rts: Enforce that mark-region isn't used with -h

As noted in #9666, the mark-region GC is not compatible with heap
profiling. Also add documentation for this flag.

Closes #9666.

docs/users_guide/runtime_control.rst

@@ -411,6 +411,17 @@ performance.
     Note that :rts-flag:`--nonmoving-gc` cannot be used with ``-G1``,
     :rts-flag:`profiling <-hc>` nor :rts-flag:`-c`.
 
+.. rts-flag:: -w
+
+    :default: off
+    :since: a long time ago
+    :reverse: none
+
+    Uses a mark-region garbage collection strategy for the oldest-generation heap.
+    Note that this cannot be used in conjunction with heap profiling
+    (:rts-flag:`-hT`) unless linked against the profiling runtime system with
+    :ghc-flag:`-prof`.
+
 .. rts-flag:: -xn
 
     :default: off

rts/RtsFlags.c

@@ -1850,6 +1850,16 @@ static void normaliseRtsOpts (void)
         barf("The non-moving collector doesn't support -G1");
     }
 
+#if !defined(PROFILING) && !defined(DEBUG)
+    // The mark-region collector is incompatible with heap census unless
+    // we zero slop of blackhole'd thunks, which doesn't happen in the
+    // vanilla way. See #9666.
+    if (RtsFlags.ProfFlags.doHeapProfile && RtsFlags.GcFlags.sweep) {
+        barf("The mark-region collector can only be used with profiling\n"
+             "when linked against the profiled RTS.");
+    }
+#endif
+
     if (RtsFlags.ProfFlags.doHeapProfile != NO_HEAP_PROFILING &&
             RtsFlags.GcFlags.useNonmoving) {
         barf("The non-moving collector doesn't support profiling");