When determining if a trait has no entries for the purposes of omitting vptrs from subtrait vtables, consider its transitive supertraits' entries, instead of just its own entries.

compiler/rustc_trait_selection/src/traits/vtable.rs

@@ -153,7 +153,12 @@ 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);
+            // We don't need to emit a vptr for "truly-empty" supertraits, but we *do* need to emit a
+            // vptr for supertraits that have no methods, but that themselves have supertraits
+            // with methods, so we check if any transitive supertrait has entries here (this includes
+            // the trait itself).
+            let has_entries = ty::elaborate::supertrait_def_ids(tcx, inner_most_trait_ref.def_id)
+                .any(|def_id| has_own_existential_vtable_entries(tcx, def_id));
 
             segment_visitor(VtblSegment::TraitOwnEntries {
                 trait_ref: inner_most_trait_ref,

tests/ui/traits/vtable/empty-supertrait-with-nonempty-supersupertrait.dump.stderr

@@ -0,0 +1,16 @@
+error: vtable entries: [
+           MetadataDropInPlace,
+           MetadataSize,
+           MetadataAlign,
+           Method(<dyn OneTwo as One>::one - shim(reify)),
+           Method(<dyn OneTwo as Two>::two - shim(reify)),
+           TraitVPtr(<dyn OneTwo as Two>),
+           TraitVPtr(<dyn OneTwo as TwoAgain>),
+       ]
+  --> $DIR/empty-supertrait-with-nonempty-supersupertrait.rs:40:1
+   |
+LL | type T = dyn OneTwo;
+   | ^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/traits/vtable/empty-supertrait-with-nonempty-supersupertrait.rs

@@ -0,0 +1,49 @@
+//@ revisions: run dump
+//@[run] run-pass
+//@[dump] check-fail
+//! Regression test for #145752
+//! Ensure that `OneTwo` contains a vptr for `TwoAgain`
+#![allow(unused)]
+#![cfg_attr(dump, feature(rustc_attrs))]
+
+trait One {
+    fn one(&self) {
+        panic!("don't call this");
+    }
+}
+impl One for () {}
+
+trait Two {
+    fn two(&self) {
+        println!("good");
+    }
+}
+impl Two for () {}
+
+trait TwoAgain: Two {}
+impl<T: Two> TwoAgain for T {}
+
+trait OneTwo: One + TwoAgain {}
+impl<T: One + Two> OneTwo for T {}
+
+fn main() {
+    (&()).two();
+    (&() as &dyn OneTwo).two();
+    (&() as &dyn OneTwo as &dyn Two).two();
+
+    // these two used to panic because they called `one` due to #145752
+    (&() as &dyn OneTwo as &dyn TwoAgain).two();
+    (&() as &dyn OneTwo as &dyn TwoAgain as &dyn Two).two();
+}
+
+#[cfg_attr(dump, rustc_dump_vtable)]
+type T = dyn OneTwo;
+//[dump]~^ ERROR vtable entries: [
+//[dump]~| ERROR            MetadataDropInPlace,
+//[dump]~| ERROR            MetadataSize,
+//[dump]~| ERROR            MetadataAlign,
+//[dump]~| ERROR            Method(<dyn OneTwo as One>::one - shim(reify)),
+//[dump]~| ERROR            Method(<dyn OneTwo as Two>::two - shim(reify)),
+//[dump]~| ERROR            TraitVPtr(<dyn OneTwo as Two>),
+//[dump]~| ERROR            TraitVPtr(<dyn OneTwo as TwoAgain>),
+//[dump]~| ERROR        ]

tests/ui/traits/vtable/multiple-auto.rs

@@ -0,0 +1,50 @@
+// Related to <https://github.com/rust-lang/rust/issues/113840>
+//
+// This test makes sure that multiple auto traits can reuse the
+// same pointer for upcasting (e.g. `Send`/`Sync`)
+
+#![crate_type = "lib"]
+#![feature(rustc_attrs, auto_traits)]
+
+// Markers
+auto trait M0 {}
+auto trait M1 {}
+auto trait M2 {}
+
+// Just a trait with a method
+trait T {
+    fn method(&self) {}
+}
+
+trait A: M0 + M1 + M2 + T {}
+
+trait B: M0 + M1 + T + M2 {}
+
+trait C: M0 + T + M1 + M2 {}
+
+trait D: T + M0 + M1 + M2 {}
+
+struct S;
+
+impl M0 for S {}
+impl M1 for S {}
+impl M2 for S {}
+impl T for S {}
+
+#[rustc_dump_vtable]
+impl A for S {}
+//~^ ERROR vtable entries
+
+#[rustc_dump_vtable]
+impl B for S {}
+//~^ ERROR vtable entries
+
+#[rustc_dump_vtable]
+impl C for S {}
+//~^ ERROR vtable entries
+
+#[rustc_dump_vtable]
+impl D for S {}
+//~^ ERROR vtable entries
+
+fn main() {}

tests/ui/traits/vtable/multiple-auto.stderr

@@ -0,0 +1,46 @@
+error: vtable entries: [
+           MetadataDropInPlace,
+           MetadataSize,
+           MetadataAlign,
+           Method(<S as T>::method),
+       ]
+  --> $DIR/multiple-auto.rs:35:1
+   |
+LL | impl A for S {}
+   | ^^^^^^^^^^^^
+
+error: vtable entries: [
+           MetadataDropInPlace,
+           MetadataSize,
+           MetadataAlign,
+           Method(<S as T>::method),
+       ]
+  --> $DIR/multiple-auto.rs:39:1
+   |
+LL | impl B for S {}
+   | ^^^^^^^^^^^^
+
+error: vtable entries: [
+           MetadataDropInPlace,
+           MetadataSize,
+           MetadataAlign,
+           Method(<S as T>::method),
+       ]
+  --> $DIR/multiple-auto.rs:43:1
+   |
+LL | impl C for S {}
+   | ^^^^^^^^^^^^
+
+error: vtable entries: [
+           MetadataDropInPlace,
+           MetadataSize,
+           MetadataAlign,
+           Method(<S as T>::method),
+       ]
+  --> $DIR/multiple-auto.rs:47:1
+   |
+LL | impl D for S {}
+   | ^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+