Allow Unpin impls for local extern type

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -153,6 +153,8 @@ fn visit_implementation_of_unpin(checker: &Checker<'_>) -> Result<(), ErrorGuara
                 }));
             }
             ty::Adt(_, _) => {}
+            // `extern type`s have no fields, so they can't be structurally pinned.
+            ty::Foreign(_) => {}
             _ => {
                 return Err(tcx.dcx().span_delayed_bug(span, "impl of `Unpin` for a non-adt type"));
             }

tests/ui/pin-ergonomics/impl-unpin-extern-type.rs

@@ -0,0 +1,18 @@
+//! Verify that a local `extern type` can manually implement `Unpin` with `pin_ergonomics` enabled.
+//! Unlike a `#[pin_v2]` ADT, an extern type has no fields that could be structually pinned.
+//!
+//! Regression test for <https://github.com/rust-lang/rust/issues/155053>.
+
+//@ check-pass
+
+#![feature(pin_ergonomics)]
+#![feature(extern_types)]
+#![allow(incomplete_features)]
+
+unsafe extern "C" {
+    type ExternType;
+}
+
+impl Unpin for ExternType {}
+
+fn main() {}