Enable `use feature 'module_true'`

Per RFC 18, whenever `use feature 'module_true';` is enabled in a scope,
any file required with `require` has an implicit return value of true
and will not trigger the "did not return a true value" error condition.

This includes logic to use the OPf_SPECIAL flag for OP_RETURN listops to
indicate that the module_true feature is in effect when it executes.
This flag plays no role unless the OP_RETURN tail calls the pp_leaveeval
logic, so it doesn't affect normal sub returns.

1 files changed, 24 insertions, 11 deletions

diff --git a/feature.h b/feature.h
index 08061e12c8..900a1b990e 100644
--- a/feature.h
+++ b/feature.h
@@ -22,16 +22,17 @@
 #define FEATURE_FC_BIT                      0x0080
 #define FEATURE_INDIRECT_BIT                0x0100
 #define FEATURE_ISA_BIT                     0x0200
-#define FEATURE_MULTIDIMENSIONAL_BIT        0x0400
-#define FEATURE_POSTDEREF_QQ_BIT            0x0800
-#define FEATURE_REFALIASING_BIT             0x1000
-#define FEATURE_SAY_BIT                     0x2000
-#define FEATURE_SIGNATURES_BIT              0x4000
-#define FEATURE_STATE_BIT                   0x8000
-#define FEATURE_SWITCH_BIT                  0x10000
-#define FEATURE_TRY_BIT                     0x20000
-#define FEATURE_UNIEVAL_BIT                 0x40000
-#define FEATURE_UNICODE_BIT                 0x80000
+#define FEATURE_MODULE_TRUE_BIT             0x0400
+#define FEATURE_MULTIDIMENSIONAL_BIT        0x0800
+#define FEATURE_POSTDEREF_QQ_BIT            0x1000
+#define FEATURE_REFALIASING_BIT             0x2000
+#define FEATURE_SAY_BIT                     0x4000
+#define FEATURE_SIGNATURES_BIT              0x8000
+#define FEATURE_STATE_BIT                   0x10000
+#define FEATURE_SWITCH_BIT                  0x20000
+#define FEATURE_TRY_BIT                     0x40000
+#define FEATURE_UNIEVAL_BIT                 0x80000
+#define FEATURE_UNICODE_BIT                 0x100000
 
 #define FEATURE_BUNDLE_DEFAULT	0
 #define FEATURE_BUNDLE_510	1
@@ -151,6 +152,13 @@
 	 FEATURE_IS_ENABLED_MASK(FEATURE___SUB___BIT)) \
     )
 
+#define FEATURE_MODULE_TRUE_IS_ENABLED \
+    ( \
+	CURRENT_FEATURE_BUNDLE == FEATURE_BUNDLE_537 \
+     || (CURRENT_FEATURE_BUNDLE == FEATURE_BUNDLE_CUSTOM && \
+	 FEATURE_IS_ENABLED_MASK(FEATURE_MODULE_TRUE_BIT)) \
+    )
+
 #define FEATURE_REFALIASING_IS_ENABLED \
     ( \
 	CURRENT_FEATURE_BUNDLE == FEATURE_BUNDLE_CUSTOM && \
@@ -333,7 +341,12 @@ S_magic_sethint_feature(pTHX_ SV *keysv, const char *keypv, STRLEN keylen,
             return;
 
         case 'm':
-            if (keylen == sizeof("feature_more_delims")-1
+            if (keylen == sizeof("feature_module_true")-1
+                 && memcmp(subf+1, "odule_true", keylen - sizeof("feature_")) == 0) {
+                mask = FEATURE_MODULE_TRUE_BIT;
+                break;
+            }
+            else if (keylen == sizeof("feature_more_delims")-1
                  && memcmp(subf+1, "ore_delims", keylen - sizeof("feature_")) == 0) {
                 mask = FEATURE_MORE_DELIMS_BIT;
                 break;