Rollup merge of #47529 - nikomatsakis:impl-trait-issue-38064, r=cramertj

track recursion limit when expanding existential impl trait

r? @cramertj

src/test/compile-fail/impl-trait/infinite-impl-trait-issue-38064.rs

@@ -0,0 +1,39 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that attempts to construct infinite types via impl trait fail
+// in a graceful way.
+//
+// Regression test for #38064.
+
+// error-pattern:overflow evaluating the requirement `impl Quux`
+
+#![feature(conservative_impl_trait)]
+
+trait Quux {}
+
+fn foo() -> impl Quux {
+    struct Foo<T>(T);
+    impl<T> Quux for Foo<T> {}
+    Foo(bar())
+}
+
+fn bar() -> impl Quux {
+    struct Bar<T>(T);
+    impl<T> Quux for Bar<T> {}
+    Bar(foo())
+}
+
+// effectively:
+//     struct Foo(Bar);
+//     struct Bar(Foo);
+// should produce an error about infinite size
+
+fn main() { foo(); }