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, 1 insertions, 0 deletions

diff --git a/cop.h b/cop.h
index fd0e12a84a..f7885887eb 100644
--- a/cop.h
+++ b/cop.h
@@ -1047,6 +1047,7 @@ struct context {
 #define CXt_FORMAT     10
 #define CXt_EVAL       11 /* eval'', eval{}, try{} */
 #define CXt_SUBST      12
+#define CXt_DEFER      13
 /* SUBST doesn't feature in all switch statements.  */
 
 /* private flags for CXt_SUB and CXt_FORMAT */