Fix missing impl trait display as ret type

Author: GuillaumeGomez

src/librustdoc/clean/mod.rs

@@ -2378,6 +2378,7 @@ impl From<ast::FloatTy> for PrimitiveType {
 impl Clean<Type> for hir::Ty {
     fn clean(&self, cx: &DocContext) -> Type {
         use rustc::hir::*;
+
         match self.node {
             TyKind::Never => Never,
             TyKind::Ptr(ref m) => RawPointer(m.mutbl.clean(cx), box m.ty.clean(cx)),
@@ -2415,6 +2416,14 @@ impl Clean<Type> for hir::Ty {
                     if let Some(bounds) = cx.impl_trait_bounds.borrow_mut().remove(&did) {
                         return ImplTrait(bounds);
                     }
+                } else if let Def::Existential(did) = path.def {
+                    // This block is for returned impl trait only.
+                    if let Some(node_id) = cx.tcx.hir.as_local_node_id(did) {
+                        let item = cx.tcx.hir.expect_item(node_id);
+                        if let hir::ItemKind::Existential(ref ty) = item.node {
+                            return ImplTrait(ty.bounds.clean(cx));
+                        }
+                    }
                 }
 
                 let mut alias = None;

src/test/rustdoc/impl-everywhere.rs

@@ -0,0 +1,40 @@
+// 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.
+
+#![crate_name = "foo"]
+
+pub trait Foo {}
+pub trait Foo2 {}
+
+pub struct Bar;
+
+impl Foo for Bar {}
+impl Foo2 for Bar {}
+
+// @!has foo/fn.foo.html '//section[@id="main"]//pre' "x: &\'x impl Foo"
+// @!has foo/fn.foo.html '//section[@id="main"]//pre' "-> &\'x impl Foo {"
+pub fn foo<'x>(x: &'x impl Foo) -> &'x impl Foo {
+    x
+}
+
+// @!has foo/fn.foo2.html '//section[@id="main"]//pre' "x: &\'x impl Foo"
+// @!has foo/fn.foo2.html '//section[@id="main"]//pre' '-> impl Foo2 {'
+pub fn foo2<'x>(_x: &'x impl Foo) -> impl Foo2 {
+    Bar
+}
+
+// @!has foo/fn.foo_foo.html '//section[@id="main"]//pre' '-> impl Foo + Foo2 {'
+pub fn foo_foo() -> impl Foo + Foo2 {
+    Bar
+}
+
+// @!has foo/fn.foo2.html '//section[@id="main"]//pre' "x: &'x (impl Foo + Foo2)"
+pub fn foo_foo_foo<'x>(_x: &'x (impl Foo + Foo2)) {
+}