNonPod, importcpp: "Foo3", header: "mtlsemulation.h".} = object
+      x: cint
+
+    proc sub(i: int) =
+      var g1 {.threadvar.}: Foo1
+      var g2 {.threadvar.}: Foo2
+      var g3 {.threadvar.}: Foo3
+      discard g1
+      discard g2
+
+      # echo (g3.x, ctorCalls, dtorCalls)
+      when compileOption("tlsEmulation"):
+        # xxx bug
+        discard
+      else:
+        doAssert g3.x.int == 10 + i
+        doAssert ctorCalls == 2
+      doAssert dtorCalls == 1
+      g3.x.inc
+
+    proc main() =
+      doAssert ctorCalls == 0
+      doAssert dtorCalls == 0
+      block:
+        var f3: Foo3
+        doAssert f3.x == 10
+      doAssert ctorCalls == 1
+      doAssert dtorCalls == 1
+
+      for i in 0..<3:
+        sub(i)
+    main()