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-rw-r--r--compiler/rustc_const_eval/src/transform/check_consts/mod.rs2
-rw-r--r--compiler/rustc_hir/src/hir.rs6
-rw-r--r--compiler/rustc_hir_analysis/src/check/compare_method.rs4
-rw-r--r--compiler/rustc_middle/src/traits/mod.rs10
-rw-r--r--compiler/rustc_trait_selection/src/traits/object_safety.rs21
-rw-r--r--src/test/ui/async-await/in-trait/object-safety.rs13
-rw-r--r--src/test/ui/async-await/in-trait/object-safety.stderr27
-rw-r--r--src/test/ui/impl-trait/in-trait/object-safety.stderr12
-rw-r--r--src/tools/clippy/clippy_lints/src/manual_async_fn.rs4
-rw-r--r--src/tools/clippy/clippy_lints/src/unused_async.rs4
-rw-r--r--src/tools/clippy/clippy_utils/src/lib.rs10
11 files changed, 86 insertions, 27 deletions
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/mod.rs b/compiler/rustc_const_eval/src/transform/check_consts/mod.rs
index 25b420bed17..b90e0962ce6 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/mod.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/mod.rs
@@ -62,7 +62,7 @@ impl<'mir, 'tcx> ConstCx<'mir, 'tcx> {
     }
 
     fn is_async(&self) -> bool {
-        self.tcx.asyncness(self.def_id()) == hir::IsAsync::Async
+        self.tcx.asyncness(self.def_id()).is_async()
     }
 }
 
diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index 7d8b859a6b4..e0a38645065 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -2720,6 +2720,12 @@ pub enum IsAsync {
     NotAsync,
 }
 
+impl IsAsync {
+    pub fn is_async(self) -> bool {
+        self == IsAsync::Async
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, Encodable, Decodable, HashStable_Generic)]
 pub enum Defaultness {
     Default { has_value: bool },
diff --git a/compiler/rustc_hir_analysis/src/check/compare_method.rs b/compiler/rustc_hir_analysis/src/check/compare_method.rs
index 0b9209771cd..e68df228c6b 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_method.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_method.rs
@@ -684,9 +684,7 @@ fn report_trait_method_mismatch<'tcx>(
                 // Suggestion to change output type. We do not suggest in `async` functions
                 // to avoid complex logic or incorrect output.
                 match tcx.hir().expect_impl_item(impl_m.def_id.expect_local()).kind {
-                    ImplItemKind::Fn(ref sig, _)
-                        if sig.header.asyncness == hir::IsAsync::NotAsync =>
-                    {
+                    ImplItemKind::Fn(ref sig, _) if !sig.header.asyncness.is_async() => {
                         let msg = "change the output type to match the trait";
                         let ap = Applicability::MachineApplicable;
                         match sig.decl.output {
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 05382bd887c..1890c0e24bb 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -924,10 +924,13 @@ impl ObjectSafetyViolation {
             }
             ObjectSafetyViolation::Method(
                 name,
-                MethodViolationCode::ReferencesImplTraitInTrait,
+                MethodViolationCode::ReferencesImplTraitInTrait(_),
                 _,
             ) => format!("method `{}` references an `impl Trait` type in its return type", name)
                 .into(),
+            ObjectSafetyViolation::Method(name, MethodViolationCode::AsyncFn, _) => {
+                format!("method `{}` is `async`", name).into()
+            }
             ObjectSafetyViolation::Method(
                 name,
                 MethodViolationCode::WhereClauseReferencesSelf,
@@ -1035,7 +1038,10 @@ pub enum MethodViolationCode {
     ReferencesSelfOutput,
 
     /// e.g., `fn foo(&self) -> impl Sized`
-    ReferencesImplTraitInTrait,
+    ReferencesImplTraitInTrait(Span),
+
+    /// e.g., `async fn foo(&self)`
+    AsyncFn,
 
     /// e.g., `fn foo(&self) where Self: Clone`
     WhereClauseReferencesSelf,
diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index eaca3eaef0c..9745e0137ee 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -375,6 +375,7 @@ fn object_safety_violation_for_method(
         let span = match (&v, node) {
             (MethodViolationCode::ReferencesSelfInput(Some(span)), _) => *span,
             (MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
+            (MethodViolationCode::ReferencesImplTraitInTrait(span), _) => *span,
             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
                 node.fn_decl().map_or(method.ident(tcx).span, |decl| decl.output.span())
             }
@@ -437,8 +438,8 @@ fn virtual_call_violation_for_method<'tcx>(
     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
         return Some(MethodViolationCode::ReferencesSelfOutput);
     }
-    if contains_illegal_impl_trait_in_trait(tcx, sig.output()) {
-        return Some(MethodViolationCode::ReferencesImplTraitInTrait);
+    if let Some(code) = contains_illegal_impl_trait_in_trait(tcx, method.def_id, sig.output()) {
+        return Some(code);
     }
 
     // We can't monomorphize things like `fn foo<A>(...)`.
@@ -864,16 +865,24 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
 
 pub fn contains_illegal_impl_trait_in_trait<'tcx>(
     tcx: TyCtxt<'tcx>,
+    fn_def_id: DefId,
     ty: ty::Binder<'tcx, Ty<'tcx>>,
-) -> bool {
+) -> Option<MethodViolationCode> {
+    // This would be caught below, but rendering the error as a separate
+    // `async-specific` message is better.
+    if tcx.asyncness(fn_def_id).is_async() {
+        return Some(MethodViolationCode::AsyncFn);
+    }
+
     // FIXME(RPITIT): Perhaps we should use a visitor here?
-    ty.skip_binder().walk().any(|arg| {
+    ty.skip_binder().walk().find_map(|arg| {
         if let ty::GenericArgKind::Type(ty) = arg.unpack()
             && let ty::Projection(proj) = ty.kind()
+            && tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
         {
-            tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
+            Some(MethodViolationCode::ReferencesImplTraitInTrait(tcx.def_span(proj.item_def_id)))
         } else {
-            false
+            None
         }
     })
 }
diff --git a/src/test/ui/async-await/in-trait/object-safety.rs b/src/test/ui/async-await/in-trait/object-safety.rs
new file mode 100644
index 00000000000..a8bc35f7e0c
--- /dev/null
+++ b/src/test/ui/async-await/in-trait/object-safety.rs
@@ -0,0 +1,13 @@
+// edition:2021
+
+#![feature(async_fn_in_trait)]
+//~^ WARN the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
+
+trait Foo {
+    async fn foo(&self);
+}
+
+fn main() {
+    let x: &dyn Foo = todo!();
+    //~^ ERROR the trait `Foo` cannot be made into an object
+}
diff --git a/src/test/ui/async-await/in-trait/object-safety.stderr b/src/test/ui/async-await/in-trait/object-safety.stderr
new file mode 100644
index 00000000000..0b318f71f39
--- /dev/null
+++ b/src/test/ui/async-await/in-trait/object-safety.stderr
@@ -0,0 +1,27 @@
+warning: the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/object-safety.rs:3:12
+   |
+LL | #![feature(async_fn_in_trait)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0038]: the trait `Foo` cannot be made into an object
+  --> $DIR/object-safety.rs:11:12
+   |
+LL |     let x: &dyn Foo = todo!();
+   |            ^^^^^^^^ `Foo` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $DIR/object-safety.rs:7:14
+   |
+LL | trait Foo {
+   |       --- this trait cannot be made into an object...
+LL |     async fn foo(&self);
+   |              ^^^ ...because method `foo` is `async`
+   = help: consider moving `foo` to another trait
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/impl-trait/in-trait/object-safety.stderr b/src/test/ui/impl-trait/in-trait/object-safety.stderr
index 9a1554b5e1c..ca0e760ff6d 100644
--- a/src/test/ui/impl-trait/in-trait/object-safety.stderr
+++ b/src/test/ui/impl-trait/in-trait/object-safety.stderr
@@ -5,12 +5,12 @@ LL |     let i = Box::new(42_u32) as Box<dyn Foo>;
    |                                 ^^^^^^^^^^^^ `Foo` cannot be made into an object
    |
 note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
-  --> $DIR/object-safety.rs:7:8
+  --> $DIR/object-safety.rs:7:22
    |
 LL | trait Foo {
    |       --- this trait cannot be made into an object...
 LL |     fn baz(&self) -> impl Debug;
-   |        ^^^ ...because method `baz` references an `impl Trait` type in its return type
+   |                      ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
    = help: consider moving `baz` to another trait
 
 error[E0038]: the trait `Foo` cannot be made into an object
@@ -20,12 +20,12 @@ LL |     let s = i.baz();
    |             ^^^^^^^ `Foo` cannot be made into an object
    |
 note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
-  --> $DIR/object-safety.rs:7:8
+  --> $DIR/object-safety.rs:7:22
    |
 LL | trait Foo {
    |       --- this trait cannot be made into an object...
 LL |     fn baz(&self) -> impl Debug;
-   |        ^^^ ...because method `baz` references an `impl Trait` type in its return type
+   |                      ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
    = help: consider moving `baz` to another trait
 
 error[E0038]: the trait `Foo` cannot be made into an object
@@ -35,12 +35,12 @@ LL |     let i = Box::new(42_u32) as Box<dyn Foo>;
    |             ^^^^^^^^^^^^^^^^ `Foo` cannot be made into an object
    |
 note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
-  --> $DIR/object-safety.rs:7:8
+  --> $DIR/object-safety.rs:7:22
    |
 LL | trait Foo {
    |       --- this trait cannot be made into an object...
 LL |     fn baz(&self) -> impl Debug;
-   |        ^^^ ...because method `baz` references an `impl Trait` type in its return type
+   |                      ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
    = help: consider moving `baz` to another trait
    = note: required for `Box<u32>` to implement `CoerceUnsized<Box<dyn Foo>>`
    = note: required by cast to type `Box<dyn Foo>`
diff --git a/src/tools/clippy/clippy_lints/src/manual_async_fn.rs b/src/tools/clippy/clippy_lints/src/manual_async_fn.rs
index 090f9f8ff73..5c6a342b3d0 100644
--- a/src/tools/clippy/clippy_lints/src/manual_async_fn.rs
+++ b/src/tools/clippy/clippy_lints/src/manual_async_fn.rs
@@ -6,7 +6,7 @@ use rustc_errors::Applicability;
 use rustc_hir::intravisit::FnKind;
 use rustc_hir::{
     AsyncGeneratorKind, Block, Body, Closure, Expr, ExprKind, FnDecl, FnRetTy, GeneratorKind, GenericArg, GenericBound,
-    HirId, IsAsync, ItemKind, LifetimeName, Term, TraitRef, Ty, TyKind, TypeBindingKind,
+    HirId, ItemKind, LifetimeName, Term, TraitRef, Ty, TyKind, TypeBindingKind,
 };
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
@@ -49,7 +49,7 @@ impl<'tcx> LateLintPass<'tcx> for ManualAsyncFn {
     ) {
         if_chain! {
             if let Some(header) = kind.header();
-            if header.asyncness == IsAsync::NotAsync;
+            if !header.asyncness.is_async();
             // Check that this function returns `impl Future`
             if let FnRetTy::Return(ret_ty) = decl.output;
             if let Some((trait_ref, output_lifetimes)) = future_trait_ref(cx, ret_ty);
diff --git a/src/tools/clippy/clippy_lints/src/unused_async.rs b/src/tools/clippy/clippy_lints/src/unused_async.rs
index bf487c7ca20..3538bef6e06 100644
--- a/src/tools/clippy/clippy_lints/src/unused_async.rs
+++ b/src/tools/clippy/clippy_lints/src/unused_async.rs
@@ -1,6 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_help;
 use rustc_hir::intravisit::{walk_expr, walk_fn, FnKind, Visitor};
-use rustc_hir::{Body, Expr, ExprKind, FnDecl, HirId, IsAsync, YieldSource};
+use rustc_hir::{Body, Expr, ExprKind, FnDecl, HirId, YieldSource};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::hir::nested_filter;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
@@ -68,7 +68,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
         span: Span,
         hir_id: HirId,
     ) {
-        if !span.from_expansion() && fn_kind.asyncness() == IsAsync::Async {
+        if !span.from_expansion() && fn_kind.asyncness().is_async() {
             let mut visitor = AsyncFnVisitor { cx, found_await: false };
             walk_fn(&mut visitor, fn_kind, fn_decl, body.id(), hir_id);
             if !visitor.found_await {
diff --git a/src/tools/clippy/clippy_utils/src/lib.rs b/src/tools/clippy/clippy_utils/src/lib.rs
index d32cf1a7936..bb91317d67f 100644
--- a/src/tools/clippy/clippy_utils/src/lib.rs
+++ b/src/tools/clippy/clippy_utils/src/lib.rs
@@ -87,10 +87,10 @@ use rustc_hir::hir_id::{HirIdMap, HirIdSet};
 use rustc_hir::intravisit::{walk_expr, FnKind, Visitor};
 use rustc_hir::LangItem::{OptionNone, ResultErr, ResultOk};
 use rustc_hir::{
-    def, Arm, ArrayLen, BindingAnnotation, Block, BlockCheckMode, Body, Closure, Constness, Destination, Expr,
-    ExprKind, FnDecl, HirId, Impl, ImplItem, ImplItemKind, IsAsync, Item, ItemKind, LangItem, Local, MatchSource,
-    Mutability, Node, Param, Pat, PatKind, Path, PathSegment, PrimTy, QPath, Stmt, StmtKind, TraitItem, TraitItemKind,
-    TraitRef, TyKind, UnOp,
+    def, Arm, ArrayLen, BindingAnnotation, Block, BlockCheckMode, Body, Closure, Constness,
+    Destination, Expr, ExprKind, FnDecl, HirId, Impl, ImplItem, ImplItemKind, Item, ItemKind,
+    LangItem, Local, MatchSource, Mutability, Node, Param, Pat, PatKind, Path, PathSegment, PrimTy,
+    QPath, Stmt, StmtKind, TraitItem, TraitItemKind, TraitRef, TyKind, UnOp,
 };
 use rustc_lexer::{tokenize, TokenKind};
 use rustc_lint::{LateContext, Level, Lint, LintContext};
@@ -1861,7 +1861,7 @@ pub fn if_sequence<'tcx>(mut expr: &'tcx Expr<'tcx>) -> (Vec<&'tcx Expr<'tcx>>,
 
 /// Checks if the given function kind is an async function.
 pub fn is_async_fn(kind: FnKind<'_>) -> bool {
-    matches!(kind, FnKind::ItemFn(_, _, header) if header.asyncness == IsAsync::Async)
+    matches!(kind, FnKind::ItemFn(_, _, header) if header.asyncness.is_async())
 }
 
 /// Peels away all the compiler generated code surrounding the body of an async function,
generated by cgit-pink 1.4.1-3-g733a5 (git 2.37.1) at 2026-01-13 12:33:22 +0000