Fix the order of operands for generator structs (rust-lang#2436)

kani-compiler/src/codegen_cprover_gotoc/codegen/rvalue.rs

@@ -331,7 +331,6 @@ impl<'tcx> GotocCtx<'tcx> {
         };
         let overall_t = self.codegen_ty(ty);
         let direct_fields = overall_t.lookup_field("direct_fields", &self.symbol_table).unwrap();
-        let mut operands_iter = operands.iter();
         let direct_fields_expr = Expr::struct_expr_from_values(
             direct_fields.typ(),
             layout
@@ -342,13 +341,12 @@ impl<'tcx> GotocCtx<'tcx> {
                     if idx == *discriminant_field {
                         Expr::int_constant(0, self.codegen_ty(field_ty))
                     } else {
-                        self.codegen_operand(operands_iter.next().unwrap())
+                        self.codegen_operand(&operands[idx])
                     }
                 })
                 .collect(),
             &self.symbol_table,
         );
-        assert!(operands_iter.next().is_none());
         Expr::union_expr(overall_t, "direct_fields", direct_fields_expr, &self.symbol_table)
     }
 

tests/kani/Generator/issue-2434.rs

@@ -0,0 +1,53 @@
+// Copyright Kani Contributors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+//
+// compile-flags: --edition 2018
+
+//! Regression test for https://github.com/model-checking/kani/issues/2434
+//! The problem was an incorrect order for the operands
+use core::{future::Future, pin::Pin};
+
+type BoxFuture = Pin<Box<dyn Future<Output = ()> + Sync + 'static>>;
+
+pub struct Scheduler {
+    task: Option<BoxFuture>,
+}
+
+impl Scheduler {
+    /// Adds a future to the scheduler's task list, returning a JoinHandle
+    pub fn spawn<F: Future<Output = ()> + Sync + 'static>(&mut self, fut: F) {
+        self.task = Some(Box::pin(fut));
+    }
+}
+
+/// Polls the given future and the tasks it may spawn until all of them complete
+///
+/// Contrary to block_on, this allows `spawn`ing other futures
+pub fn spawnable_block_on<F: Future<Output = ()> + Sync + 'static>(
+    scheduler: &mut Scheduler,
+    fut: F,
+) {
+    scheduler.spawn(fut);
+}
+
+/// Sender of a channel.
+pub struct Sender {}
+
+impl Sender {
+    pub async fn send(&self) {}
+}
+
+#[kani::proof]
+fn check() {
+    let mut scheduler = Scheduler { task: None };
+    spawnable_block_on(&mut scheduler, async {
+        let num: usize = 1;
+        let tx = Sender {};
+
+        let _task1 = async move {
+            for _i in 0..num {
+                tx.send().await;
+            }
+        };
+    });
+}