Introduce support for async bound modifier on Fn* traits

compiler/rustc_ast/src/ast.rs

@@ -291,12 +291,16 @@ pub use crate::node_id::{NodeId, CRATE_NODE_ID, DUMMY_NODE_ID};
 #[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug)]
 pub struct TraitBoundModifiers {
     pub constness: BoundConstness,
+    pub asyncness: BoundAsyncness,
     pub polarity: BoundPolarity,
 }
 
 impl TraitBoundModifiers {
-    pub const NONE: Self =
-        Self { constness: BoundConstness::Never, polarity: BoundPolarity::Positive };
+    pub const NONE: Self = Self {
+        constness: BoundConstness::Never,
+        asyncness: BoundAsyncness::Normal,
+        polarity: BoundPolarity::Positive,
+    };
 }
 
 /// The AST represents all type param bounds as types.
@@ -2562,6 +2566,25 @@ impl BoundConstness {
     }
 }
 
+/// The asyncness of a trait bound.
+#[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug)]
+#[derive(HashStable_Generic)]
+pub enum BoundAsyncness {
+    /// `Type: Trait`
+    Normal,
+    /// `Type: async Trait`
+    Async(Span),
+}
+
+impl BoundAsyncness {
+    pub fn as_str(self) -> &'static str {
+        match self {
+            Self::Normal => "",
+            Self::Async(_) => "async",
+        }
+    }
+}
+
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum FnRetTy {
     /// Returns type is not specified.
@@ -3300,7 +3323,7 @@ mod size_asserts {
     static_assert_size!(ForeignItem, 96);
     static_assert_size!(ForeignItemKind, 24);
     static_assert_size!(GenericArg, 24);
-    static_assert_size!(GenericBound, 72);
+    static_assert_size!(GenericBound, 88);
     static_assert_size!(Generics, 40);
     static_assert_size!(Impl, 136);
     static_assert_size!(Item, 136);

compiler/rustc_ast_lowering/messages.ftl

@@ -11,6 +11,12 @@ ast_lowering_argument = argument
 ast_lowering_assoc_ty_parentheses =
     parenthesized generic arguments cannot be used in associated type constraints
 
+ast_lowering_async_bound_not_on_trait =
+    `async` bound modifier only allowed on trait, not `{$descr}`
+
+ast_lowering_async_bound_only_for_fn_traits =
+    `async` bound modifier only allowed on `Fn`/`FnMut`/`FnOnce` traits
+
 ast_lowering_async_coroutines_not_supported =
     `async` coroutines are not yet supported
 

compiler/rustc_ast_lowering/src/errors.rs

@@ -395,3 +395,18 @@ pub(crate) struct GenericParamDefaultInBinder {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_lowering_async_bound_not_on_trait)]
+pub(crate) struct AsyncBoundNotOnTrait {
+    #[primary_span]
+    pub span: Span,
+    pub descr: &'static str,
+}
+
+#[derive(Diagnostic)]
+#[diag(ast_lowering_async_bound_only_for_fn_traits)]
+pub(crate) struct AsyncBoundOnlyForFnTraits {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_ast_lowering/src/expr.rs

@@ -100,6 +100,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         ParenthesizedGenericArgs::Err,
                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         None,
+                        // Method calls can't have bound modifiers
+                        None,
                     ));
                     let receiver = self.lower_expr(receiver);
                     let args =

compiler/rustc_ast_lowering/src/item.rs

@@ -343,14 +343,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let itctx = ImplTraitContext::Universal;
                 let (generics, (trait_ref, lowered_ty)) =
                     self.lower_generics(ast_generics, *constness, id, &itctx, |this| {
-                        let constness = match *constness {
-                            Const::Yes(span) => BoundConstness::Maybe(span),
-                            Const::No => BoundConstness::Never,
+                        let modifiers = TraitBoundModifiers {
+                            constness: match *constness {
+                                Const::Yes(span) => BoundConstness::Maybe(span),
+                                Const::No => BoundConstness::Never,
+                            },
+                            asyncness: BoundAsyncness::Normal,
+                            // we don't use this in bound lowering
+                            polarity: BoundPolarity::Positive,
                         };
 
                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
                             this.lower_trait_ref(
-                                constness,
+                                modifiers,
                                 trait_ref,
                                 &ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
                             )

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) }
     }
 

compiler/rustc_ast_lowering/src/path.rs

@@ -1,17 +1,21 @@
 use crate::ImplTraitPosition;
 
-use super::errors::{GenericTypeWithParentheses, UseAngleBrackets};
+use super::errors::{
+    AsyncBoundNotOnTrait, AsyncBoundOnlyForFnTraits, GenericTypeWithParentheses, UseAngleBrackets,
+};
 use super::ResolverAstLoweringExt;
 use super::{GenericArgsCtor, LifetimeRes, ParenthesizedGenericArgs};
 use super::{ImplTraitContext, LoweringContext, ParamMode};
 
 use rustc_ast::{self as ast, *};
+use rustc_data_structures::sync::Lrc;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, PartialRes, Res};
+use rustc_hir::def_id::DefId;
 use rustc_hir::GenericArg;
 use rustc_middle::span_bug;
 use rustc_span::symbol::{kw, sym, Ident};
-use rustc_span::{BytePos, Span, DUMMY_SP};
+use rustc_span::{BytePos, DesugaringKind, Span, Symbol, DUMMY_SP};
 
 use smallvec::{smallvec, SmallVec};
 
@@ -24,8 +28,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         p: &Path,
         param_mode: ParamMode,
         itctx: &ImplTraitContext,
-        // constness of the impl/bound if this is a trait path
-        constness: Option<ast::BoundConstness>,
+        // modifiers of the impl/bound if this is a trait path
+        modifiers: Option<ast::TraitBoundModifiers>,
     ) -> hir::QPath<'hir> {
         let qself_position = qself.as_ref().map(|q| q.position);
         let qself = qself.as_ref().map(|q| self.lower_ty(&q.ty, itctx));
@@ -35,10 +39,36 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let base_res = partial_res.base_res();
         let unresolved_segments = partial_res.unresolved_segments();
 
+        let mut res = self.lower_res(base_res);
+
+        // When we have an `async` kw on a bound, map the trait it resolves to.
+        let mut bound_modifier_allowed_features = None;
+        if let Some(TraitBoundModifiers { asyncness: BoundAsyncness::Async(_), .. }) = modifiers {
+            match res {
+                Res::Def(DefKind::Trait, def_id) => {
+                    if let Some((async_def_id, features)) = self.map_trait_to_async_trait(def_id) {
+                        res = Res::Def(DefKind::Trait, async_def_id);
+                        bound_modifier_allowed_features = Some(features);
+                    } else {
+                        self.dcx().emit_err(AsyncBoundOnlyForFnTraits { span: p.span });
+                    }
+                }
+                Res::Err => {
+                    // No additional error.
+                }
+                _ => {
+                    // This error isn't actually emitted AFAICT, but it's best to keep
+                    // it around in case the resolver doesn't always check the defkind
+                    // of an item or something.
+                    self.dcx().emit_err(AsyncBoundNotOnTrait { span: p.span, descr: res.descr() });
+                }
+            }
+        }
+
         let path_span_lo = p.span.shrink_to_lo();
         let proj_start = p.segments.len() - unresolved_segments;
         let path = self.arena.alloc(hir::Path {
-            res: self.lower_res(base_res),
+            res,
             segments: self.arena.alloc_from_iter(p.segments[..proj_start].iter().enumerate().map(
                 |(i, segment)| {
                     let param_mode = match (qself_position, param_mode) {
@@ -77,7 +107,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         parenthesized_generic_args,
                         itctx,
                         // if this is the last segment, add constness to the trait path
-                        if i == proj_start - 1 { constness } else { None },
+                        if i == proj_start - 1 { modifiers.map(|m| m.constness) } else { None },
+                        bound_modifier_allowed_features.clone(),
                     )
                 },
             )),
@@ -88,6 +119,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             ),
         });
 
+        if let Some(bound_modifier_allowed_features) = bound_modifier_allowed_features {
+            path.span = self.mark_span_with_reason(
+                DesugaringKind::BoundModifier,
+                path.span,
+                Some(bound_modifier_allowed_features),
+            );
+        }
+
         // Simple case, either no projections, or only fully-qualified.
         // E.g., `std::mem::size_of` or `<I as Iterator>::Item`.
         if unresolved_segments == 0 {
@@ -125,6 +164,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 ParenthesizedGenericArgs::Err,
                 itctx,
                 None,
+                None,
             ));
             let qpath = hir::QPath::TypeRelative(ty, hir_segment);
 
@@ -166,6 +206,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     ParenthesizedGenericArgs::Err,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     None,
+                    None,
                 )
             })),
             span: self.lower_span(p.span),
@@ -180,6 +221,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         parenthesized_generic_args: ParenthesizedGenericArgs,
         itctx: &ImplTraitContext,
         constness: Option<ast::BoundConstness>,
+        // Additional features ungated with a bound modifier like `async`.
+        // This is passed down to the implicit associated type binding in
+        // parenthesized bounds.
+        bound_modifier_allowed_features: Option<Lrc<[Symbol]>>,
     ) -> hir::PathSegment<'hir> {
         debug!("path_span: {:?}, lower_path_segment(segment: {:?})", path_span, segment);
         let (mut generic_args, infer_args) = if let Some(generic_args) = segment.args.as_deref() {
@@ -188,9 +233,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     self.lower_angle_bracketed_parameter_data(data, param_mode, itctx)
                 }
                 GenericArgs::Parenthesized(data) => match parenthesized_generic_args {
-                    ParenthesizedGenericArgs::ParenSugar => {
-                        self.lower_parenthesized_parameter_data(data, itctx)
-                    }
+                    ParenthesizedGenericArgs::ParenSugar => self
+                        .lower_parenthesized_parameter_data(
+                            data,
+                            itctx,
+                            bound_modifier_allowed_features,
+                        ),
                     ParenthesizedGenericArgs::Err => {
                         // Suggest replacing parentheses with angle brackets `Trait(params...)` to `Trait<params...>`
                         let sub = if !data.inputs.is_empty() {
@@ -357,6 +405,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         &mut self,
         data: &ParenthesizedArgs,
         itctx: &ImplTraitContext,
+        bound_modifier_allowed_features: Option<Lrc<[Symbol]>>,
     ) -> (GenericArgsCtor<'hir>, bool) {
         // Switch to `PassThrough` mode for anonymous lifetimes; this
         // means that we permit things like `&Ref<T>`, where `Ref` has
@@ -392,7 +441,19 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             FnRetTy::Default(_) => self.arena.alloc(self.ty_tup(*span, &[])),
         };
         let args = smallvec![GenericArg::Type(self.arena.alloc(self.ty_tup(*inputs_span, inputs)))];
-        let binding = self.assoc_ty_binding(sym::Output, output_ty.span, output_ty);
+
+        // If we have a bound like `async Fn() -> T`, make sure that we mark the
+        // `Output = T` associated type bound with the right feature gates.
+        let mut output_span = output_ty.span;
+        if let Some(bound_modifier_allowed_features) = bound_modifier_allowed_features {
+            output_span = self.mark_span_with_reason(
+                DesugaringKind::BoundModifier,
+                output_span,
+                Some(bound_modifier_allowed_features),
+            );
+        }
+        let binding = self.assoc_ty_binding(sym::Output, output_span, output_ty);
+
         (
             GenericArgsCtor {
                 args,
@@ -429,4 +490,23 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             kind,
         }
     }
+
+    /// When a bound is annotated with `async`, it signals to lowering that the trait
+    /// that the bound refers to should be mapped to the "async" flavor of the trait.
+    ///
+    /// This only needs to be done until we unify `AsyncFn` and `Fn` traits into one
+    /// that is generic over `async`ness, if that's ever possible, or modify the
+    /// lowering of `async Fn()` bounds to desugar to another trait like `LendingFn`.
+    fn map_trait_to_async_trait(&self, def_id: DefId) -> Option<(DefId, Lrc<[Symbol]>)> {
+        let lang_items = self.tcx.lang_items();
+        if Some(def_id) == lang_items.fn_trait() {
+            Some((lang_items.async_fn_trait()?, self.allow_async_fn_traits.clone()))
+        } else if Some(def_id) == lang_items.fn_mut_trait() {
+            Some((lang_items.async_fn_mut_trait()?, self.allow_async_fn_traits.clone()))
+        } else if Some(def_id) == lang_items.fn_once_trait() {
+            Some((lang_items.async_fn_once_trait()?, self.allow_async_fn_traits.clone()))
+        } else {
+            None
+        }
+    }
 }