pretty small

src/lib.rs

@@ -0,0 +1,69 @@
+trait SecondTrait {
+    type Item2;
+}
+
+pub trait FirstTrait {
+    type Item;
+    type Extra: Extra<(), Error = Self::Item>;
+}
+
+trait ThirdTrait: SecondTrait {
+    type Item3;
+}
+
+trait FourthTrait {
+    type Item4;
+}
+
+impl<First> SecondTrait for First
+where
+    First: FirstTrait,
+{
+    type Item2 = First::Extra;
+}
+
+impl<Second, T> ThirdTrait for Second
+where
+    Second: SecondTrait<Item2 = T>,
+{
+    type Item3 = T;
+}
+
+impl<S, Third: ?Sized> FourthTrait for Third
+where
+    Third: ThirdTrait<Item3 = S>,
+{
+    type Item4 = S;
+}
+
+pub trait Extra<Request> {
+    type Error;
+}
+
+struct ImplShoulExist<D, Req> {
+    _gen: (D, Req),
+}
+
+impl<D, Req> ImplShoulExist<D, Req>
+where
+    D: FourthTrait,
+    D::Item4: Extra<Req>,
+    <D::Item4 as Extra<Req>>::Error: Into<()>,
+{
+    fn access_fn(_: D) {
+        todo!()
+    }
+}
+
+impl<D, Req> Extra<Req> for ImplShoulExist<D, Req> {
+    type Error = ();
+}
+
+pub fn broken<MS>(ms: MS)
+where
+    MS: FirstTrait,
+    MS::Item: Into<()>,
+{
+    // Error: Apparently Balance::new doesn't exist during MIR validation
+    let _ = ImplShoulExist::<MS, ()>::access_fn(ms);
+}

src/main.rs

@@ -1,73 +0,0 @@
-trait Stream {
-    type Item;
-}
-
-trait TryStream: Stream {
-    type TryItem;
-}
-
-impl<S, T> TryStream for S
-where
-    S: Stream<Item = T>,
-{
-    type TryItem = T;
-}
-
-trait Discover {
-    type Service;
-}
-
-impl<S, D: ?Sized> Discover for D
-where
-    D: TryStream<TryItem = S>,
-{
-    type Service = S;
-}
-
-pub trait Service<Request> {
-    type Error;
-}
-
-pub trait MakeService {
-    type Error;
-    type Service: Service<(), Error = Self::Error>;
-}
-
-struct Balance<D, Req> {
-    _dreq: (D, Req),
-}
-
-impl<D, Req> Balance<D, Req>
-where
-    D: Discover,
-    D::Service: Service<Req>,
-    <D::Service as Service<Req>>::Error: Into<()>,
-{
-    fn new(_: D) -> Self {
-        todo!()
-    }
-}
-
-impl<D, Req> Service<Req> for Balance<D, Req> {
-    type Error = ();
-}
-
-impl<MS> Stream for MS
-where
-    MS: MakeService,
-{
-    type Item = MS::Service;
-}
-
-pub fn broken<MS>(ms: MS)
-where
-    MS: MakeService,
-    MS::Error: Into<()>,
-{
-    // Error: Apparently Balance::new doesn't exist during MIR validation
-    let _ = Balance::<MS, ()>::new(ms);
-}
-
-fn main() {
-    println!("Hello, world!");
-}