Regression test for issue 60431.

Author: pnkfelix

src/test/ui/layout/issue-60431-unsized-tail-behind-projection.rs

@@ -0,0 +1,35 @@
+// rust-lang/rust#60431: This is a scenario where to determine the size of
+// `&Ref<Obstack>`, we need to know the concrete type of the last field in
+// `Ref<Obstack>` (i.e. its "struct tail"), and determining that concrete type
+// requires normalizing `Obstack::Dyn`.
+//
+// The old "struct tail" computation did not perform such normalization, and so
+// the compiler would ICE when trying to figure out if `Ref<Obstack>` is a
+// dynamically-sized type (DST).
+
+// run-pass
+
+use std::mem;
+
+pub trait Arena {
+    type Dyn : ?Sized;
+}
+
+pub struct DynRef {
+    _dummy: [()],
+}
+
+pub struct Ref<A: Arena> {
+    _value: u8,
+    _dyn_arena: A::Dyn,
+}
+
+pub struct Obstack;
+
+impl Arena for Obstack {
+    type Dyn = DynRef;
+}
+
+fn main() {
+    assert_eq!(mem::size_of::<&Ref<Obstack>>(), mem::size_of::<&[()]>());
+}