Rollup merge of #59459 - JohnTitor:add-tests, r=Centril

Add some tests

close #52977

It seems that there are no tests for this issue, so I opened this PR.
off-topic: I noticed [this test](https://github.com/rust-lang/rust/blob/master/src/test/ui/existential_types/nested_existential_types.rs)'s indents are bad, could I include commit to fix this, or should I separate?

r? @oli-obk

src/test/ui/existential_types/existential-types-with-cycle-error.rs

@@ -0,0 +1,12 @@
+#![feature(existential_type)]
+
+existential type Foo: Fn() -> Foo;
+//~^ ERROR: cycle detected when processing `Foo`
+
+fn crash(x: Foo) -> Foo {
+    x
+}
+
+fn main() {
+
+}

src/test/ui/existential_types/existential-types-with-cycle-error.stderr

@@ -0,0 +1,30 @@
+error[E0391]: cycle detected when processing `Foo`
+  --> $DIR/existential-types-with-cycle-error.rs:3:1
+   |
+LL | existential type Foo: Fn() -> Foo;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: ...which requires processing `crash`...
+  --> $DIR/existential-types-with-cycle-error.rs:6:25
+   |
+LL |   fn crash(x: Foo) -> Foo {
+   |  _________________________^
+LL | |     x
+LL | | }
+   | |_^
+   = note: ...which again requires processing `Foo`, completing the cycle
+note: cycle used when collecting item types in top-level module
+  --> $DIR/existential-types-with-cycle-error.rs:1:1
+   |
+LL | / #![feature(existential_type)]
+LL | |
+LL | | existential type Foo: Fn() -> Foo;
+LL | |
+...  |
+LL | |
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.

src/test/ui/existential_types/existential-types-with-cycle-error2.rs

@@ -0,0 +1,16 @@
+#![feature(existential_type)]
+
+pub trait Bar<T> {
+    type Item;
+}
+
+existential type Foo: Bar<Foo, Item = Foo>;
+//~^ ERROR: cycle detected when processing `Foo`
+
+fn crash(x: Foo) -> Foo {
+    x
+}
+
+fn main() {
+
+}

src/test/ui/existential_types/existential-types-with-cycle-error2.stderr

@@ -0,0 +1,30 @@
+error[E0391]: cycle detected when processing `Foo`
+  --> $DIR/existential-types-with-cycle-error2.rs:7:1
+   |
+LL | existential type Foo: Bar<Foo, Item = Foo>;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: ...which requires processing `crash`...
+  --> $DIR/existential-types-with-cycle-error2.rs:10:25
+   |
+LL |   fn crash(x: Foo) -> Foo {
+   |  _________________________^
+LL | |     x
+LL | | }
+   | |_^
+   = note: ...which again requires processing `Foo`, completing the cycle
+note: cycle used when collecting item types in top-level module
+  --> $DIR/existential-types-with-cycle-error2.rs:1:1
+   |
+LL | / #![feature(existential_type)]
+LL | |
+LL | | pub trait Bar<T> {
+LL | |     type Item;
+...  |
+LL | |
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.