trait_sel: delay bug for removed pointeesized

This code path isn't hit during normal compilation, only in error
reporting, so delaying the bug is sufficient. It might be possible to
trigger this with `dyn PointeeSized` and code that doesn't error but no
such example could be found at the time of writing.

Author: davidtwco

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

@@ -101,7 +101,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     );
                 }
                 Some(LangItem::PointeeSized) => {
-                    bug!("`PointeeSized` is removed during lowering");
+                    tcx.dcx().delayed_bug("`PointeeSized` is removed during lowering");
                 }
                 Some(LangItem::Unsize) => {
                     self.assemble_candidates_for_unsizing(obligation, &mut candidates);

tests/ui/sized-hierarchy/dyn-pointeesized-issue-142652.rs

@@ -0,0 +1,8 @@
+// check-pass
+#![feature(sized_hierarchy)]
+use std::marker::PointeeSized;
+fn main() {
+      let x = main;
+      let y: Box<dyn PointeeSized> = x;
+//~^ ERROR mismatched types
+}

tests/ui/sized-hierarchy/dyn-pointeesized-issue-142652.stderr

@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/dyn-pointeesized-issue-142652.rs:6:38
+   |
+LL |       let y: Box<dyn PointeeSized> = x;
+   |              ---------------------   ^ expected `Box<dyn PointeeSized>`, found fn item
+   |              |
+   |              expected due to this
+   |
+   = note: expected struct `Box<dyn PointeeSized>`
+             found fn item `fn() {main}`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.