array: fix arr << arr bug

vlib/builtin/array.v

@@ -66,19 +66,20 @@ fn new_array_from_c_array_no_alloc(len, cap, elm_size int, c_array voidptr) arra
 
 // Private function. Doubles array capacity if needed
 fn (a mut array) ensure_cap(required int) {
-	if required > a.cap {
-		mut cap := if a.cap == 0 { 2 } else { a.cap * 2 }
-		for required > cap && true {
-			cap *= 2
-		}
-		if a.cap == 0 {
-			a.data = calloc(cap * a.element_size)
-		}
-		else {
-			a.data = C.realloc(a.data, cap * a.element_size)
-		}
-		a.cap = cap
+	if required <= a.cap {
+		return
 	}
+	mut cap := if a.cap == 0 { 2 } else { a.cap * 2 }
+	for required > cap {
+		cap *= 2
+	}
+	if a.cap == 0 {
+		a.data = calloc(cap * a.element_size)
+	}
+	else {
+		a.data = C.realloc(a.data, cap * a.element_size)
+	}
+	a.cap = cap
 }
 
 // array.repeat returns new array with the given array elements
@@ -280,8 +281,16 @@ fn (a mut array) push(val voidptr) {
 // `val` is array.data
 // TODO make private, right now it's used by strings.Builder
 pub fn (a mut array) push_many(val voidptr, size int) {
-	a.ensure_cap(a.len + size)
-	C.memcpy(a.data + a.element_size * a.len, val, a.element_size * size)
+	// handle `arr << arr`
+	if a.data == val {
+		copy := a.clone()
+		a.ensure_cap(a.len + size)
+		//C.memcpy(a.data, copy.data, copy.element_size * copy.len)
+		C.memcpy(a.data + a.element_size * a.len, copy.data, a.element_size * size)
+	} else {
+		a.ensure_cap(a.len + size)
+		C.memcpy(a.data + a.element_size * a.len, val, a.element_size * size)
+	}
 	a.len += size
 }