Change rb_str_cat to avoid conversion to and from native strings.

Author: aardvark179

CHANGELOG.md

@@ -12,6 +12,10 @@ Compatibility:
 * Added missing `Enumerable#filter` and `Enumerator::Lazy#filter` aliases to the respective `select` method (#1610).
 * Implement more `Ripper` methods as no-ops (#1694).
 
+Performance:
+
+* `rb_str_cat` has been changed to improve performance. THe C string is now concatentated without first being converted to a Ruby string or having its encoding checked. As a side effect the behaviour of `rb_str_cat` should now more closely match that of MRI.
+
 # 20.0.0 beta 1
 
 Bug fixes:

src/main/c/cext/ruby.c

@@ -879,13 +879,22 @@ ID rb_intern_str(VALUE string) {
 }
 
 VALUE rb_str_cat(VALUE string, const char *to_concat, long length) {
-  polyglot_invoke(rb_tr_unwrap(string), "concat", rb_tr_unwrap(rb_enc_str_new(to_concat, length, STR_ENC_GET(string))));
+  if (length == 0) {
+    return string;
+  }
+  if (length < 0) {
+	rb_raise(rb_eArgError, "negative string size (or size too big)");
+  }
+  int old_length = RSTRING_LEN(string);
+  rb_str_resize(string, old_length + length);
+  // Resizing the string will clear out the code range, so there is no
+  // need to do it explicitly.
+  memcpy(RSTRING_PTR(string) + old_length, to_concat, length);
   return string;
 }
 
 VALUE rb_str_cat2(VALUE string, const char *to_concat) {
-  polyglot_invoke(rb_tr_unwrap(string), "concat", rb_tr_unwrap(rb_str_new_cstr(to_concat)));
-  return string;
+  return rb_str_cat(string, to_concat, strlen(to_concat));
 }
 
 VALUE rb_str_to_str(VALUE string) {