Avoid ICE in printing RPITIT type

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -927,7 +927,7 @@ pub trait PrettyPrinter<'tcx>:
                         // unless we can find out what generator return type it comes from.
                         let term = if let Some(ty) = term.skip_binder().ty()
                             && let ty::Projection(proj) = ty.kind()
-                            && let assoc = tcx.associated_item(proj.item_def_id)
+                            && let Some(assoc) = tcx.opt_associated_item(proj.item_def_id)
                             && assoc.trait_container(tcx) == tcx.lang_items().gen_trait()
                             && assoc.name == rustc_span::sym::Return
                         {

src/test/ui/impl-trait/in-trait/issue-102571.rs

@@ -0,0 +1,24 @@
+#![feature(return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+use std::fmt::Display;
+use std::ops::Deref;
+
+trait Foo {
+    fn bar(self) -> impl Deref<Target = impl Display + ?Sized>;
+}
+
+struct A;
+
+impl Foo for A {
+    fn bar(self) -> &'static str {
+        "Hello, world"
+    }
+}
+
+fn foo<T: Foo>(t: T) {
+    let () = t.bar();
+    //~^ ERROR mismatched types
+}
+
+fn main() {}

src/test/ui/impl-trait/in-trait/issue-102571.stderr

@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-102571.rs:20:9
+   |
+LL |     let () = t.bar();
+   |         ^^   ------- this expression has type `impl Deref<Target = impl std::fmt::Display + ?Sized>`
+   |         |
+   |         expected associated type, found `()`
+   |
+   = note: expected associated type `impl Deref<Target = impl std::fmt::Display + ?Sized>`
+                    found unit type `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.