utf8_to_uv_msgs: Create a common macro

Previous commits have allowed the beginning of several of the case
statements in this switch() to have the same code.  This commit creates
a macro encapsulating that code and changes the cases to use it.

The macro continues the enclosing loop if no message needs to be
generated.  This allows the removal of various conditional blocks.  And
it means that these conditions don't break to the bottom of the switch()
if no message is needed.

Braces are needed in one case: so as to not run afoul of C++
initialization crossing

Author: khwilliamson

utf8.c

@@ -2177,54 +2177,50 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
                 /* NOTREACHED */
                 break;
 
+/* If this condition is allowed, no message is to be generated.  Similarly, if
+ * warnings for it aren't enabled.  All of these are controlled only by 'utf8'
+ * warnings.  This macro relies on the GOT and ACCEPT flags being identical. */
+#define COMMON_DEFAULT_REJECTS(p1, p2)                                      \
+                if (   (! (this_problem & rejects))                         \
+                    || ((pack_warn = PACK_WARN(WARN_UTF8,p1,p2)) == 0))     \
+                {                                                           \
+                    continue;                                               \
+                }                                                           \
+
               case UTF8_GOT_EMPTY:
-                if (! (flags & UTF8_ALLOW_EMPTY)) {
+                COMMON_DEFAULT_REJECTS(,);
 
                     /* This so-called malformation is now treated as a bug in
                      * the caller.  If you have nothing to decode, skip calling
                      * this function */
                     assert(0);
-
-                    if (PACK_WARN(WARN_UTF8,,)) {
                         message = Perl_form(aTHX_ "%s (empty string)",
                                                    malformed_text);
-                    }
-                }
-
                 break;
 
               case UTF8_GOT_CONTINUATION:
-                if (! (flags & UTF8_ALLOW_CONTINUATION)) {
-                    if (PACK_WARN(WARN_UTF8,,)) {
+                COMMON_DEFAULT_REJECTS(,);
                         message = Perl_form(aTHX_
                                 "%s: %s (unexpected continuation byte 0x%02x,"
                                 " with no preceding start byte)",
                                 malformed_text,
                                 _byte_dump_string(s0, 1, 0), *s0);
-                    }
-                }
-
                 break;
 
               case UTF8_GOT_SHORT:
-                if (! (flags & UTF8_ALLOW_SHORT)) {
-                    if (PACK_WARN(WARN_UTF8,,)) {
+                COMMON_DEFAULT_REJECTS(,);
                         message = Perl_form(aTHX_
                              "%s: %s (too short; %d byte%s available, need %d)",
                              malformed_text,
                              _byte_dump_string(s0, send - s0, 0),
                              (int)avail_len,
                              avail_len == 1 ? "" : "s",
                              (int)expectlen);
-                    }
-                }
-
                 break;
 
               case UTF8_GOT_NON_CONTINUATION:
-                if (! (flags & UTF8_ALLOW_NON_CONTINUATION)) {
-                    if (PACK_WARN(WARN_UTF8,,)) {
-
+               {
+                COMMON_DEFAULT_REJECTS(,);
                         /* If we don't know for sure that the input length is
                          * valid, avoid as much as possible reading past the
                          * end of the buffer */
@@ -2236,10 +2232,8 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
                                                             printlen,
                                                             s - s0,
                                                             (int) expectlen));
-                    }
-                }
-
                 break;
+               }
 
               case UTF8_GOT_LONG: