[Test/SILOptimizer] Add test cases against non-class protocol let constant mutation

Author: timbodeit

test/SILOptimizer/definite_init_lvalue_let_witness_methods.swift

@@ -0,0 +1,61 @@
+// RUN: %target-swift-frontend -emit-sil -disable-objc-attr-requires-foundation-module -verify %s
+
+// High-level tests that DI rejects passing let constants to
+// mutating witness methods
+
+
+// Mark: General Definitions
+
+protocol TestProtocol {
+    var foo: Int { get set }
+}
+
+struct TestStruct: TestProtocol {
+    var foo: Int
+}
+
+// Mark: - Case1: Illegaly mutating let property of class in initializer
+
+class TestClass {
+    let testObject: TestProtocol // expected-note {{change 'let' to 'var' to make it mutable}}
+    init() {
+        testObject = TestStruct(foo: 42)
+        testObject.foo = 666 // expected-error {{cannot perform mutating operation: 'self.testObject' is a 'let' constant}}
+    }
+}
+
+// Mark: - Case2: Illegaly mutating global let constant
+
+let testObject: TestProtocol  // expected-note {{change 'let' to 'var' to make it mutable}}
+testObject = TestStruct(foo: 42)
+
+testObject.foo = 666 // expected-error {{cannot perform mutating operation: 'testObject' is a 'let' constant}}
+
+extension TestProtocol {
+    mutating func messThingsUp() {
+        foo = 666
+    }
+}
+
+// Mark: - Case3: Illegaly muatating let constant in a function scope
+
+let testObject2: TestProtocol  // expected-note {{change 'let' to 'var' to make it mutable}}
+testObject2 = TestStruct(foo: 42)
+testObject2.messThingsUp() // expected-error {{cannot perform mutating operation: 'testObject2' is a 'let' constant}}
+
+func testFunc() {
+    let testObject: TestProtocol // expected-note {{change 'let' to 'var' to make it mutable}}
+
+    testObject = TestStruct(foo: 42)
+    testObject.foo = 666 // expected-error {{cannot perform mutating operation: 'testObject' is a 'let' constant}}
+}
+
+// Mark: - Case4: Illegaly passing a let constants property as an inout parameter
+
+let testObject3: TestProtocol  // expected-note {{change 'let' to 'var' to make it mutable}}
+testObject3 = TestStruct(foo: 42)
+
+func mutateThis(mutatee: inout Int) {
+    mutatee = 666
+}
+mutateThis(mutatee: &testObject3.foo) // expected-error {{cannot perform mutating operation: 'testObject3' is a 'let' constant}}