Set RXf_RTRIM if the pattern *is* /[[:space:]]+\z/u

We can be more flexible in our selection of patterns soon, but for now,
start with a minimal testcase.

We don't actually *do* anything yet, so we must "fail" in re_intuit_start.

regcomp.c

@@ -8488,6 +8488,15 @@ Perl_re_op_compile(pTHX_ SV ** const patternp, int pat_count,
                   && *(STRING(first)) == ' '
                   && OP(regnext(first)) == END )
             RExC_rx->extflags |= (RXf_SKIPWHITE|RXf_WHITE);
+        else if (fop == PLUS
+                 && nop == POSIXU && FLAGS(next) == _CC_SPACE) {
+            regnode *second = regnext(first);
+            regnode *third = OP(second) == EOS ? regnext(second) : NULL;
+            if (third && OP(third) == END) {
+                /* /[[:space:]]+\z/u */
+                RExC_rx->extflags |= RXf_RTRIM;
+            }
+        }
 
     }
 

regexec.c

@@ -921,6 +921,12 @@ Perl_re_intuit_start(pTHX_
     /* not actually used within intuit, but zero for safety anyway */
     reginfo->poscache_maxiter = 0;
 
+    if(prog->extflags & RXf_RTRIM) {
+        DEBUG_EXECUTE_r(Perl_re_printf( aTHX_
+                              "  rtrim intuit not yet implemented...\n"));
+        return rx_origin;
+    }
+
     if (utf8_target) {
         if ((!prog->anchored_utf8 && prog->anchored_substr)
                 || (!prog->float_utf8 && prog->float_substr))