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.)

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

@@ -0,0 +1,16 @@
+// compile-flags: -C debuginfo=2
+
+// no-prefer-dynamic
+#![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/b_reexport_obj.rs

@@ -0,0 +1,7 @@
+// compile-flags: -C debuginfo=2 -C prefer-dynamic
+
+#![crate_type="dylib"]
+
+extern crate a_def_obj;
+
+pub use a_def_obj::Object;

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

@@ -0,0 +1,12 @@
+// no-prefer-dynamic
+// compile-flags: -C debuginfo=2
+#![crate_type="rlib"]
+
+extern crate b_reexport_obj;
+use b_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/d_chain_of_rlibs_and_dylibs.rs

@@ -0,0 +1,9 @@
+// compile-flags: -C debuginfo=2 -C prefer-dynamic
+
+#![crate_type="rlib"]
+
+extern crate c_another_vtable_for_obj;
+
+pub fn chain() {
+    c_another_vtable_for_obj::another_dyn_debug();
+}

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

@@ -0,0 +1,17 @@
+// run-pass
+
+// note that these aux-build directives must be in this order: the
+// later crates depend on the earlier ones. (The particular bug that
+// is being exercised here used to exhibit itself during the build of
+// `chain_of_rlibs_and_dylibs.dylib`)
+
+// aux-build:a_def_obj.rs
+// aux-build:b_reexport_obj.rs
+// aux-build:c_another_vtable_for_obj.rs
+// aux-build:d_chain_of_rlibs_and_dylibs.rs
+
+extern crate d_chain_of_rlibs_and_dylibs;
+
+pub fn main() {
+    d_chain_of_rlibs_and_dylibs::chain();
+}