Fix rustdoc ICE for impl trait in argument position

Author: GuillaumeGomez

src/librustdoc/clean/mod.rs

@@ -4048,7 +4048,6 @@ where
 
     let old_bounds = mem::replace(&mut *cx.impl_trait_bounds.borrow_mut(), bounds);
     let r = f();
-    assert!(cx.impl_trait_bounds.borrow().is_empty());
     *cx.impl_trait_bounds.borrow_mut() = old_bounds;
     r
 }

src/test/rustdoc/ice-impl-trait-arg-position.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 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.
+
+use std::io::Read;
+
+pub struct D<R>(R);
+
+pub struct A;
+impl A {
+    pub fn a(&self, d: impl IntoIterator<Item = D<impl Read>>) -> Result<(), ()> {
+        unimplemented!()
+    }
+
+    pub fn b(&self, d: impl IntoIterator<Item = D<()>>) -> Result<(), ()> {
+        unimplemented!()
+    }
+}