rustdoc: synthetic auto trait impls: accept unresolved region vars for now

tests/rustdoc-ui/synthetic-auto-trait-impls/lifetime-generic-user-impl-normalize.rs

@@ -0,0 +1,17 @@
+// We used to ICE here while trying to synthesize auto trait impls.
+// issue: 112242
+//@ check-pass
+//@ compile-flags: -Znormalize-docs
+
+pub trait MyTrait<'a> {
+    type MyItem;
+}
+pub struct Inner<Q>(Q);
+pub struct Outer<Q>(Inner<Q>);
+
+impl<'a, Q> std::marker::Unpin for Inner<Q>
+where
+    Q: MyTrait<'a>,
+    <Q as MyTrait<'a>>::MyItem: Copy,
+{
+}