Fix "unused mut" warnings created by generated code. (rust-lang#3247)

This change adds "unused_mut" to the list of suppressed lints for
wrappers generated by the contracts macros. This will get rid of
spurious errors caused by mutable parameters to functions.

This fixes the example from
model-checking/kani#3010 .

It can be tested by adding the example from the issues to
tests/expected/test_macros/gcd.rs, creating a file
tests/expected/test_macros/gcd.expected, then running
```bash
cargo build-dev
RUST_BACKTRACE=1 cargo run -p compiletest -- --logfile logfile.txt --suite expected --mode expected --ignored --no-fail-fast --src-base tests/expected/test_macros
 ```
 
 RESOLVES rust-lang#3010 

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

---------

Co-authored-by: Jacob Salzberg <[email protected]>

Author: jsalzbergedu

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

@@ -85,7 +85,7 @@ impl<'a> ContractConditionsHandler<'a> {
 
         let result = Ident::new(INTERNAL_RESULT_IDENT, Span::call_site());
         self.output.extend(quote!(
-            #[allow(dead_code, unused_variables)]
+            #[allow(dead_code, unused_variables, unused_mut)]
             #[kanitool::is_contract_generated(recursion_wrapper)]
             #wrapper_sig {
                 static mut REENTRY: bool = false;

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

@@ -50,7 +50,7 @@ impl<'a> ContractConditionsHandler<'a> {
     pub fn emit_common_header(&mut self) {
         if self.function_state.emit_tag_attr() {
             self.output.extend(quote!(
-                #[allow(dead_code, unused_variables)]
+                #[allow(dead_code, unused_variables, unused_mut)]
             ));
         }
         self.output.extend(self.annotated_fn.attrs.iter().flat_map(Attribute::to_token_stream));

tests/expected/function-contract/gcd_success.rs

@@ -1,28 +1,22 @@
 // Copyright Kani Contributors
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 // kani-flags: -Zfunction-contracts
+#![deny(warnings)]
 type T = u8;
 
 /// Euclid's algorithm for calculating the GCD of two numbers
 #[kani::requires(x != 0 && y != 0)]
 #[kani::ensures(|result : &T| *result != 0 && x % *result == 0 && y % *result == 0)]
-fn gcd(x: T, y: T) -> T {
-    let mut max = x;
-    let mut min = y;
-    if min > max {
-        let val = max;
-        max = min;
-        min = val;
-    }
-
+fn gcd(mut x: T, mut y: T) -> T {
+    (x, y) = (if x > y { x } else { y }, if x > y { y } else { x });
     loop {
-        let res = max % min;
+        let res = x % y;
         if res == 0 {
-            return min;
+            return y;
         }
 
-        max = min;
-        min = res;
+        x = y;
+        y = res;
     }
 }