Implement doAssertRaises (#6819)

lib/system.nim

@@ -3752,6 +3752,7 @@ template assert*(cond: bool, msg = "") =
   ## that ``AssertionError`` is hidden from the effect system, so it doesn't
   ## produce ``{.raises: [AssertionError].}``. This exception is only supposed
   ## to be caught by unit testing frameworks.
+  ##
   ## The compiler may not generate any code at all for ``assert`` if it is
   ## advised to do so through the ``-d:release`` or ``--assertions:off``
   ## `command line switches <nimc.html#command-line-switches>`_.
@@ -4009,3 +4010,21 @@ when defined(nimHasRunnableExamples):
 else:
   template runnableExamples*(body: untyped) =
     discard
+
+template doAssertRaises*(exception, code: untyped): typed =
+  ## Raises ``AssertionError`` if specified ``code`` does not raise the
+  ## specified exception.
+  runnableExamples:
+    doAssertRaises(ValueError):
+      raise newException(ValueError, "Hello World")
+
+  try:
+    block:
+      code
+    raiseAssert(astToStr(exception) & " wasn't raised by:\n" & astToStr(code))
+  except exception:
+    discard
+  except Exception as exc:
+    raiseAssert(astToStr(exception) &
+                " wasn't raised, another error was raised instead by:\n"&
+                astToStr(code))