RPITITs are considered object-safe, they're always on Self:Sized methods

compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs

@@ -354,6 +354,12 @@ pub(in crate::solve) fn predicates_for_object_candidate<'tcx>(
         // FIXME(associated_const_equality): Also add associated consts to
         // the requirements here.
         if item.kind == ty::AssocKind::Type {
+            // RPITITs are not checked here, since they are not (currently) object-safe
+            // and cannot be named from a non-`Self: Sized` method.
+            if item.is_impl_trait_in_trait() {
+                continue;
+            }
+
             requirements
                 .extend(tcx.item_bounds(item.def_id).iter_instantiated(tcx, trait_ref.args));
         }

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -535,6 +535,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let assoc_types: Vec<_> = tcx
             .associated_items(trait_predicate.def_id())
             .in_definition_order()
+            // RPITITs are not checked here, since they are not (currently) object-safe
+            // and cannot be named from a non-`Self: Sized` method.
+            .filter(|item| !item.is_impl_trait_in_trait())
             .filter_map(
                 |item| if item.kind == ty::AssocKind::Type { Some(item.def_id) } else { None },
             )

tests/ui/impl-trait/in-trait/issue-102140.stderr

@@ -6,7 +6,11 @@ LL |         MyTrait::foo(&self)
    |         |
    |         required by a bound introduced by this call
    |
-   = help: the trait `MyTrait` is implemented for `Outer`
+help: consider removing the leading `&`-reference
+   |
+LL -         MyTrait::foo(&self)
+LL +         MyTrait::foo(self)
+   |
 
 error[E0277]: the trait bound `&dyn MyTrait: MyTrait` is not satisfied
   --> $DIR/issue-102140.rs:23:9

tests/ui/impl-trait/in-trait/object-safety-sized.rs

@@ -0,0 +1,23 @@
+// check-pass
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+
+#![feature(return_position_impl_trait_in_trait)]
+
+fn main() {
+    let vec: Vec<Box<dyn Trait>> = Vec::new();
+
+    for i in vec {
+        i.fn_2();
+    }
+}
+
+trait OtherTrait {}
+
+trait Trait {
+    fn fn_1(&self) -> impl OtherTrait
+    where
+        Self: Sized;
+
+    fn fn_2(&self) -> bool;
+}

tests/ui/impl-trait/in-trait/object-safety.rs

@@ -19,5 +19,4 @@ fn main() {
     //~| ERROR the trait `Foo` cannot be made into an object
     let s = i.baz();
     //~^ ERROR the trait `Foo` cannot be made into an object
-    //~| ERROR the trait `Foo` cannot be made into an object
 }

tests/ui/impl-trait/in-trait/object-safety.stderr

@@ -13,21 +13,6 @@ LL |     fn baz(&self) -> impl Debug;
    |                      ^^^^^^^^^^ ...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
-  --> $DIR/object-safety.rs:20:15
-   |
-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:7:22
-   |
-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
-   = help: consider moving `baz` to another trait
-
 error[E0038]: the trait `Foo` cannot be made into an object
   --> $DIR/object-safety.rs:20:13
    |
@@ -59,6 +44,6 @@ LL |     fn baz(&self) -> impl Debug;
    = help: consider moving `baz` to another trait
    = note: required for the cast from `Box<u32>` to `Box<dyn Foo>`
 
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0038`.