Never emit vptr for empty/auto traits

Author: lrh2000

compiler/rustc_trait_selection/src/traits/vtable.rs

@@ -154,18 +154,17 @@ fn prepare_vtable_segments_inner<'tcx, T>(
 
         // emit innermost item, move to next sibling and stop there if possible, otherwise jump to outer level.
         while let Some((inner_most_trait_ref, emit_vptr, mut siblings)) = stack.pop() {
+            let has_entries =
+                has_own_existential_vtable_entries(tcx, inner_most_trait_ref.def_id());
+
             segment_visitor(VtblSegment::TraitOwnEntries {
                 trait_ref: inner_most_trait_ref,
-                emit_vptr: emit_vptr && !tcx.sess.opts.unstable_opts.no_trait_vptr,
+                emit_vptr: emit_vptr && has_entries && !tcx.sess.opts.unstable_opts.no_trait_vptr,
             })?;
 
             // If we've emitted (fed to `segment_visitor`) a trait that has methods present in the vtable,
             // we'll need to emit vptrs from now on.
-            if !emit_vptr_on_new_entry
-                && has_own_existential_vtable_entries(tcx, inner_most_trait_ref.def_id())
-            {
-                emit_vptr_on_new_entry = true;
-            }
+            emit_vptr_on_new_entry |= has_entries;
 
             if let Some(next_inner_most_trait_ref) =
                 siblings.find(|&sibling| visited.insert(sibling.upcast(tcx)))

tests/ui/traits/upcast_reorder.rs

@@ -0,0 +1,29 @@
+//@ run-pass
+//
+// issue: <https://github.com/rust-lang/rust/issues/131813>
+
+#![feature(trait_upcasting)]
+
+trait Pollable {
+    #[allow(unused)]
+    fn poll(&self) {}
+}
+trait FileIo: Pollable + Send + Sync {
+    fn read(&self) {}
+}
+trait Terminal: Send + Sync + FileIo {}
+
+struct A;
+
+impl Pollable for A {}
+impl FileIo for A {}
+impl Terminal for A {}
+
+fn main() {
+    let a = A;
+
+    let b = &a as &dyn Terminal;
+    let c = b as &dyn FileIo;
+
+    c.read();
+}