test: Add guard feature test for expression simplification

Introduces a new test file demonstrating the use of guard in pattern matching for simplifying arithmetic expressions. The test showcases simplification rules like handling zero and identity elements, and verifies the simplification of equivalent subexpressions.

Author: bobzhang

builtin/guard_feature_test.mbt

@@ -0,0 +1,47 @@
+// Copyright 2025 International Digital Economy Academy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+///|
+enum Expr {
+  Add(Expr, Expr)
+  Sub(Expr, Expr)
+  Mul(Expr, Expr)
+  Div(Expr, Expr)
+  Lit(Int)
+  Var(String)
+} derive(Show, ToJson, Eq)
+
+///|
+fn simplify(e : Expr) -> Expr {
+  match e {
+    Add(e1, Lit(0)) => simplify(e1)
+    Add(Lit(0), e2) => simplify(e2)
+    Mul(e1, Lit(1)) => simplify(e1)
+    Mul(Lit(1), e2) => simplify(e2)
+    Sub(e1, e2) if e1 == e2 => Lit(0)
+    // guard is useful for in-exhaustive match
+    // otherwise the else condition will be duplicated
+    Mul(Lit(0), _) => Lit(0)
+    Mul(_, Lit(0)) => Lit(0)
+    _ => e
+  }
+}
+
+///|
+test "simplify" {
+  let _ignored = Add(Div(Lit(1), Lit(2)), Div(Lit(1), Lit(2)))
+  let e = Sub(Mul(Lit(1), Var("x")), Mul(Lit(1), Var("x")))
+  let simplified = simplify(e)
+  inspect!(simplified, content="Lit(0)")
+}