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 {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

4 files changed, 17 insertions, 9 deletions

diff --git a/tests/ui/pattern/issue-115599.rs b/tests/ui/pattern/issue-115599.rs
index 1521d728d95..47fe6b87da9 100644
--- a/tests/ui/pattern/issue-115599.rs
+++ b/tests/ui/pattern/issue-115599.rs
@@ -3,5 +3,5 @@ const CONST_STRING: String = String::new();
 fn main() {
     let empty_str = String::from("");
     if let CONST_STRING = empty_str {}
-    //~^ ERROR to use a constant of type `Vec<u8>` in a pattern, `Vec<u8>` must be annotated with `#[derive(PartialEq)]`
+    //~^ ERROR constant of non-structural type `Vec<u8>` in a pattern
 }
diff --git a/tests/ui/pattern/issue-115599.stderr b/tests/ui/pattern/issue-115599.stderr
index 65e3b08ac48..69d10728ccd 100644
--- a/tests/ui/pattern/issue-115599.stderr
+++ b/tests/ui/pattern/issue-115599.stderr
@@ -1,13 +1,15 @@
-error: to use a constant of type `Vec<u8>` in a pattern, `Vec<u8>` must be annotated with `#[derive(PartialEq)]`
+error: constant of non-structural type `Vec<u8>` in a pattern
   --> $DIR/issue-115599.rs:5:12
    |
 LL | const CONST_STRING: String = String::new();
    | -------------------------- constant defined here
 ...
 LL |     if let CONST_STRING = empty_str {}
-   |            ^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^ constant of non-structural type
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+   |
+   = note: `Vec<u8>` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
    |
-   = note: the traits must be derived, manual `impl`s are not sufficient
    = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
 
 error: aborting due to 1 previous error
diff --git a/tests/ui/pattern/usefulness/const-partial_eq-fallback-ice.rs b/tests/ui/pattern/usefulness/const-partial_eq-fallback-ice.rs
index f34093ef149..f09dcf8498f 100644
--- a/tests/ui/pattern/usefulness/const-partial_eq-fallback-ice.rs
+++ b/tests/ui/pattern/usefulness/const-partial_eq-fallback-ice.rs
@@ -12,7 +12,7 @@ const CONSTANT: &&MyType = &&MyType;
 
 fn main() {
     if let CONSTANT = &&MyType {
-        //~^ ERROR must be annotated with `#[derive(PartialEq)]`
+        //~^ ERROR constant of non-structural type `MyType` in a pattern
         println!("did match!");
     }
 }
diff --git a/tests/ui/pattern/usefulness/const-partial_eq-fallback-ice.stderr b/tests/ui/pattern/usefulness/const-partial_eq-fallback-ice.stderr
index bcbcd0bc280..f9da0430f2e 100644
--- a/tests/ui/pattern/usefulness/const-partial_eq-fallback-ice.stderr
+++ b/tests/ui/pattern/usefulness/const-partial_eq-fallback-ice.stderr
@@ -1,14 +1,20 @@
-error: to use a constant of type `MyType` in a pattern, `MyType` must be annotated with `#[derive(PartialEq)]`
+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
    |
-   = note: the traits must be derived, manual `impl`s are not sufficient
-   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
+LL | impl PartialEq<usize> for MyType {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error