S_parse_ident: Add ability to start parse in middle

S_scan_ident would like to call this function, already having looked at
the first character of an identifier, and deciding it is legal.  It
wants this function to finish the scan.

This commit adds a flag to S_parse_ident to accommodate this.

Author: khwilliamson

toke.c

@@ -181,6 +181,7 @@ static const char ident_var_zero_multi_digit[] = "Numeric variables with more th
 #define STOP_AT_FIRST_NON_DIGIT     (1 << 4)
 #define CHECK_ONLY                  (1 << 5)
 #define CHECK_UNARY                 (1 << 6)
+#define IDCONT_first_OK             (1 << 7)
 
 #ifdef DEBUGGING
 static const char* const lex_state_names[] = {
@@ -10597,6 +10598,11 @@ S_parse_ident(pTHX_ const char *s, const char * const s_end,
      * in things like Foo::$bar */
     const bool check_dollar = flags & CHECK_DOLLAR;
 
+    /* There is a use case for calling this function in the middle of having
+     * parsed a portion of an identifier.  Therefore it should be able to
+     * accept the first character being an IDCont, and not necessarily an
+     * IDFIRST.  The 'IDCONT_first_OK' flag is used to indicate this */
+
     while (s < s_end) {
 
         /* For non-UTF8, variables that match ASCII \w are a superset of
@@ -10605,7 +10611,10 @@ S_parse_ident(pTHX_ const char *s, const char * const s_end,
          * for the subset before checking for the superset. */
         Size_t advance;
         if (   (is_utf8 || idfirst_only)
-            && (advance = isIDFIRST_lazy_if_safe(s, s_end, is_utf8)))
+            && (advance = (flags & IDCONT_first_OK)
+                          ? isIDCONT_lazy_if_safe((U8 *) s, (U8 *) s_end,
+                                                  is_utf8)
+                          : isIDFIRST_lazy_if_safe(s, s_end, is_utf8)))
         {
             const char *this_start = s;
             s += advance;