Auto merge of #116184 - compiler-errors:afit-lint, r=tmandry

Add `async_fn_in_trait` lint

cc https://github.com/rust-lang/rust/pull/115822#issuecomment-1731168465

Mostly unsure what the messaging should be. Feedback required.

r? `@tmandry`

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
+    }
+}