Test illustrating that the nested_impl_trait lint should only catch shallow cases.

src/test/ui/impl-trait/issue-57979-deeply-nested-impl-trait-in-assoc-proj.rs

+// rust-lang/rust#57979 : the initial support for `impl Trait` didn't
+// properly check syntax hidden behind an associated type projection,
+// but it did catch *some cases*. This is checking that we continue to
+// properly emit errors for those, even with the new
+// future-incompatibility warnings.
+//
+// issue-57979-nested-impl-trait-in-assoc-proj.rs shows the main case
+// that we were previously failing to catch.
+
+struct Deeper<T>(T);
+
+mod allowed {
+    #![allow(nested_impl_trait)]
+
+    pub trait Foo<T> { }
+    pub trait Bar { }
+    pub trait Quux { type Assoc; }
+    pub fn demo(_: impl Quux<Assoc=super::Deeper<impl Foo<impl Bar>>>) { }
+    //~^ ERROR nested `impl Trait` is not allowed
+}
+
+mod warned {
+    #![warn(nested_impl_trait)]
+
+    pub trait Foo<T> { }
+    pub trait Bar { }
+    pub trait Quux { type Assoc; }
+    pub fn demo(_: impl Quux<Assoc=super::Deeper<impl Foo<impl Bar>>>) { }
+    //~^ ERROR nested `impl Trait` is not allowed
+}
+
+mod denied {
+    #![deny(nested_impl_trait)]
+
+    pub trait Foo<T> { }
+    pub trait Bar { }
+    pub trait Quux { type Assoc; }
+    pub fn demo(_: impl Quux<Assoc=super::Deeper<impl Foo<impl Bar>>>) { }
+    //~^ ERROR nested `impl Trait` is not allowed
+}
+
+fn main() { }

src/test/ui/impl-trait/issue-57979-deeply-nested-impl-trait-in-assoc-proj.stderr

+error[E0666]: nested `impl Trait` is not allowed
+  --> $DIR/issue-57979-deeply-nested-impl-trait-in-assoc-proj.rs:18:59
+   |
+LL |     pub fn demo(_: impl Quux<Assoc=super::Deeper<impl Foo<impl Bar>>>) { }
+   |                                                  ---------^^^^^^^^-
+   |                                                  |        |
+   |                                                  |        nested `impl Trait` here
+   |                                                  outer `impl Trait`
+
+error[E0666]: nested `impl Trait` is not allowed
+  --> $DIR/issue-57979-deeply-nested-impl-trait-in-assoc-proj.rs:28:59
+   |
+LL |     pub fn demo(_: impl Quux<Assoc=super::Deeper<impl Foo<impl Bar>>>) { }
+   |                                                  ---------^^^^^^^^-
+   |                                                  |        |
+   |                                                  |        nested `impl Trait` here
+   |                                                  outer `impl Trait`
+
+error[E0666]: nested `impl Trait` is not allowed
+  --> $DIR/issue-57979-deeply-nested-impl-trait-in-assoc-proj.rs:38:59
+   |
+LL |     pub fn demo(_: impl Quux<Assoc=super::Deeper<impl Foo<impl Bar>>>) { }
+   |                                                  ---------^^^^^^^^-
+   |                                                  |        |
+   |                                                  |        nested `impl Trait` here
+   |                                                  outer `impl Trait`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0666`.