Add test for rust-lang#1767 (rust-lang#1864)

Author: zhassan-aws

tests/kani/DynTrait/encode_utf8.rs

@@ -0,0 +1,39 @@
+// Copyright Kani Contributors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+/// This test checks that CBMC's unwinding terminates on a program that involves
+/// dyn trait and `char::encode_utf8` (a minimal reproducer from
+/// https://github.com/model-checking/kani/issues/1767)
+
+pub trait Trait {
+    fn f(&self);
+}
+
+struct Foo {}
+
+impl Trait for Foo {
+    fn f(&self) {
+        let _ = 'x'.encode_utf8(&mut [0; 4]);
+    }
+}
+
+pub struct Formatter {}
+
+fn nn(_x: &u8, _f: &Formatter) {}
+
+pub struct ArgumentV1 {
+    formatter: fn(&u8, &Formatter) -> (),
+}
+
+#[kani::proof]
+#[kani::unwind(2)]
+fn dyn_trait_with_encode_utf8() {
+    let f = Foo {};
+    let a = [ArgumentV1 { formatter: nn }];
+
+    let _output = &f as &dyn Trait;
+    let formatter = Formatter {};
+
+    let mut iter = a.iter();
+    let _ = (iter.next().unwrap().formatter)(&5, &formatter);
+}