From d5f98fbb27f6f31a7d211353b016941fa45f699d Mon Sep 17 00:00:00 2001
From: Waffle Lapkin <waffle.lapkin@gmail.com>
Date: Mon, 6 Apr 2026 17:01:55 +0200
Subject: [PATCH] implement `StructuralPartialEq` for `MaybeDangling`

This fixes a stable-to-stable regression where constants of type
`ManuallyDrop<T>` would not be allowed to be used as a pattern due to
`MaybeDangling<T>` in `ManuallyDrop<T>` not implementing
`StructuralPartialEq`.
---
 library/core/src/mem/maybe_dangling.rs         |  3 +++
 tests/ui/consts/manually_drop_structural_eq.rs | 16 ++++++++++++++++
 2 files changed, 19 insertions(+)
 create mode 100644 tests/ui/consts/manually_drop_structural_eq.rs

diff --git a/library/core/src/mem/maybe_dangling.rs b/library/core/src/mem/maybe_dangling.rs
index c85576b5778cc..2c9c435c24085 100644
--- a/library/core/src/mem/maybe_dangling.rs
+++ b/library/core/src/mem/maybe_dangling.rs
@@ -1,5 +1,6 @@
 #![unstable(feature = "maybe_dangling", issue = "118166")]
 
+use crate::marker::StructuralPartialEq;
 use crate::{mem, ptr};
 
 /// Allows wrapped [references] and [boxes] to dangle.
@@ -109,3 +110,5 @@ impl<P: ?Sized> MaybeDangling<P> {
         x
     }
 }
+
+impl<T: ?Sized> StructuralPartialEq for MaybeDangling<T> {}
diff --git a/tests/ui/consts/manually_drop_structural_eq.rs b/tests/ui/consts/manually_drop_structural_eq.rs
new file mode 100644
index 0000000000000..1768ebd5f9c4e
--- /dev/null
+++ b/tests/ui/consts/manually_drop_structural_eq.rs
@@ -0,0 +1,16 @@
+// Check that `ManuallyDrop` types can be used as a constant when matching.
+// I.e. that `ManuallyDrop` implements `StructuralPartialEq`.
+//
+// Regression test for <https://github.com/rust-lang/rust/issues/154890>.
+//
+//@ check-pass
+use std::mem::ManuallyDrop;
+
+fn main() {
+    const X: ManuallyDrop<u32> = ManuallyDrop::new(1);
+
+    match ManuallyDrop::new(1) {
+        X => println!("blah"),
+        _ => println!("bleh"),
+    }
+}