Add existing behaviour test for deref suggestions.

This commit adds a test that demonstrates the current behaviour where
suggestions to add a dereference aren't given for non-references.

Author: davidtwco

src/test/ui/suggestions/issue-59819.rs

@@ -0,0 +1,20 @@
+#![allow(warnings)]
+
+// Test that suggestion to add `*` characters applies to implementations of `Deref` as well as
+// references.
+
+struct Foo(i32);
+
+impl std::ops::Deref for Foo {
+    type Target = i32;
+    fn deref(&self) -> &i32 {
+        &self.0
+    }
+}
+
+fn main() {
+    let x = Foo(42);
+    let y: i32 = x; //~ ERROR mismatched types
+    let a = &42;
+    let b: i32 = a; //~ ERROR mismatched types
+}

src/test/ui/suggestions/issue-59819.stderr

@@ -0,0 +1,24 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-59819.rs:17:18
+   |
+LL |     let y: i32 = x;
+   |                  ^ expected i32, found struct `Foo`
+   |
+   = note: expected type `i32`
+              found type `Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-59819.rs:19:18
+   |
+LL |     let b: i32 = a;
+   |                  ^
+   |                  |
+   |                  expected i32, found &{integer}
+   |                  help: consider dereferencing the borrow: `*a`
+   |
+   = note: expected type `i32`
+              found type `&{integer}`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.