From 0935df7829c466f8ef94248429095ea79dc0fa73 Mon Sep 17 00:00:00 2001
From: yukang <moorekang@gmail.com>
Date: Wed, 15 Oct 2025 09:05:09 +0800
Subject: [PATCH] Fix ICE when using contracts on async functions

---
 compiler/rustc_builtin_macros/src/contracts.rs  |  8 ++++++++
 .../contracts/async-fn-contract-ice-145333.rs   | 10 ++++++++++
 .../async-fn-contract-ice-145333.stderr         | 17 +++++++++++++++++
 3 files changed, 35 insertions(+)
 create mode 100644 tests/ui/contracts/async-fn-contract-ice-145333.rs
 create mode 100644 tests/ui/contracts/async-fn-contract-ice-145333.stderr

diff --git a/compiler/rustc_builtin_macros/src/contracts.rs b/compiler/rustc_builtin_macros/src/contracts.rs
index 6a24af361fe78..13c63268e1e87 100644
--- a/compiler/rustc_builtin_macros/src/contracts.rs
+++ b/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 {
diff --git a/tests/ui/contracts/async-fn-contract-ice-145333.rs b/tests/ui/contracts/async-fn-contract-ice-145333.rs
new file mode 100644
index 0000000000000..a6de8a786af96
--- /dev/null
+++ b/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
+}
diff --git a/tests/ui/contracts/async-fn-contract-ice-145333.stderr b/tests/ui/contracts/async-fn-contract-ice-145333.stderr
new file mode 100644
index 0000000000000..77f5379e6fb54
--- /dev/null
+++ 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
+