add incomplete feature async_fn_in_trait

Doesn't do much yet, just disables the check preventing the use of the
syntax. Attempting to use it just results in ICE 🧊.

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -286,8 +286,10 @@ impl<'a> AstValidator<'a> {
     }
 
     fn check_trait_fn_not_async(&self, fn_span: Span, asyncness: Async) {
-        if let Async::Yes { span, .. } = asyncness {
-            self.session.emit_err(TraitFnAsync { fn_span, span });
+        if !self.session.features_untracked().async_fn_in_trait {
+            if let Async::Yes { span, .. } = asyncness {
+                self.session.emit_err(TraitFnAsync { fn_span, span });
+            }
         }
     }
 

compiler/rustc_feature/src/active.rs

@@ -310,6 +310,8 @@ declare_features! (
     (active, associated_type_defaults, "1.2.0", Some(29661), None),
     /// Allows `async || body` closures.
     (active, async_closure, "1.37.0", Some(62290), None),
+    /// Alows async functions to be declared, implemented, and used in traits.
+    (incomplete, async_fn_in_trait, "1.65.0", Some(91611), None),
     /// Allows `extern "C-unwind" fn` to enable unwinding across ABI boundaries.
     (active, c_unwind, "1.52.0", Some(74990), None),
     /// Allows using C-variadics.

compiler/rustc_span/src/symbol.rs

@@ -389,6 +389,7 @@ symbols! {
         assume_init,
         async_await,
         async_closure,
+        async_fn_in_trait,
         atomic,
         atomic_mod,
         atomics,

src/test/ui/async-await/feature-gate-async_fn_in_trait.rs

@@ -0,0 +1,7 @@
+// edition:2021
+
+trait T {
+    async fn foo(); //~ ERROR functions in traits cannot be declared `async`
+}
+
+fn main() {}

src/test/ui/async-await/feature-gate-async_fn_in_trait.stderr

@@ -0,0 +1,14 @@
+error[E0706]: functions in traits cannot be declared `async`
+  --> $DIR/feature-gate-async_fn_in_trait.rs:4:5
+   |
+LL |     async fn foo();
+   |     -----^^^^^^^^^^
+   |     |
+   |     `async` because of this
+   |
+   = note: `async` trait functions are not currently supported
+   = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0706`.