markused: fix const as fn mark as used (fix #25888) (#25895)

Author: felipensp

vlib/v/markused/walker.v

@@ -454,7 +454,9 @@ fn (mut w Walker) expr(node_ ast.Expr) {
 		}
 		ast.CallExpr {
 			w.call_expr(mut node)
-			if node.name == 'json.decode' {
+			if node.is_fn_a_const {
+				w.mark_const_as_used(node.name)
+			} else if node.name == 'json.decode' {
 				w.mark_by_type((node.args[0].expr as ast.TypeNode).typ)
 			} else if node.name == 'json.encode' && node.args[0].typ != 0 {
 				sym := w.table.final_sym(node.args[0].typ)

vlib/v/tests/consts/const_markused_test.v

@@ -0,0 +1,6 @@
+import my_utils
+
+fn test_main() {
+	v := f64(1.0 / 8)
+	assert my_utils.my_round(v, 2) == '0.13'
+}

vlib/v/tests/consts/my_utils/my_utils.v

@@ -0,0 +1,5 @@
+module my_utils
+
+import strconv
+
+pub const my_round = strconv.f64_to_str_lnd1