Add a failing rustdoc-ui test for public infinite recursive type

tests/rustdoc-ui/error-in-impl-trait/infinite-recursive-type-2.rs

@@ -0,0 +1,9 @@
+// check-pass
+
+pub fn f() -> impl Sized {
+    pub enum E {
+        V(E),
+    }
+
+    unimplemented!()
+}

tests/rustdoc-ui/error-in-impl-trait/infinite-recursive-type-impl-trait-return.rs

@@ -0,0 +1,14 @@
+// normalize-stderr-test: "`.*`" -> "`DEF_ID`"
+// normalize-stdout-test: "`.*`" -> "`DEF_ID`"
+// edition:2018
+
+pub async fn f() -> impl std::fmt::Debug {
+    #[derive(Debug)]
+    enum E { //~ ERROR
+        This(E),
+        Unit,
+    }
+    E::Unit
+}
+
+fn main() {}

tests/rustdoc-ui/error-in-impl-trait/infinite-recursive-type-impl-trait-return.stderr

@@ -0,0 +1,16 @@
+error[E0072]: recursive type `DEF_ID` has infinite size
+  --> $DIR/infinite-recursive-type-impl-trait-return.rs:7:5
+   |
+LL |     enum E {
+   |     ^^^^^^
+LL |         This(E),
+   |              - recursive without indirection
+   |
+help: insert some indirection (e.g., a `DEF_ID`) to break the cycle
+   |
+LL |         This(Box<E>),
+   |              ++++ +
+
+error: aborting due to previous error
+
+For more information about this error, try `DEF_ID`.

tests/rustdoc-ui/error-in-impl-trait/infinite-recursive-type.rs

@@ -0,0 +1,9 @@
+// check-pass
+
+fn f() -> impl Sized {
+    enum E {
+        V(E),
+    }
+
+    unimplemented!()
+}