Add regression ui test for unconditional_recursion lint on PartialEq

tests/ui/unconditional_recursion.rs

@@ -264,6 +264,28 @@ impl S13 {
     }
 }
 
-fn main() {
-    // test code goes here
+struct S14 {
+    field: String,
+}
+
+impl PartialEq for S14 {
+    fn eq(&self, other: &Self) -> bool {
+        // Should not warn!
+        self.field.eq(&other.field)
+    }
+}
+
+struct S15<'a> {
+    field: &'a S15<'a>,
 }
+
+impl PartialEq for S15<'_> {
+    fn eq(&self, other: &Self) -> bool {
+        //~^ ERROR: function cannot return without recursing
+        let mine = &self.field;
+        let theirs = &other.field;
+        mine.eq(theirs)
+    }
+}
+
+fn main() {}

tests/ui/unconditional_recursion.stderr

@@ -340,5 +340,22 @@ note: recursive call site
 LL |         Self::default()
    |         ^^^^^^^^^^^^^^^
 
-error: aborting due to 26 previous errors
+error: function cannot return without recursing
+  --> $DIR/unconditional_recursion.rs:283:5
+   |
+LL | /     fn eq(&self, other: &Self) -> bool {
+LL | |
+LL | |         let mine = &self.field;
+LL | |         let theirs = &other.field;
+LL | |         mine.eq(theirs)
+LL | |     }
+   | |_____^
+   |
+note: recursive call site
+  --> $DIR/unconditional_recursion.rs:287:9
+   |
+LL |         mine.eq(theirs)
+   |         ^^^^^^^^^^^^^^^
+
+error: aborting due to 27 previous errors