Treat errors in signatures as an object safety violation

Author: oli-obk

compiler/rustc_middle/src/traits/mod.rs

@@ -820,6 +820,9 @@ impl ObjectSafetyViolation {
             ) => {
                 format!("method `{name}` references an `impl Trait` type in its return type").into()
             }
+            ObjectSafetyViolation::Method(name, MethodViolationCode::AlreadyErrored, _) => {
+                format!("method `{name}` has errors in its signature").into()
+            }
             ObjectSafetyViolation::Method(name, MethodViolationCode::AsyncFn, _) => {
                 format!("method `{name}` is `async`").into()
             }
@@ -974,6 +977,9 @@ pub enum MethodViolationCode {
 
     /// the method's receiver (`self` argument) can't be dispatched on
     UndispatchableReceiver(Option<Span>),
+
+    /// The signature contained type errors, e.g. due to cycle errors
+    AlreadyErrored,
 }
 
 /// These are the error cases for `codegen_select_candidate`.

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -419,7 +419,10 @@ fn virtual_call_violations_for_method<'tcx>(
     trait_def_id: DefId,
     method: ty::AssocItem,
 ) -> Vec<MethodViolationCode> {
-    let sig = tcx.fn_sig(method.def_id).instantiate_identity();
+    let sig: ty::Binder<'_, ty::FnSig<'_>> = tcx.fn_sig(method.def_id).instantiate_identity();
+    if sig.references_error() {
+        return vec![MethodViolationCode::AlreadyErrored];
+    }
 
     // The method's first parameter must be named `self`
     if !method.fn_has_self_parameter {

tests/ui/async-await/inference_var_self_argument.rs

@@ -0,0 +1,13 @@
+//! This is a regression test for an ICE.
+// edition: 2021
+
+trait Foo {
+    async fn foo(self: &Foo) {
+        //~^ ERROR: `Foo` cannot be made into an object
+        //~| ERROR invalid `self` parameter type: &dyn Foo
+        //~| ERROR trait objects must include the `dyn` keyword
+        todo!()
+    }
+}
+
+fn main() {}

tests/ui/async-await/inference_var_self_argument.stderr

@@ -0,0 +1,39 @@
+error[E0038]: the trait `Foo` cannot be made into an object
+  --> $DIR/inference_var_self_argument.rs:5:5
+   |
+LL |     async fn foo(self: &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/inference_var_self_argument.rs:5:14
+   |
+LL | trait Foo {
+   |       --- this trait cannot be made into an object...
+LL |     async fn foo(self: &Foo) {
+   |              ^^^ ...because method `foo` has errors in its signature
+   = help: consider moving `foo` to another trait
+
+error[E0307]: invalid `self` parameter type: &dyn Foo
+  --> $DIR/inference_var_self_argument.rs:5:24
+   |
+LL |     async fn foo(self: &Foo) {
+   |                        ^^^^
+   |
+   = note: type of `self` must be `Self` or a type that dereferences to it
+   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/inference_var_self_argument.rs:5:25
+   |
+LL |     async fn foo(self: &Foo) {
+   |                         ^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL |     async fn foo(self: &dyn Foo) {
+   |                         +++
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0038, E0307, E0782.
+For more information about an error, try `rustc --explain E0038`.