add regression test for #57979

Author: nikomatsakis

src/test/ui/issues/issue-57979.rs

@@ -0,0 +1,42 @@
+// Regression test for #57979. This situation is meant to be an error.
+// As noted in the issue thread, we decided to forbid nested impl
+// trait of this kind:
+//
+// ```rust
+// fn foo() -> impl Foo<impl Bar> { .. }
+// ```
+//
+// Basically there are two hidden variables here, let's call them `X`
+// and `Y`, and we must prove that:
+//
+// ```
+// X: Foo<Y>
+// Y: Bar
+// ```
+//
+// However, the user is only giving us the return type `X`. It's true
+// that in some cases, we can infer `Y` from `X`, because `X` only
+// implements `Foo` for one type (and indeed the compiler does
+// inference of this kind), but I do recall that we intended to forbid
+// this -- in part because such inference is fragile, and there is not
+// necessarily a way for the user to be more explicit should the
+// inference fail (so you could get stuck with no way to port your
+// code forward if, for example, more impls are added to an existing
+// type).
+//
+// The same seems to apply in this situation. Here there are three impl traits, so we have
+//
+// ```
+// X: IntoIterator<Item = Y>
+// Y: Borrow<Data<Z>>
+// Z: AsRef<[u8]>
+// ```
+
+use std::borrow::Borrow;
+
+pub struct Data<TBody>(TBody);
+
+pub fn collect(_: impl IntoIterator<Item = impl Borrow<Data<impl AsRef<[u8]>>>>) {
+    //~^ ERROR
+    unimplemented!()
+}