Use end of char boundary in start_with?

Previously we used the next character following the found prefix to
determine if the match ended on a broken character.

This had caused surprising behaviour when a valid character was followed
by a UTF-8 continuation byte.

This commit changes the behaviour to instead look for the end of the
last character in the prefix.

[Bug #19784]

Co-authored-by: ywenc <ywenc@github.com>
Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

internal/string.h

@@ -125,6 +125,15 @@ at_char_boundary(const char *s, const char *p, const char *e, rb_encoding *enc)
     return rb_enc_left_char_head(s, p, e, enc) == p;
 }
 
+static inline bool
+at_char_right_boundary(const char *s, const char *p, const char *e, rb_encoding *enc)
+{
+    RUBY_ASSERT(s <= p);
+    RUBY_ASSERT(p <= e);
+
+    return rb_enc_right_char_head(s, p, e, enc) == p;
+}
+
 /* expect tail call optimization */
 // YJIT needs this function to never allocate and never raise
 static inline VALUE

string.c

@@ -10472,7 +10472,7 @@ rb_str_start_with(int argc, VALUE *argv, VALUE str)
             p = RSTRING_PTR(str);
             e = p + slen;
             s = p + tlen;
-            if (!at_char_boundary(p, s, e, enc))
+            if (!at_char_right_boundary(p, s, e, enc))
                 continue;
             if (memcmp(p, RSTRING_PTR(tmp), tlen) == 0)
                 return Qtrue;
@@ -10554,7 +10554,7 @@ deleted_prefix_length(VALUE str, VALUE prefix)
         }
         const char *strend = strptr + olen;
         const char *after_prefix = strptr + prefixlen;
-        if (!at_char_boundary(strptr, after_prefix, strend, enc)) {
+        if (!at_char_right_boundary(strptr, after_prefix, strend, enc)) {
             /* prefix does not end at char-boundary */
             return 0;
         }