ui test formulation of regression test for issue 64872.

(Many thanks to alex for 1. making this even smaller than what I had
originally minimized, and 2. pointing out that there is precedent for
having ui tests with crate dependency chains of length > 2, thus
allowing me to avoid encoding this as a run-make test.)

Author: pnkfelix

src/test/ui/cross-crate/issue-64872/auxiliary/another_vtable_for_obj.rs

@@ -0,0 +1,10 @@
+// compile-flags: -C debuginfo=2 --crate-type=rlib
+
+extern crate reexport_obj;
+use reexport_obj::Object;
+
+pub fn another_dyn_debug() {
+    let ref u = 1_u32;
+    let _d = &u as &dyn crate::Object;
+    _d.method()
+}

src/test/ui/cross-crate/issue-64872/auxiliary/def_obj.rs

@@ -0,0 +1,13 @@
+// compile-flags: -C debuginfo=2 --crate-type=rlib
+
+pub trait Object { fn method(&self) { } }
+
+impl Object for u32 { }
+impl Object for () { }
+impl<T> Object for &T { }
+
+pub fn unused() {
+    let ref u = 0_u32;
+    let _d = &u as &dyn crate::Object;
+    _d.method()
+}

src/test/ui/cross-crate/issue-64872/auxiliary/reexport_obj.rs

@@ -0,0 +1,5 @@
+// compile-flags: -C debuginfo=2 -C prefer-dynamic --crate-type=dylib
+
+extern crate def_obj;
+
+pub use def_obj::Object;

src/test/ui/cross-crate/issue-64872/chain-of-rlibs-and-dylibs.rs

@@ -0,0 +1,12 @@
+// note that these aux-build directives must be in this order: the later crates
+// depend on the earlier ones.
+
+// aux-build:def_obj.rs
+// aux-build:rexport_obj.rs
+// aux-build:another_vtable_for_obj.rs
+
+extern crate another_vtable_for_obj;
+
+pub fn main() {
+    another_vtable_for_obj::another_dyn_debug();
+}