Add async_fn_in_trait lint

4 files changed, 86 insertions, 0 deletions

diff --git a/compiler/rustc_lint/messages.ftl b/compiler/rustc_lint/messages.ftl
index 7377c6e2f35..ad6c381a581 100644
--- a/compiler/rustc_lint/messages.ftl
+++ b/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 = usage of `async fn` in trait is discouraged because they do not automatically have auto trait bounds
+    .note = you can suppress this lint if you plan to use the trait locally, for concrete types, or do not care about auto traits like `Send` on the future
+    .suggestion = you can alternatively desugar the `async fn` and any add additional traits such as `Send` to the signature
+
 lint_atomic_ordering_fence = memory fences cannot have `Relaxed` ordering
     .help = consider using ordering modes `Acquire`, `Release`, `AcqRel` or `SeqCst`
 
diff --git a/compiler/rustc_lint/src/async_fn_in_trait.rs b/compiler/rustc_lint/src/async_fn_in_trait.rs
new file mode 100644
index 00000000000..0ad129d43cd
--- /dev/null
+++ b/compiler/rustc_lint/src/async_fn_in_trait.rs
@@ -0,0 +1,58 @@
+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! {
+    /// TODO
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// fn foo<T: Drop>() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// TODO
+    pub ASYNC_FN_IN_TRAIT,
+    Warn,
+    "TODO"
+}
+
+declare_lint_pass!(
+    // TODO:
+    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
+        {
+            if cx.tcx.features().return_type_notation {
+                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
+            });
+        }
+    }
+}
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index 72c103f2d4a..af2132fb899 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/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,
         ]
     ]
 );
diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index c091c260a47..12694aa0bed 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/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
+    }
+}