implement sizeof and alignof operator (manually squashed #5664) (#9356)

lib/system.nim

@@ -682,13 +682,30 @@ proc sizeof*[T](x: T): int {.magic: "SizeOf", noSideEffect.}
   ## that one never needs to know ``x``'s size. As a special semantic rule,
   ## ``x`` may also be a type identifier (``sizeof(int)`` is valid).
   ##
-  ## Limitations: If used within nim VM context ``sizeof`` will only work
-  ## for simple types.
+  ## Limitations: If used for types that are imported from C or C++,
+  ## sizeof should fallback to the ``sizeof`` in the C compiler. The
+  ## result isn't available for the Nim compiler and therefore can't
+  ## be used inside of macros.
   ##
   ## .. code-block:: nim
   ##  sizeof('A') #=> 1
   ##  sizeof(2) #=> 8
 
+when defined(nimHasalignOf):
+  proc alignof*[T](x: T): int {.magic: "AlignOf", noSideEffect.}
+  proc alignof*(x: typedesc): int {.magic: "AlignOf", noSideEffect.}
+
+  proc offsetOfDotExpr(typeAccess: typed): int {.magic: "OffsetOf", noSideEffect, compileTime.}
+
+  template offsetOf*[T](t: typedesc[T]; member: untyped): int =
+    var tmp: T
+    offsetOfDotExpr(tmp.member)
+
+  template offsetOf*[T](value: T; member: untyped): int =
+    offsetOfDotExpr(value.member)
+
+  #proc offsetOf*(memberaccess: typed): int {.magic: "OffsetOf", noSideEffect.}
+
 when defined(nimtypedescfixed):
   proc sizeof*(x: typedesc): int {.magic: "SizeOf", noSideEffect.}