Add additional context for non-sructural type constant used in pattern

- Point at type that should derive `PartialEq` to be structural.
- Point at manual `impl PartialEq`, explaining that it is not sufficient to be structural.

```
error: constant of non-structural type `MyType` in a pattern
  --> $DIR/const-partial_eq-fallback-ice.rs:14:12
   |
LL | struct MyType;
   | ------------- `MyType` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL | const CONSTANT: &&MyType = &&MyType;
   | ------------------------ constant defined here
...
LL |     if let CONSTANT = &&MyType {
   |            ^^^^^^^^ constant of non-structural type
   |
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
  --> $DIR/const-partial_eq-fallback-ice.rs:5:1
   |
LL | impl PartialEq<usize> for MyType {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/ui/rfcs/rfc-1445-restrict-constants-in-patterns/cant-hide-behind-indirect-struct-param.rs b/tests/ui/rfcs/rfc-1445-restrict-constants-in-patterns/cant-hide-behind-indirect-struct-param.rs
index cbfabec6819..9226f9c3ecd 100644
--- a/tests/ui/rfcs/rfc-1445-restrict-constants-in-patterns/cant-hide-behind-indirect-struct-param.rs
+++ b/tests/ui/rfcs/rfc-1445-restrict-constants-in-patterns/cant-hide-behind-indirect-struct-param.rs
@@ -20,7 +20,7 @@ const WRAP_INDIRECT_PARAM: & &WrapParam<NoDerive> = & &WrapParam(NoDerive(0));
 fn main() {
     match WRAP_INDIRECT_PARAM {
         WRAP_INDIRECT_PARAM => { panic!("WRAP_INDIRECT_PARAM matched itself"); }
-        //~^ ERROR must be annotated with `#[derive(PartialEq)]`
+        //~^ ERROR constant of non-structural type `NoDerive` in a pattern
         _ => { println!("WRAP_INDIRECT_PARAM correctly did not match itself"); }
     }
 }