Auto merge of rust-lang#120392 - compiler-errors:async-bound-modifier, r=<try>

Introduce support for `async` bound modifier on `Fn*` traits

Adds `async` to the list of `TraitBoundModifiers`, which instructs AST lowering to map the trait to an async flavor of the trait. For now, this is only supported for `Fn*` to `AsyncFn*`, and I expect that this manual mapping via lang items will be replaced with a better system in the future.

The motivation for adding these bounds is to separate the users of async closures from the exact trait desugaring of their callable bounds. Instead of users needing to be concerned with the `AsyncFn` trait, they should be able to write `async Fn()` and it will desugar to whatever underlying trait we decide is best for the lowering of async closures.

Author: bors

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
+        }
+    }
 }

compiler/rustc_expand/src/build.rs

@@ -141,6 +141,7 @@ impl<'a> ExtCtxt<'a> {
                 } else {
                     ast::BoundConstness::Never
                 },
+                asyncness: ast::BoundAsyncness::Normal,
             },
         )
     }