Add async_fn_in_trait lint

compiler/rustc_lint/messages.ftl

@@ -5,6 +5,10 @@ lint_array_into_iter =
     .use_explicit_into_iter_suggestion =
         or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicitly iterate by value
 
+lint_async_fn_in_trait = use of `async fn` in public traits is discouraged as auto trait bounds cannot be specified
+    .note = you can suppress this lint if you plan to use the trait only in your own code, or do not care about auto traits like `Send` on the `Future`
+    .suggestion = you can alternatively desugar to a normal `fn` that returns `impl Future` and add any desired bounds such as `Send`
+
 lint_atomic_ordering_fence = memory fences cannot have `Relaxed` ordering
     .help = consider using ordering modes `Acquire`, `Release`, `AcqRel` or `SeqCst`
 

compiler/rustc_lint/src/async_fn_in_trait.rs

@@ -0,0 +1,128 @@
+use crate::lints::AsyncFnInTraitDiag;
+use crate::LateContext;
+use crate::LateLintPass;
+use rustc_hir as hir;
+use rustc_trait_selection::traits::error_reporting::suggestions::suggest_desugaring_async_fn_to_impl_future_in_trait;
+
+declare_lint! {
+    /// The `async_fn_in_trait` lint detects use of `async fn` in the
+    /// definition of a publicly-reachable trait.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// # #![feature(async_fn_in_trait)]
+    /// pub trait Trait {
+    ///     async fn method(&self);
+    /// }
+    /// # fn main() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// When `async fn` is used in a trait definition, the trait does not
+    /// promise that the opaque [`Future`] returned by the associated function
+    /// or method will implement any [auto traits] such as [`Send`]. This may
+    /// be surprising and may make the associated functions or methods on the
+    /// trait less useful than intended. On traits exposed publicly from a
+    /// crate, this may affect downstream crates whose authors cannot alter
+    /// the trait definition.
+    ///
+    /// For example, this code is invalid:
+    ///
+    /// ```rust,compile_fail
+    /// # #![feature(async_fn_in_trait)]
+    /// pub trait Trait {
+    ///     async fn method(&self) {}
+    /// }
+    ///
+    /// fn test<T: Trait>(x: T) {
+    ///     fn spawn<T: Send>(_: T) {}
+    ///     spawn(x.method()); // Not OK.
+    /// }
+    /// ```
+    ///
+    /// This lint exists to warn authors of publicly-reachable traits that
+    /// they may want to consider desugaring the `async fn` to a normal `fn`
+    /// that returns an opaque `impl Future<..> + Send` type.
+    ///
+    /// For example, instead of:
+    ///
+    /// ```rust
+    /// # #![feature(async_fn_in_trait)]
+    /// pub trait Trait {
+    ///     async fn method(&self) {}
+    /// }
+    /// ```
+    ///
+    /// The author of the trait may want to write:
+    ///
+    ///
+    /// ```rust
+    /// # #![feature(return_position_impl_trait_in_trait)]
+    /// use core::future::Future;
+    /// pub trait Trait {
+    ///     fn method(&self) -> impl Future<Output = ()> + Send { async {} }
+    /// }
+    /// ```
+    ///
+    /// This still allows the use of `async fn` within impls of the trait.
+    /// However, it also means that the trait will never be compatible with
+    /// impls where the returned [`Future`] of the method does not implement
+    /// `Send`.
+    ///
+    /// Conversely, if the trait is used only locally, if it is never used in
+    /// generic functions, or if it is only used in single-threaded contexts
+    /// that do not care whether the returned [`Future`] implements [`Send`],
+    /// then the lint may be suppressed.
+    ///
+    /// [`Future`]: https://doc.rust-lang.org/core/future/trait.Future.html
+    /// [`Send`]: https://doc.rust-lang.org/core/marker/trait.Send.html
+    /// [auto traits]: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
+    pub ASYNC_FN_IN_TRAIT,
+    Warn,
+    "use of `async fn` in definition of a publicly-reachable trait"
+}
+
+declare_lint_pass!(
+    /// Lint for use of `async fn` in the definition of a publicly-reachable
+    /// trait.
+    AsyncFnInTrait => [ASYNC_FN_IN_TRAIT]
+);
+
+impl<'tcx> LateLintPass<'tcx> for AsyncFnInTrait {
+    fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::TraitItem<'tcx>) {
+        if let hir::TraitItemKind::Fn(sig, body) = item.kind
+            && let hir::IsAsync::Async(async_span) = sig.header.asyncness
+        {
+            // RTN can be used to bound `async fn` in traits in a better way than "always"
+            if cx.tcx.features().return_type_notation {
+                return;
+            }
+
+            // Only need to think about library implications of reachable traits
+            if !cx.tcx.effective_visibilities(()).is_reachable(item.owner_id.def_id) {
+                return;
+            }
+
+            let hir::FnRetTy::Return(hir::Ty { kind: hir::TyKind::OpaqueDef(def, ..), .. }) =
+                sig.decl.output
+            else {
+                // This should never happen, but let's not ICE.
+                return;
+            };
+            let sugg = suggest_desugaring_async_fn_to_impl_future_in_trait(
+                cx.tcx,
+                sig,
+                body,
+                def.owner_id.def_id,
+                " + Send",
+            );
+            cx.tcx.emit_spanned_lint(ASYNC_FN_IN_TRAIT, item.hir_id(), async_span, AsyncFnInTraitDiag {
+                sugg
+            });
+        }
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -50,6 +50,7 @@ extern crate rustc_session;
 extern crate tracing;
 
 mod array_into_iter;
+mod async_fn_in_trait;
 pub mod builtin;
 mod context;
 mod deref_into_dyn_supertrait;
@@ -96,6 +97,7 @@ use rustc_session::lint::builtin::{
 };
 
 use array_into_iter::ArrayIntoIter;
+use async_fn_in_trait::AsyncFnInTrait;
 use builtin::*;
 use deref_into_dyn_supertrait::*;
 use drop_forget_useless::*;
@@ -234,6 +236,7 @@ late_lint_methods!(
             MapUnitFn: MapUnitFn,
             MissingDebugImplementations: MissingDebugImplementations,
             MissingDoc: MissingDoc,
+            AsyncFnInTrait: AsyncFnInTrait,
         ]
     ]
 );

compiler/rustc_lint/src/lints.rs

@@ -1818,3 +1818,24 @@ pub struct UnusedAllocationDiag;
 #[derive(LintDiagnostic)]
 #[diag(lint_unused_allocation_mut)]
 pub struct UnusedAllocationMutDiag;
+
+pub struct AsyncFnInTraitDiag {
+    pub sugg: Option<Vec<(Span, String)>>,
+}
+
+impl<'a> DecorateLint<'a, ()> for AsyncFnInTraitDiag {
+    fn decorate_lint<'b>(
+        self,
+        diag: &'b mut rustc_errors::DiagnosticBuilder<'a, ()>,
+    ) -> &'b mut rustc_errors::DiagnosticBuilder<'a, ()> {
+        diag.note(fluent::lint_note);
+        if let Some(sugg) = self.sugg {
+            diag.multipart_suggestion(fluent::lint_suggestion, sugg, Applicability::MaybeIncorrect);
+        }
+        diag
+    }
+
+    fn msg(&self) -> rustc_errors::DiagnosticMessage {
+        fluent::lint_async_fn_in_trait
+    }
+}

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -4000,14 +4000,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 
         // ... whose signature is `async` (i.e. this is an AFIT)
         let (sig, body) = item.expect_fn();
-        let hir::IsAsync::Async(async_span) = sig.header.asyncness else {
-            return;
-        };
-        let Ok(async_span) =
-            self.tcx.sess.source_map().span_extend_while(async_span, |c| c.is_whitespace())
-        else {
-            return;
-        };
         let hir::FnRetTy::Return(hir::Ty { kind: hir::TyKind::OpaqueDef(def, ..), .. }) =
             sig.decl.output
         else {
@@ -4021,55 +4013,17 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             return;
         }
 
-        let future = self.tcx.hir().item(*def).expect_opaque_ty();
-        let Some(hir::GenericBound::LangItemTrait(_, _, _, generics)) = future.bounds.get(0) else {
-            // `async fn` should always lower to a lang item bound... but don't ICE.
-            return;
-        };
-        let Some(hir::TypeBindingKind::Equality { term: hir::Term::Ty(future_output_ty) }) =
-            generics.bindings.get(0).map(|binding| binding.kind)
-        else {
-            // Also should never happen.
+        let Some(sugg) = suggest_desugaring_async_fn_to_impl_future_in_trait(
+            self.tcx,
+            *sig,
+            *body,
+            opaque_def_id.expect_local(),
+            &format!(" + {auto_trait}"),
+        ) else {
             return;
         };
 
         let function_name = self.tcx.def_path_str(fn_def_id);
-
-        let mut sugg = if future_output_ty.span.is_empty() {
-            vec![
-                (async_span, String::new()),
-                (
-                    future_output_ty.span,
-                    format!(" -> impl std::future::Future<Output = ()> + {auto_trait}"),
-                ),
-            ]
-        } else {
-            vec![
-                (
-                    future_output_ty.span.shrink_to_lo(),
-                    "impl std::future::Future<Output = ".to_owned(),
-                ),
-                (future_output_ty.span.shrink_to_hi(), format!("> + {auto_trait}")),
-                (async_span, String::new()),
-            ]
-        };
-
-        // If there's a body, we also need to wrap it in `async {}`
-        if let hir::TraitFn::Provided(body) = body {
-            let body = self.tcx.hir().body(*body);
-            let body_span = body.value.span;
-            let body_span_without_braces =
-                body_span.with_lo(body_span.lo() + BytePos(1)).with_hi(body_span.hi() - BytePos(1));
-            if body_span_without_braces.is_empty() {
-                sugg.push((body_span_without_braces, " async {} ".to_owned()));
-            } else {
-                sugg.extend([
-                    (body_span_without_braces.shrink_to_lo(), "async {".to_owned()),
-                    (body_span_without_braces.shrink_to_hi(), "} ".to_owned()),
-                ]);
-            }
-        }
-
         err.multipart_suggestion(
             format!(
                 "`{auto_trait}` can be made part of the associated future's \
@@ -4321,3 +4275,65 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ReplaceImplTraitFolder<'tcx> {
         self.tcx
     }
 }
+
+pub fn suggest_desugaring_async_fn_to_impl_future_in_trait<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    sig: hir::FnSig<'tcx>,
+    body: hir::TraitFn<'tcx>,
+    opaque_def_id: LocalDefId,
+    add_bounds: &str,
+) -> Option<Vec<(Span, String)>> {
+    let hir::IsAsync::Async(async_span) = sig.header.asyncness else {
+        return None;
+    };
+    let Ok(async_span) = tcx.sess.source_map().span_extend_while(async_span, |c| c.is_whitespace())
+    else {
+        return None;
+    };
+
+    let future = tcx.hir().get_by_def_id(opaque_def_id).expect_item().expect_opaque_ty();
+    let Some(hir::GenericBound::LangItemTrait(_, _, _, generics)) = future.bounds.get(0) else {
+        // `async fn` should always lower to a lang item bound... but don't ICE.
+        return None;
+    };
+    let Some(hir::TypeBindingKind::Equality { term: hir::Term::Ty(future_output_ty) }) =
+        generics.bindings.get(0).map(|binding| binding.kind)
+    else {
+        // Also should never happen.
+        return None;
+    };
+
+    let mut sugg = if future_output_ty.span.is_empty() {
+        vec![
+            (async_span, String::new()),
+            (
+                future_output_ty.span,
+                format!(" -> impl std::future::Future<Output = ()>{add_bounds}"),
+            ),
+        ]
+    } else {
+        vec![
+            (future_output_ty.span.shrink_to_lo(), "impl std::future::Future<Output = ".to_owned()),
+            (future_output_ty.span.shrink_to_hi(), format!(">{add_bounds}")),
+            (async_span, String::new()),
+        ]
+    };
+
+    // If there's a body, we also need to wrap it in `async {}`
+    if let hir::TraitFn::Provided(body) = body {
+        let body = tcx.hir().body(body);
+        let body_span = body.value.span;
+        let body_span_without_braces =
+            body_span.with_lo(body_span.lo() + BytePos(1)).with_hi(body_span.hi() - BytePos(1));
+        if body_span_without_braces.is_empty() {
+            sugg.push((body_span_without_braces, " async {} ".to_owned()));
+        } else {
+            sugg.extend([
+                (body_span_without_braces.shrink_to_lo(), "async {".to_owned()),
+                (body_span_without_braces.shrink_to_hi(), "} ".to_owned()),
+            ]);
+        }
+    }
+
+    Some(sugg)
+}

tests/ui/async-await/in-trait/async-associated-types.rs

@@ -9,12 +9,14 @@ use std::fmt::Debug;
 trait MyTrait<'a, 'b, T> where Self: 'a, T: Debug + Sized + 'b {
     type MyAssoc;
 
+    #[allow(async_fn_in_trait)]
     async fn foo(&'a self, key: &'b T) -> Self::MyAssoc;
 }
 
 impl<'a, 'b, T: Debug + Sized + 'b, U: 'a> MyTrait<'a, 'b, T> for U {
     type MyAssoc = (&'a U, &'b T);
 
+    #[allow(async_fn_in_trait)]
     async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
         (self, key)
     }

tests/ui/async-await/in-trait/async-default-fn-overridden.rs

@@ -6,10 +6,12 @@
 use std::future::Future;
 
 trait AsyncTrait {
+    #[allow(async_fn_in_trait)]
     async fn default_impl() {
         assert!(false);
     }
 
+    #[allow(async_fn_in_trait)]
     async fn call_default_impl() {
         Self::default_impl().await
     }

tests/ui/async-await/in-trait/async-example-desugared-extra.rs

@@ -10,6 +10,7 @@ use std::pin::Pin;
 use std::task::Poll;
 
 pub trait MyTrait {
+    #[allow(async_fn_in_trait)]
     async fn foo(&self) -> i32;
 }
 

tests/ui/async-await/in-trait/async-example-desugared.rs

@@ -8,6 +8,7 @@
 use std::future::Future;
 
 trait MyTrait {
+    #[allow(async_fn_in_trait)]
     async fn foo(&self) -> i32;
 }
 

tests/ui/async-await/in-trait/async-example.rs

@@ -5,7 +5,10 @@
 #![allow(incomplete_features)]
 
 trait MyTrait {
+    #[allow(async_fn_in_trait)]
     async fn foo(&self) -> i32;
+
+    #[allow(async_fn_in_trait)]
     async fn bar(&self) -> i32;
 }
 

tests/ui/async-await/in-trait/async-lifetimes-and-bounds.rs

@@ -7,6 +7,7 @@
 use std::fmt::Debug;
 
 trait MyTrait<'a, 'b, T> {
+    #[allow(async_fn_in_trait)]
     async fn foo(&'a self, key: &'b T) -> (&'a Self, &'b T) where T: Debug + Sized;
 }