rts: Fix use of sized array in Heap.h

Sized arrays cannot be used in headers that might be imported from C++.

Fixes #20199.

3 files changed, 9 insertions, 5 deletions

diff --git a/includes/rts/storage/Heap.h b/includes/rts/storage/Heap.h
index b950df76e5..b3b1efaf9f 100644
--- a/includes/rts/storage/Heap.h
+++ b/includes/rts/storage/Heap.h
@@ -18,10 +18,14 @@ void heap_view_closure_ptrs_in_pap_payload(StgClosure *ptrs[], StgWord *nptrs
 StgWord heap_view_closureSize(StgClosure *closure);
 
 /*
- * Collect the pointers of a closure into the given array. `size` should be
+ * Collect the pointers of a closure into the given array. The given array should be
  * large enough to hold all collected pointers e.g.
  * `heap_view_closureSize(closure)`. Returns the number of pointers collected.
  * The caller must ensure that `closure` is not modified (or moved by the GC)
  * for the duration of the call to `collect_pointers`.
+ *
+ * In principle this is
+ *   StgWord collect_pointers(StgClosure *closure, StgWord size, StgClosure *ptrs[size]);
+ * but we cannot write this and retain C++ compatibility.
  */
-StgWord collect_pointers(StgClosure *closure, StgWord size, StgClosure *ptrs[size]);
+StgWord collect_pointers(StgClosure *closure, StgClosure *ptrs[]);
diff --git a/libraries/ghc-heap/tests/create_tso.c b/libraries/ghc-heap/tests/create_tso.c
index 4b00333197..064a88d47b 100644
--- a/libraries/ghc-heap/tests/create_tso.c
+++ b/libraries/ghc-heap/tests/create_tso.c
@@ -17,7 +17,7 @@ void unpack_closure
     StgWord closureSizeW = heap_view_closureSize(inClosure);
     int closureSizeB = sizeof(StgWord) * closureSizeW;
     StgClosure ** pointers = malloc(closureSizeB);
-    *outPointersSize = collect_pointers(inClosure, closureSizeW, pointers);
+    *outPointersSize = collect_pointers(inClosure, pointers);
     *outPointers = pointers;
 
     // Copy the heap rep.
diff --git a/rts/Heap.c b/rts/Heap.c
index 627b91d5db..9f9154de27 100644
--- a/rts/Heap.c
+++ b/rts/Heap.c
@@ -77,7 +77,7 @@ void heap_view_closure_ptrs_in_pap_payload(StgClosure *ptrs[], StgWord *nptrs
 }
 
 // See Heap.h
-StgWord collect_pointers(StgClosure *closure, StgWord size, StgClosure *ptrs[size]) {
+StgWord collect_pointers(StgClosure *closure, StgClosure *ptrs[]) {
     StgClosure **end;
     const StgInfoTable *info = get_itbl(closure);
     StgWord nptrs = 0;
@@ -250,7 +250,7 @@ StgMutArrPtrs *heap_view_closurePtrs(Capability *cap, StgClosure *closure) {
     // the closure and then we can allocate space on the heap and copy them
     // there
     StgClosure *ptrs[size];
-    StgWord nptrs = collect_pointers(closure, size, ptrs);
+    StgWord nptrs = collect_pointers(closure, ptrs);
 
     size = nptrs + mutArrPtrsCardTableSize(nptrs);
     StgMutArrPtrs *arr =