From a7bdcceadb2c8b455939a6c55aa070e428d54cbe Mon Sep 17 00:00:00 2001
From: "Tim (Theemathas Chirananthavat)" <theemathas@gmail.com>
Date: Mon, 8 Jun 2026 20:46:55 +0700
Subject: [PATCH] Add a test where subtyping inhibits coercion.

---
 .../subtyping-inhibits-deref-coercion.rs      | 35 +++++++++++++++++++
 .../subtyping-inhibits-deref-coercion.stderr  | 12 +++++++
 2 files changed, 47 insertions(+)
 create mode 100644 tests/ui/coercion/subtyping-inhibits-deref-coercion.rs
 create mode 100644 tests/ui/coercion/subtyping-inhibits-deref-coercion.stderr

diff --git a/tests/ui/coercion/subtyping-inhibits-deref-coercion.rs b/tests/ui/coercion/subtyping-inhibits-deref-coercion.rs
new file mode 100644
index 0000000000000..8232dcc6b1f18
--- /dev/null
+++ b/tests/ui/coercion/subtyping-inhibits-deref-coercion.rs
@@ -0,0 +1,35 @@
+// For some reason, subtyping due to higher ranked function pointers,
+// even in an invariant position, causes deref coercion to not happen.
+// This might be a compiler bug. However, as of this writing,
+// the `pin!()` macro's soundness relies on code like this not compiling.
+// See https://github.com/rust-lang/rust/issues/153438#issuecomment-4517609119
+
+use std::marker::PhantomData;
+use std::ops::{Deref, DerefMut};
+
+// `Sub` is a subtype of `Sup`
+type Sup = fn(&'static ());
+type Sub = for<'a> fn(&'a ());
+
+struct Thing<S>(PhantomData<S>);
+
+impl Deref for Thing<Sub> {
+    type Target = Thing<Sup>;
+    fn deref(&self) -> &Thing<Sup> {
+        unimplemented!()
+    }
+}
+
+impl DerefMut for Thing<Sub> {
+    fn deref_mut(&mut self) -> &mut Thing<Sup> {
+        unimplemented!()
+    }
+}
+
+// This could, in theory, compile due to a deref coercion. However, it does not.
+fn foo(x: &mut Thing<Sub>) -> &mut Thing<Sup> {
+    x
+    //~^ ERROR mismatched types
+}
+
+fn main() {}
diff --git a/tests/ui/coercion/subtyping-inhibits-deref-coercion.stderr b/tests/ui/coercion/subtyping-inhibits-deref-coercion.stderr
new file mode 100644
index 0000000000000..c2a105e75fe82
--- /dev/null
+++ b/tests/ui/coercion/subtyping-inhibits-deref-coercion.stderr
@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/subtyping-inhibits-deref-coercion.rs:31:5
+   |
+LL |     x
+   |     ^ one type is more general than the other
+   |
+   = note: expected mutable reference `&mut Thing<fn(&())>`
+              found mutable reference `&mut Thing<for<'a> fn(&'a ())>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.