Rollup merge of #56456 - oli-obk:private_impl_trait, r=cramertj

Handle existential types in dead code analysis

fixes #55124

r? @cramertj

src/test/ui/existential_types/private_unused.rs

@@ -0,0 +1,13 @@
+// compile-pass
+
+#[deny(warnings)]
+
+enum Empty { }
+trait Bar<T> {}
+impl Bar<Empty> for () {}
+
+fn boo() -> impl Bar<Empty> {}
+
+fn main() {
+    boo();
+}