Error on projection of dyn noncompat type in old trait solver

compiler/rustc_trait_selection/src/traits/project.rs

@@ -813,6 +813,12 @@ fn assemble_candidates_from_object_ty<'cx, 'tcx>(
         }
         _ => return,
     };
+
+    // Projecting `dyn Trait` is only valid when `Trait` is dyn-compatible.
+    if data.principal_def_id().is_some_and(|def_id| !tcx.is_dyn_compatible(def_id)) {
+        return;
+    }
+
     let env_predicates = data
         .projection_bounds()
         .filter(|bound| bound.item_def_id() == obligation.predicate.def_id)

tests/ui/self/dispatch-dyn-incompatible-that-does-not-deref.rs

@@ -12,7 +12,7 @@ trait Foo: Deref<Target = W> {
 fn test(x: &dyn Foo) {
     //~^ ERROR the trait `Foo` is not dyn compatible
     x.method();
-    //~^ ERROR the trait `Foo` is not dyn compatible
+    //~^ ERROR no method named `method` found for reference `&dyn Foo`
 }
 
 fn main() {}

tests/ui/self/dispatch-dyn-incompatible-that-does-not-deref.stderr

@@ -25,25 +25,16 @@ LL |     fn method(self: &W) {}
    = 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[E0038]: the trait `Foo` is not dyn compatible
-  --> $DIR/dispatch-dyn-incompatible-that-does-not-deref.rs:14:5
+error[E0599]: no method named `method` found for reference `&dyn Foo` in the current scope
+  --> $DIR/dispatch-dyn-incompatible-that-does-not-deref.rs:14:7
    |
 LL |     fn method(self: &W) {}
-   |                     -- help: consider changing method `method`'s `self` parameter to be `&self`: `&Self`
+   |                     -- the method might not be found because of this arbitrary self type
 ...
 LL |     x.method();
-   |     ^^^^^^^^^^ `Foo` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/dispatch-dyn-incompatible-that-does-not-deref.rs:8:21
-   |
-LL | trait Foo: Deref<Target = W> {
-   |       --- this trait is not dyn compatible...
-LL |     fn method(self: &W) {}
-   |                     ^^ ...because method `method`'s `self` parameter cannot be dispatched on
+   |       ^^^^^^ method not found in `&dyn Foo`
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0038, E0307.
+Some errors have detailed explanations: E0038, E0307, E0599.
 For more information about an error, try `rustc --explain E0038`.

tests/ui/traits/ice-with-dyn-pointee-errors.rs

@@ -8,7 +8,8 @@ fn unknown_sized_object_ptr_in(_: &(impl Pointee<Metadata = ()> + ?Sized)) {}
 fn raw_pointer_in(x: &dyn Pointee<Metadata = ()>) {
     //~^ ERROR the trait `Pointee` is not dyn compatible
     unknown_sized_object_ptr_in(x)
-    //~^ ERROR the trait `Pointee` is not dyn compatible
+    //~^ ERROR type mismatch resolving `<dyn Pointee<Metadata = ()> as Pointee>::Metadata == ()`
+    //~| ERROR the trait `Pointee` is not dyn compatible
 }
 
 fn main() {

tests/ui/traits/ice-with-dyn-pointee-errors.stderr

@@ -10,6 +10,22 @@ note: for a trait to be dyn compatible it needs to allow building a vtable
    |
    = note: the trait is not dyn compatible because it opted out of dyn-compatibility
 
+error[E0271]: type mismatch resolving `<dyn Pointee<Metadata = ()> as Pointee>::Metadata == ()`
+  --> $DIR/ice-with-dyn-pointee-errors.rs:10:33
+   |
+LL |     unknown_sized_object_ptr_in(x)
+   |     --------------------------- ^ expected `()`, found `DynMetadata<dyn Pointee<Metadata = ()>>`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: expected unit type `()`
+                 found struct `DynMetadata<dyn Pointee<Metadata = ()>>`
+note: required by a bound in `unknown_sized_object_ptr_in`
+  --> $DIR/ice-with-dyn-pointee-errors.rs:6:50
+   |
+LL | fn unknown_sized_object_ptr_in(_: &(impl Pointee<Metadata = ()> + ?Sized)) {}
+   |                                                  ^^^^^^^^^^^^^ required by this bound in `unknown_sized_object_ptr_in`
+
 error[E0038]: the trait `Pointee` is not dyn compatible
   --> $DIR/ice-with-dyn-pointee-errors.rs:10:5
    |
@@ -23,7 +39,7 @@ note: for a trait to be dyn compatible it needs to allow building a vtable
    = note: the trait is not dyn compatible because it opted out of dyn-compatibility
 
 error[E0038]: the trait `Pointee` is not dyn compatible
-  --> $DIR/ice-with-dyn-pointee-errors.rs:15:20
+  --> $DIR/ice-with-dyn-pointee-errors.rs:16:20
    |
 LL |     raw_pointer_in(&42)
    |                    ^^^ `Pointee` is not dyn compatible
@@ -34,6 +50,7 @@ note: for a trait to be dyn compatible it needs to allow building a vtable
    |
    = note: the trait is not dyn compatible because it opted out of dyn-compatibility
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0038`.
+Some errors have detailed explanations: E0038, E0271.
+For more information about an error, try `rustc --explain E0038`.