Create `defer` syntax and `OP_PUSHDEFER` opcode

Adds syntax `defer { BLOCK }` to create a deferred block; code that is
deferred until the scope exits. This syntax is guarded by

  use feature 'defer';

Adds a new opcode, `OP_PUSHDEFER`, which is a LOGOP whose `op_other` field
gives the start of an optree to be deferred until scope exit. That op
pointer will be stored on the save stack and invoked as part of scope
unwind.

Included is support for `B::Deparse` to deparse the optree back into
syntax.

1 files changed, 38 insertions, 0 deletions

diff --git a/op.c b/op.c
index 0a4f2b803c..d93b62a6d6 100644
--- a/op.c
+++ b/op.c
@@ -10612,6 +10612,43 @@ Perl_newWHENOP(pTHX_ OP *cond, OP *block)
     return newGIVWHENOP(cond_op, block, OP_ENTERWHEN, OP_LEAVEWHEN, 0);
 }
 
+/*
+=for apidoc newDEFEROP
+
+Constructs and returns a deferred-block statement that implements the
+C<defer> semantics.  The C<block> optree is consumed by this function and
+becomes part of the returned optree.
+
+The C<flags> argument is currently ignored.
+
+=cut
+ */
+
+OP *
+Perl_newDEFEROP(pTHX_ I32 flags, OP *block)
+{
+    OP *o, *start, *blockfirst;
+
+    PERL_ARGS_ASSERT_NEWDEFEROP;
+    PERL_UNUSED_ARG(flags);
+
+    start = LINKLIST(block);
+
+    /* Hide the block inside an OP_NULL with no exection */
+    block = newUNOP(OP_NULL, 0, block);
+    block->op_next = block;
+
+    o = (OP *)alloc_LOGOP(OP_PUSHDEFER, block, start);
+    o->op_flags |= OPf_WANT_VOID;
+
+    /* Terminate the block */
+    blockfirst = cUNOPx(block)->op_first;
+    assert(blockfirst->op_type == OP_SCOPE || blockfirst->op_type == OP_LEAVE);
+    blockfirst->op_next = NULL;
+
+    return o;
+}
+
 /* must not conflict with SVf_UTF8 */
 #define CV_CKPROTO_CURSTASH	0x1
 
@@ -17748,6 +17785,7 @@ Perl_rpeep(pTHX_ OP *o)
         case OP_OR:
         case OP_DOR:
         case OP_CMPCHAIN_AND:
+        case OP_PUSHDEFER:
             while (cLOGOP->op_other->op_type == OP_NULL)
                 cLOGOP->op_other = cLOGOP->op_other->op_next;
             while (o->op_next && (   o->op_type == o->op_next->op_type