Improve spans for RPITIT object-safety errors

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,

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()) {
+    if let Some(code) = contains_illegal_impl_trait_in_trait(tcx, method.def_id, sig.output()) {
-        return Some(MethodViolationCode::ReferencesImplTraitInTrait);
+        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
         }
     })
 }

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
+}

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`.

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>`