Handle cycles in impl types better

- impl Trait<Self> for S is allowed
 - impl Trait for S<Self> is an invalid cycle, but we can add cycle recovery for
   it in Salsa now

crates/ra_hir_ty/src/tests.rs

@@ -4675,6 +4675,48 @@ fn test<T, U>() where T::Item: Trait2, T: Trait<U::Item>, U: Trait<()> {
     assert_eq!(t, "u32");
 }
 
+#[test]
+fn trait_impl_self_ty() {
+    let t = type_at(
+        r#"
+//- /main.rs
+trait Trait<T> {
+   fn foo(&self);
+}
+
+struct S;
+
+impl Trait<Self> for S {}
+
+fn test() {
+    S.foo()<|>;
+}
+"#,
+    );
+    assert_eq!(t, "()");
+}
+
+#[test]
+fn trait_impl_self_ty_cycle() {
+    let t = type_at(
+        r#"
+//- /main.rs
+trait Trait {
+   fn foo(&self);
+}
+
+struct S<T>;
+
+impl Trait for S<Self> {}
+
+fn test() {
+    S.foo()<|>;
+}
+"#,
+    );
+    assert_eq!(t, "{unknown}");
+}
+
 #[test]
 // FIXME this is currently a Salsa panic; it would be nicer if it just returned
 // in Unknown, and we should be able to do that once Salsa allows us to handle