add NSString#encode which returns a properly encoded RubyString, and …

…NSString#encode! which raises an exception

NSString.m

@@ -180,14 +180,6 @@
     return range.location == NSNotFound ? Qfalse : Qtrue;
 }
 
-static id
-rstr_only(id rcv, SEL sel)
-{
-    rb_raise(rb_eArgError, "method `%s' does not work on NSStrings",
-	    sel_getName(sel));
-    return rcv; // never reached
-}
-
 static VALUE
 nsstr_to_rstr(id nsstr)
 {

encoding.h

@@ -383,6 +383,14 @@ VALUE rstr_new_path(const char *path);
 const char *nsstr_cstr(VALUE str);
 long nsstr_clen(VALUE str);
 
+static inline id
+rstr_only(id rcv, SEL sel)
+{
+    rb_raise(rb_eArgError, "method `%s' does not work on NSStrings",
+            sel_getName(sel));
+    return rcv; // never reached
+}
+
 #if defined(__cplusplus)
 } // extern "C"
 #endif

transcode.c

@@ -313,7 +313,7 @@ rstr_encode(VALUE str, SEL sel, int argc, VALUE *argv)
         }
     }
 
-    rb_str_t *self = RSTR(str);
+    rb_str_t *self = str_need_string(str);
     rb_str_t *replacement_str = NULL;
     rb_encoding_t *src_encoding, *dst_encoding;
     transcode_behavior_t behavior_for_invalid = TRANSCODE_BEHAVIOR_RAISE_EXCEPTION;
@@ -381,8 +381,11 @@ rstr_encode_bang(VALUE str, SEL sel, int argc, VALUE *argv)
 void
 Init_Transcode(void)
 {
-    rb_objc_define_method(rb_cRubyString, "encode", rstr_encode, -1);
+    // #encode works on both NSStrings and RubyStrings, #encode! only works
+    // on RubyStrings.
+    rb_objc_define_method(rb_cNSString, "encode", rstr_encode, -1);
     rb_objc_define_method(rb_cRubyString, "encode!", rstr_encode_bang, -1);
+    rb_objc_define_method(rb_cNSString, "encode!", rstr_only, -1);
 
     rb_cEncodingConverter = rb_define_class_under(rb_cEncoding, "Converter", rb_cObject);
     rb_objc_define_method(*(VALUE *)rb_cEncodingConverter, "alloc", rb_econv_alloc, 0);