Declare polyglot type for RSTRING_PTR.

Author: aardvark179

CHANGELOG.md

@@ -27,6 +27,7 @@ Bug fixes:
 * Keep the Truffle working directory in sync with the native working directory.
 * Rename `to_native` to `polyglot_to_native` to match `polyglot_pointer?` and `polyglot_address` methods. 
 * Fixed missing partial evaluation boundary in `Array#{sort,sort!}` (#1727).
+* Fixed missing polyglot type declaration for `RSTRING_PTR` to help with native/managed interop.
 
 Compatibility:
 

src/main/c/cext/ruby.c

@@ -804,7 +804,7 @@ double rb_float_value(VALUE value) {
 // String
 
 char *RSTRING_PTR_IMPL(VALUE string) {
-  return RUBY_CEXT_INVOKE_NO_WRAP("RSTRING_PTR", string);
+  return (char *)polyglot_as_i8_array(RUBY_CEXT_INVOKE_NO_WRAP("RSTRING_PTR", string));
 }
 
 char *RSTRING_END(VALUE string) {