checker: allow pass array as mut param to spawn fn (#21283)

vlib/v/checker/fn.v

@@ -2516,6 +2516,10 @@ fn (mut c Checker) spawn_expr(mut node ast.SpawnExpr) ast.Type {
 	// Make sure there are no mutable arguments
 	for arg in node.call_expr.args {
 		if arg.is_mut && !arg.typ.is_ptr() {
+			if c.table.final_sym(arg.typ).kind == .array {
+				// allow mutable []array to be passed as mut
+				continue
+			}
 			c.error('function in `spawn` statement cannot contain mutable non-reference arguments',
 				arg.expr.pos())
 		}

vlib/v/tests/spawn_array_mut_test.v

@@ -0,0 +1,14 @@
+fn mutable_array_arg(mut a []int) {
+	println(a)
+	a << 4
+	assert a.len == 4
+}
+
+fn test_main() {
+	mut a := []int{}
+	a << 1
+	a << 2
+	a << 3
+	w := spawn mutable_array_arg(mut a)
+	w.wait()
+}