Fix ICE when using contracts on async functions

compiler/rustc_builtin_macros/src/contracts.rs

@@ -71,6 +71,14 @@ fn expand_contract_clause(
             .span_err(attr_span, "contract annotations can only be used on functions"));
     }
 
+    // Contracts are not yet supported on async/gen functions
+    if new_tts.iter().any(|tt| is_kw(tt, kw::Async) || is_kw(tt, kw::Gen)) {
+        return Err(ecx.sess.dcx().span_err(
+            attr_span,
+            "contract annotations are not yet supported on async or gen functions",
+        ));
+    }
+
     // Found the `fn` keyword, now find either the `where` token or the function body.
     let next_tt = loop {
         let Some(tt) = cursor.next() else {

tests/ui/contracts/async-fn-contract-ice-145333.rs

@@ -0,0 +1,10 @@
+//@ compile-flags: --crate-type=lib
+//@ edition: 2021
+#![feature(contracts)]
+//~^ WARN the feature `contracts` is incomplete
+
+#[core::contracts::ensures(|ret| *ret)]
+//~^ ERROR contract annotations are not yet supported on async or gen functions
+async fn _always_true(b: bool) -> bool {
+    b
+}

tests/ui/contracts/async-fn-contract-ice-145333.stderr

@@ -0,0 +1,17 @@
+error: contract annotations are not yet supported on async or gen functions
+  --> $DIR/async-fn-contract-ice-145333.rs:6:1
+   |
+LL | #[core::contracts::ensures(|ret| *ret)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: the feature `contracts` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/async-fn-contract-ice-145333.rs:3:12
+   |
+LL | #![feature(contracts)]
+   |            ^^^^^^^^^
+   |
+   = note: see issue #128044 <https://github.com/rust-lang/rust/issues/128044> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: aborting due to 1 previous error; 1 warning emitted
+