Add a (failing test) for issue 113840

Author: WaffleLapkin

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

@@ -0,0 +1,47 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/113840>
+//
+// This test makes sure that multiple marker (method-less) traits can reuse the
+// same pointer for upcasting.
+//
+// build-fail
+#![crate_type = "lib"]
+#![feature(rustc_attrs)]
+
+// Markers
+trait M0 {}
+trait M1 {}
+trait M2 {}
+
+// Just a trait with a method
+trait T {
+    fn method(&self) {}
+}
+
+#[rustc_dump_vtable]
+trait A: M0 + M1 + M2 + T {} //~ error: vtable entries for `<S as A>`:
+
+#[rustc_dump_vtable]
+trait B: M0 + M1 + T + M2 {} //~ error: vtable entries for `<S as B>`:
+
+#[rustc_dump_vtable]
+trait C: M0 + T + M1 + M2 {} //~ error: vtable entries for `<S as C>`:
+
+#[rustc_dump_vtable]
+trait D: T + M0 + M1 + M2 {} //~ error: vtable entries for `<S as D>`:
+
+struct S;
+
+impl M0 for S {}
+impl M1 for S {}
+impl M2 for S {}
+impl T for S {}
+impl A for S {}
+impl B for S {}
+impl C for S {}
+impl D for S {}
+
+pub fn require_vtables() {
+    fn require_vtables(_: &dyn A, _: &dyn B, _: &dyn C, _: &dyn D) {}
+
+    require_vtables(&S, &S, &S, &S)
+}

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

@@ -0,0 +1,58 @@
+error: vtable entries for `<S as A>`: [
+           MetadataDropInPlace,
+           MetadataSize,
+           MetadataAlign,
+           TraitVPtr(<S as M1>),
+           TraitVPtr(<S as M2>),
+           Method(<S as T>::method),
+           TraitVPtr(<S as T>),
+       ]
+  --> $DIR/multiple-markers.rs:21:1
+   |
+LL | trait A: M0 + M1 + M2 + T {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: vtable entries for `<S as B>`: [
+           MetadataDropInPlace,
+           MetadataSize,
+           MetadataAlign,
+           TraitVPtr(<S as M1>),
+           Method(<S as T>::method),
+           TraitVPtr(<S as T>),
+           TraitVPtr(<S as M2>),
+       ]
+  --> $DIR/multiple-markers.rs:24:1
+   |
+LL | trait B: M0 + M1 + T + M2 {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: vtable entries for `<S as C>`: [
+           MetadataDropInPlace,
+           MetadataSize,
+           MetadataAlign,
+           Method(<S as T>::method),
+           TraitVPtr(<S as T>),
+           TraitVPtr(<S as M1>),
+           TraitVPtr(<S as M2>),
+       ]
+  --> $DIR/multiple-markers.rs:27:1
+   |
+LL | trait C: M0 + T + M1 + M2 {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: vtable entries for `<S as D>`: [
+           MetadataDropInPlace,
+           MetadataSize,
+           MetadataAlign,
+           Method(<S as T>::method),
+           TraitVPtr(<S as M0>),
+           TraitVPtr(<S as M1>),
+           TraitVPtr(<S as M2>),
+       ]
+  --> $DIR/multiple-markers.rs:30:1
+   |
+LL | trait D: T + M0 + M1 + M2 {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+