Provide dispatcher-drop-arg syscall

And fix up passing along derived captures as we delegate between
dispatchers.

src/core/callstack.h

@@ -141,15 +141,18 @@ struct MVMCallStackDispatchRecord {
     /* Commonalities of all records. */
     MVMCallStackRecord common;
 
-    /* The argument capture going into the dispatch (always an object, but
-     * stored this way to let this address be safely used as an argument
-     * buffer. */
-    MVMRegister initial_capture;
+    /* The argument capture that started out this dispatch. */
+    MVMObject *initial_capture;
 
     /* An MVMArray of derived captures, produced during the dispatch
      * process. */
     MVMObject *derived_captures;
 
+    /* Register holding the current capture. Starts out as the initial capture
+     * and then, in the case we delegate, becomes the one sent to the delegate
+     * dispatcher. */
+    MVMRegister current_capture;
+
     /* The outcome of the dispatch. */
     MVMDispProgramOutcome outcome;
     /* TODO */

src/disp/program.c

@@ -4,9 +4,10 @@
 static void run_dispatch(MVMThreadContext *tc, MVMCallStackDispatchRecord *record,
         MVMDispDefinition *disp, MVMObject *capture, MVMuint32 *thunked) {
     MVMCallsite *disp_callsite = MVM_callsite_get_common(tc, MVM_CALLSITE_ID_INV_ARG);
+    record->current_capture.o = capture;
     MVMArgs dispatch_args = {
         .callsite = disp_callsite,
-        .source = &(record->initial_capture),
+        .source = &(record->current_capture),
         .map = MVM_args_identity_map(tc, disp_callsite)
     };
     MVMObject *dispatch = disp->dispatch;
@@ -32,7 +33,7 @@ void MVM_disp_program_run_dispatch(MVMThreadContext *tc, MVMDispDefinition *disp
     /* Push a dispatch recording frame onto the callstack; this is how we'll
      * keep track of the current recording state. */
     MVMCallStackDispatchRecord *record = MVM_callstack_allocate_dispatch_record(tc);
-    record->initial_capture.o = capture;
+    record->initial_capture = capture;
     record->derived_captures = NULL;
     run_dispatch(tc, record, disp, capture, NULL);
 }
@@ -41,7 +42,7 @@ void MVM_disp_program_run_dispatch(MVMThreadContext *tc, MVMDispDefinition *disp
  * being recorded dispatch program. */
 static void ensure_known_capture(MVMThreadContext *tc, MVMCallStackDispatchRecord *record,
         MVMObject *capture) {
-    if (capture != record->initial_capture.o) {
+    if (capture != record->initial_capture) {
         int found = 0;
         if (record->derived_captures) {
             MVMint64 elems = MVM_repr_elems(tc, record->derived_captures);

src/disp/syscall.c

@@ -45,6 +45,25 @@ static MVMDispSysCall dispatcher_delegate = {
     .hash_handle = EMPTY_HASH_HANDLE
 };
 
+/* dispatcher-drop-arg */
+static void dispatcher_drop_arg_impl(MVMThreadContext *tc, MVMArgs arg_info) {
+    MVMArgProcContext arg_ctx;
+    MVM_args_proc_setup(tc, &arg_ctx, arg_info);
+    MVMObject *capture = MVM_args_get_required_pos_obj(tc, &arg_ctx, 0);
+    MVMint64 idx = MVM_args_get_required_pos_int(tc, &arg_ctx, 1);
+    MVMObject *derived = MVM_disp_program_record_capture_drop_arg(tc, capture, (MVMuint32)idx);
+    MVM_args_set_result_obj(tc, derived, MVM_RETURN_CURRENT_FRAME);
+}
+static MVMDispSysCall dispatcher_drop_arg = {
+    .c_name = "dispatcher-drop-arg",
+    .implementation = dispatcher_drop_arg_impl,
+    .min_args = 2,
+    .max_args = 2,
+    .expected_kinds = { MVM_CALLSITE_ARG_OBJ, MVM_CALLSITE_ARG_INT },
+    .expected_reprs = { MVM_REPR_ID_MVMCapture, 0 },
+    .expected_concrete = { 1, 1 },
+    .hash_handle = EMPTY_HASH_HANDLE
+};
 
 /* Add all of the syscalls into the hash. */
 MVM_STATIC_INLINE void add_to_hash(MVMThreadContext *tc, MVMDispSysCall *syscall) {
@@ -63,6 +82,7 @@ void MVM_disp_syscall_setup(MVMThreadContext *tc) {
     MVM_gc_allocate_gen2_default_set(tc);
     add_to_hash(tc, &dispatcher_register);
     add_to_hash(tc, &dispatcher_delegate);
+    add_to_hash(tc, &dispatcher_drop_arg);
     MVM_gc_allocate_gen2_default_clear(tc);
 }