Function Contracts: Closure Type Inference (rust-lang#3307)

The rust type inference for closures doesn't work in the particular use
case we are using it for ensures clauses. By creating a helper function,
we change the path the rust type inference takes and lets the type of
the closure be identified properly. This means type annotations are no
longer required within ensures clauses.

Resolves rust-lang#3304

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

library/kani/src/internal.rs

@@ -90,3 +90,10 @@ pub fn untracked_deref<T>(_: &T) -> T {
 #[doc(hidden)]
 #[rustc_diagnostic_item = "KaniInitContracts"]
 pub fn init_contracts() {}
+
+/// This should only be used within contracts. The intent is to
+/// perform type inference on a closure's argument
+#[doc(hidden)]
+pub fn apply_closure<T, U: Fn(&T) -> bool>(f: U, x: &T) -> bool {
+    f(x)
+}

library/kani_macros/src/sysroot/contracts/shared.rs

@@ -176,7 +176,7 @@ pub fn build_ensures(data: &ExprClosure) -> (TokenStream2, Expr) {
         .fold(quote!(), |collect, (ident, expr)| quote!(let #ident = #expr; #collect));
 
     let result: Ident = Ident::new(INTERNAL_RESULT_IDENT, Span::call_site());
-    (remembers_stmts, Expr::Verbatim(quote!((#expr)(&#result))))
+    (remembers_stmts, Expr::Verbatim(quote!(kani::internal::apply_closure(#expr, &#result))))
 }
 
 trait OldTrigger {

tests/expected/function-contract/simple_ensures_pass_no_annotation.expected

@@ -0,0 +1,6 @@
+assertion\
+- Status: SUCCESS\
+- Description: "|result| (*result == x) | (*result == y)"\
+in function max
+
+VERIFICATION:- SUCCESSFUL

tests/expected/function-contract/simple_ensures_pass_no_annotation.rs

@@ -0,0 +1,14 @@
+// Copyright Kani Contributors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+// kani-flags: -Zfunction-contracts
+
+#[kani::ensures(|result| (*result == x) | (*result == y))]
+fn max(x: u32, y: u32) -> u32 {
+    if x > y { x } else { y }
+}
+
+#[kani::proof_for_contract(max)]
+fn max_harness() {
+    let _ = Box::new(9_usize);
+    max(7, 6);
+}

tests/ui/function-contracts/mutating_ensures_error.expected

@@ -0,0 +1 @@
+cannot assign to `*_x`, as `Fn` closures cannot mutate their captured variables

tests/ui/function-contracts/mutating_ensures_error.rs

@@ -0,0 +1,12 @@
+// Copyright Kani Contributors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+// kani-flags: -Zfunction-contracts
+
+#[kani::ensures(|_| {*_x += 1; true})]
+fn unit(_x: &mut u32) {}
+
+#[kani::proof_for_contract(id)]
+fn harness() {
+    let mut x = kani::any();
+    unit(&mut x);
+}