Add a subsignature_append_fence_op()

A "fence op" is a miscellaneous op fragment that performs some work for
side-effects during processing of a subroutine signature. In terms of
timing, it will run at some time after any previously-defined arguments
have been assigned from argument values passed in by the caller, but
before any defaulting expressions for parameters that come after it are
run.

We specifically make no guarantees about whether parameters defined
after this op have had their values assigned, nor whether defaulting
expressions of earlier parameters have already been invoked. This is
intentional because upcoming changes will change the order of these.

The intention here is that method subroutines will use a fence op for
the `OP_METHSTART` behaviour, ensuring that subsequent defaulting
expressions can see the values of field bindings established by
processing the `$self` parameter.

Author: leonerd

embed.fnc

@@ -3110,6 +3110,8 @@ CRp	|NV	|str_to_version |NN SV *sv
 : Used in pp_ctl.c
 p	|void	|sub_crush_depth|NN CV *cv
 : Used in perly.y
+p	|void	|subsignature_append_fence_op				\
+				|NN OP *o
 p	|void	|subsignature_append_positional 			\
 				|PADOFFSET padix			\
 				|OPCODE defmode 			\

embed.h

@@ -1162,6 +1162,7 @@
 #   define sighandler1                          Perl_sighandler1
 #   define sighandler3                          Perl_sighandler3
 #   define sub_crush_depth(a)                   Perl_sub_crush_depth(aTHX_ a)
+#   define subsignature_append_fence_op(a)      Perl_subsignature_append_fence_op(aTHX_ a)
 #   define subsignature_append_positional(a,b,c) Perl_subsignature_append_positional(aTHX_ a,b,c)
 #   define subsignature_append_slurpy(a,b)      Perl_subsignature_append_slurpy(aTHX_ a,b)
 #   define subsignature_finish()                Perl_subsignature_finish(aTHX)

op.c

@@ -16479,7 +16479,7 @@ struct yy_parser_signature {
     UV          elems;      /* number of signature elements seen so far */
     UV          optelems;   /* number of optional signature elems seen */
     char        slurpy;     /* the sigil of the slurpy var (or null) */
-    OP         *elemops;    /* NULL, or an OP_LINESEQ of individual element ops */
+    OP         *elemops;    /* NULL, or an OP_LINESEQ of individual element and fence ops */
 };
 
 static void
@@ -16518,6 +16518,24 @@ Perl_subsignature_start(pTHX)
     PL_parser->signature = signature;
 }
 
+/* Appends another arbitrary optree into the accumulated set of signature-
+ * handling ops. This op will be invoked at some time after all of the
+ * parameters already present have received their values, but before any of
+ * the defaulting expressions for later parameters are executed.
+ */
+
+void
+Perl_subsignature_append_fence_op(pTHX_ OP *o)
+{
+    PERL_ARGS_ASSERT_SUBSIGNATURE_APPEND_FENCE_OP;
+    assert(PL_parser);
+    yy_parser_signature *signature = PL_parser->signature;
+    assert(signature);
+
+    signature->elemops = op_append_elem(OP_LINESEQ, signature->elemops,
+            o);
+}
+
 /* Appends another positional scalar parameter to the accumulated set of
  * subroutine params. `padix` may be zero, but if not it must be the pad
  * index of a scalar pad lexical to store the incoming argument value into.