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`.

Author: WaffleLapkin

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> {}

tests/ui/consts/manually_drop_structural_eq.rs

@@ -0,0 +1,14 @@
+// Check that `ManuallyDrop` types can be used as a constant when matching.
+// I.e. that `ManuallyDrop` implements `StructuralPartialEq`.
+//
+//@ check-pass
+use std::mem::ManuallyDrop;
+
+fn main() {
+    const X: ManuallyDrop<u32> = ManuallyDrop::new(1);
+
+    match ManuallyDrop::new(1) {
+        X => println!("blah"),
+        _ => println!("bleh"),
+    }
+}