Add universal_impl_trait feature gate

Move feature gate check to inside HIR lowering. Change error messages
and update tests.

src/librustc/hir/lowering.rs

@@ -771,9 +771,21 @@ impl<'a> LoweringContext<'a> {
                 use syntax::feature_gate::{emit_feature_err, GateIssue};
                 match itctx {
                     ImplTraitContext::Existential => {
+                        let has_feature = self.sess.features.borrow().conservative_impl_trait;
+                        if !t.span.allows_unstable() && !has_feature {
+                            emit_feature_err(&self.sess.parse_sess, "conservative_impl_trait",
+                                             t.span, GateIssue::Language,
+                                             "`impl Trait` in return position is experimental");
+                        }
                         hir::TyImplTraitExistential(self.lower_bounds(bounds, itctx))
                     },
                     ImplTraitContext::Universal(def_id) => {
+                        let has_feature = self.sess.features.borrow().universal_impl_trait;
+                        if !t.span.allows_unstable() && !has_feature {
+                            emit_feature_err(&self.sess.parse_sess, "universal_impl_trait",
+                                             t.span, GateIssue::Language,
+                                             "`impl Trait` in argument position is experimental");
+                        }
                         hir::TyImplTraitUniversal(def_id, self.lower_bounds(bounds, itctx))
                     },
                     ImplTraitContext::Disallowed => {

src/libsyntax/feature_gate.rs

@@ -275,6 +275,9 @@ declare_features! (
     // Allows `impl Trait` in function return types.
     (active, conservative_impl_trait, "1.12.0", Some(34511)),
 
+    // Allows `impl Trait` in function arguments.
+    (active, universal_impl_trait, "1.23.0", Some(34511)),
+
     // The `!` type
     (active, never_type, "1.13.0", Some(35121)),
 
@@ -1451,10 +1454,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             ast::TyKind::BareFn(ref bare_fn_ty) => {
                 self.check_abi(bare_fn_ty.abi, ty.span);
             }
-            ast::TyKind::ImplTrait(..) => {
-                gate_feature_post!(&self, conservative_impl_trait, ty.span,
-                                   "`impl Trait` is experimental");
-            }
             ast::TyKind::Never => {
                 gate_feature_post!(&self, never_type, ty.span,
                                    "The `!` type is experimental");

src/test/compile-fail/impl-trait/feature-gate-universal.rs

@@ -0,0 +1,16 @@
+// Copyright 2017 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.
+
+// gate-test-universal_impl_trait
+
+fn foo(x: impl std::fmt::Debug) { print!("{:?}", x); }
+//~^ ERROR `impl Trait` in argument position is experimental
+
+fn main() {}

src/test/compile-fail/impl-trait/feature-gate.rs

@@ -11,6 +11,6 @@
 // gate-test-conservative_impl_trait
 
 fn foo() -> impl Fn() { || {} }
-//~^ ERROR `impl Trait` is experimental
+//~^ ERROR `impl Trait` in return position is experimental
 
 fn main() {}