FIX: optimized (and fixed regression) when appending char on string

Author: Oldes

src/core/f-modify.c

@@ -157,9 +157,15 @@
 		else Trap_Arg(src_val);
 	}
 	else if (IS_CHAR(src_val)) {
-		src_ser = BUF_FORM;
-		RESET_TAIL(src_ser);
-		src_ser = Append_Byte(src_ser, VAL_CHAR(src_val)); // unicode ok too
+		if (VAL_CHAR(src_val) < 256) {
+			src_ser = BUF_FORM;
+			*SERIES_DATA(src_ser) = (REBYTE)VAL_CHAR(src_val);
+		}
+		else {
+			src_ser = BUF_UTF8;
+			*(REBUNI*)SERIES_DATA(src_ser) = (REBUNI)VAL_CHAR(src_val);
+		}
+		SERIES_TAIL(src_ser) = 1;
 	}
 	else if (IS_BLOCK(src_val)) {
 		src_ser = Form_Tight_Block(src_val);

src/tests/units/series-test.r3

@@ -179,6 +179,14 @@ Rebol [
 
 ===end-group===
 
+
+===start-group=== "APPEND string!"
+	--test-- "APPEND string! char!"
+		--assert "a" = append "" #"a"
+		--assert "←" = append "" #"^(2190)" ; wide char
+===end-group===
+
+
 ;@@ https://github.com/Oldes/Rebol-issues/issues/1791
 ===start-group=== "APPEND binary!"
 	--test-- "APPEND binary! binary!"