Rollup merge of #100734 - ComputerDruid:afit_feature, r=compiler-errors

Split out async_fn_in_trait into a separate feature

PR #101224 added support for async fn in trait desuraging behind the `return_position_impl_trait_in_trait` feature.

Split this out so that it's behind its own feature gate, since async fn in trait doesn't need to follow the same stabilization schedule.

compiler/rustc_ast_lowering/src/lib.rs

@@ -332,6 +332,15 @@ impl FnDeclKind {
             _ => false,
         }
     }
+
+    fn async_fn_allowed(&self, tcx: TyCtxt<'_>) -> bool {
+        match self {
+            FnDeclKind::Fn | FnDeclKind::Inherent => true,
+            FnDeclKind::Impl if tcx.features().async_fn_in_trait => true,
+            FnDeclKind::Trait if tcx.features().async_fn_in_trait => true,
+            _ => false,
+        }
+    }
 }
 
 #[derive(Copy, Clone)]
@@ -1692,14 +1701,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }));
 
         let output = if let Some((ret_id, span)) = make_ret_async {
-            if !kind.impl_trait_allowed(self.tcx) {
+            if !kind.async_fn_allowed(self.tcx) {
                 match kind {
                     FnDeclKind::Trait | FnDeclKind::Impl => {
                         self.tcx
                             .sess
                             .create_feature_err(
                                 TraitFnAsync { fn_span, span },
-                                sym::return_position_impl_trait_in_trait,
+                                sym::async_fn_in_trait,
                             )
                             .emit();
                     }
@@ -1917,9 +1926,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let future_bound = this.lower_async_fn_output_type_to_future_bound(
                     output,
                     span,
-                    ImplTraitContext::ReturnPositionOpaqueTy {
-                        origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
-                        in_trait,
+                    if in_trait && !this.tcx.features().return_position_impl_trait_in_trait {
+                        ImplTraitContext::Disallowed(ImplTraitPosition::TraitReturn)
+                    } else {
+                        ImplTraitContext::ReturnPositionOpaqueTy {
+                            origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
+                            in_trait,
+                        }
                     },
                 );
 

compiler/rustc_feature/src/active.rs

@@ -312,6 +312,8 @@ 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, "CURRENT_RUSTC_VERSION", 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

@@ -396,6 +396,7 @@ symbols! {
         assume_init,
         async_await,
         async_closure,
+        async_fn_in_trait,
         atomic,
         atomic_mod,
         atomics,