Add async bound modifier to enable async Fn bounds

compiler/rustc_ast_lowering/src/lib.rs

@@ -131,6 +131,7 @@ struct LoweringContext<'a, 'hir> {
     allow_gen_future: Lrc<[Symbol]>,
     allow_async_iterator: Lrc<[Symbol]>,
     allow_for_await: Lrc<[Symbol]>,
+    allow_async_fn_traits: Lrc<[Symbol]>,
 
     /// Mapping from generics `def_id`s to TAIT generics `def_id`s.
     /// For each captured lifetime (e.g., 'a), we create a new lifetime parameter that is a generic
@@ -176,6 +177,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 [sym::gen_future].into()
             },
             allow_for_await: [sym::async_iterator].into(),
+            allow_async_fn_traits: [sym::async_fn_traits].into(),
             // FIXME(gen_blocks): how does `closure_track_caller`/`async_fn_track_caller`
             // interact with `gen`/`async gen` blocks
             allow_async_iterator: [sym::gen_future, sym::async_iterator].into(),
@@ -1311,7 +1313,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         span: t.span,
                     },
                     itctx,
-                    ast::BoundConstness::Never,
+                    TraitBoundModifiers::NONE,
                 );
                 let bounds = this.arena.alloc_from_iter([bound]);
                 let lifetime_bound = this.elided_dyn_bound(t.span);
@@ -1426,7 +1428,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                         itctx,
                                         // Still, don't pass along the constness here; we don't want to
                                         // synthesize any host effect args, it'd only cause problems.
-                                        ast::BoundConstness::Never,
+                                        TraitBoundModifiers {
+                                            constness: BoundConstness::Never,
+                                            ..*modifiers
+                                        },
                                     ))
                                 }
                                 BoundPolarity::Maybe(_) => None,
@@ -2019,7 +2024,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     ) -> hir::GenericBound<'hir> {
         match tpb {
             GenericBound::Trait(p, modifiers) => hir::GenericBound::Trait(
-                self.lower_poly_trait_ref(p, itctx, modifiers.constness.into()),
+                self.lower_poly_trait_ref(p, itctx, *modifiers),
                 self.lower_trait_bound_modifiers(*modifiers),
             ),
             GenericBound::Outlives(lifetime) => {
@@ -2192,7 +2197,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
     fn lower_trait_ref(
         &mut self,
-        constness: ast::BoundConstness,
+        modifiers: ast::TraitBoundModifiers,
         p: &TraitRef,
         itctx: &ImplTraitContext,
     ) -> hir::TraitRef<'hir> {
@@ -2202,7 +2207,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             &p.path,
             ParamMode::Explicit,
             itctx,
-            Some(constness),
+            Some(modifiers),
         ) {
             hir::QPath::Resolved(None, path) => path,
             qpath => panic!("lower_trait_ref: unexpected QPath `{qpath:?}`"),
@@ -2215,11 +2220,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         &mut self,
         p: &PolyTraitRef,
         itctx: &ImplTraitContext,
-        constness: ast::BoundConstness,
+        modifiers: ast::TraitBoundModifiers,
     ) -> hir::PolyTraitRef<'hir> {
         let bound_generic_params =
             self.lower_lifetime_binder(p.trait_ref.ref_id, &p.bound_generic_params);
-        let trait_ref = self.lower_trait_ref(constness, &p.trait_ref, itctx);
+        let trait_ref = self.lower_trait_ref(modifiers, &p.trait_ref, itctx);
         hir::PolyTraitRef { bound_generic_params, trait_ref, span: self.lower_span(p.span) }
     }