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, 29 insertions, 14 deletions

diff --git a/feature.h b/feature.h
index 29522eedb5..23cc5f3d31 100644
--- a/feature.h
+++ b/feature.h
@@ -16,20 +16,21 @@
 #define FEATURE_BITWISE_BIT              0x0002
 #define FEATURE___SUB___BIT              0x0004
 #define FEATURE_MYREF_BIT                0x0008
-#define FEATURE_EVALBYTES_BIT            0x0010
-#define FEATURE_FC_BIT                   0x0020
-#define FEATURE_INDIRECT_BIT             0x0040
-#define FEATURE_ISA_BIT                  0x0080
-#define FEATURE_MULTIDIMENSIONAL_BIT     0x0100
-#define FEATURE_POSTDEREF_QQ_BIT         0x0200
-#define FEATURE_REFALIASING_BIT          0x0400
-#define FEATURE_SAY_BIT                  0x0800
-#define FEATURE_SIGNATURES_BIT           0x1000
-#define FEATURE_STATE_BIT                0x2000
-#define FEATURE_SWITCH_BIT               0x4000
-#define FEATURE_TRY_BIT                  0x8000
-#define FEATURE_UNIEVAL_BIT              0x10000
-#define FEATURE_UNICODE_BIT              0x20000
+#define FEATURE_DEFER_BIT                0x0010
+#define FEATURE_EVALBYTES_BIT            0x0020
+#define FEATURE_FC_BIT                   0x0040
+#define FEATURE_INDIRECT_BIT             0x0080
+#define FEATURE_ISA_BIT                  0x0100
+#define FEATURE_MULTIDIMENSIONAL_BIT     0x0200
+#define FEATURE_POSTDEREF_QQ_BIT         0x0400
+#define FEATURE_REFALIASING_BIT          0x0800
+#define FEATURE_SAY_BIT                  0x1000
+#define FEATURE_SIGNATURES_BIT           0x2000
+#define FEATURE_STATE_BIT                0x4000
+#define FEATURE_SWITCH_BIT               0x8000
+#define FEATURE_TRY_BIT                  0x10000
+#define FEATURE_UNIEVAL_BIT              0x20000
+#define FEATURE_UNICODE_BIT              0x40000
 
 #define FEATURE_BUNDLE_DEFAULT	0
 #define FEATURE_BUNDLE_510	1
@@ -80,6 +81,12 @@
 	 FEATURE_IS_ENABLED_MASK(FEATURE_TRY_BIT) \
     )
 
+#define FEATURE_DEFER_IS_ENABLED \
+    ( \
+	CURRENT_FEATURE_BUNDLE == FEATURE_BUNDLE_CUSTOM && \
+	 FEATURE_IS_ENABLED_MASK(FEATURE_DEFER_BIT) \
+    )
+
 #define FEATURE_STATE_IS_ENABLED \
     ( \
 	(CURRENT_FEATURE_BUNDLE >= FEATURE_BUNDLE_510 && \
@@ -268,6 +275,14 @@ S_magic_sethint_feature(pTHX_ SV *keysv, const char *keypv, STRLEN keylen,
             }
             return;
 
+        case 'd':
+            if (keylen == sizeof("feature_defer")-1
+                 && memcmp(subf+1, "efer", keylen - sizeof("feature_")) == 0) {
+                mask = FEATURE_DEFER_BIT;
+                break;
+            }
+            return;
+
         case 'e':
             if (keylen == sizeof("feature_evalbytes")-1
                  && memcmp(subf+1, "valbytes", keylen - sizeof("feature_")) == 0) {