add some more positive tests

It'd be good to have a positive test for each case where it is
allowed, I should think.

Author: nikomatsakis

src/test/run-pass/impl-trait/universal_hrtb_anon.rs

@@ -0,0 +1,20 @@
+// 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.
+
+#![feature(universal_impl_trait)]
+
+fn hrtb(f: impl Fn(&u32) -> u32) -> u32 {
+    f(&22) + f(&44)
+}
+
+fn main() {
+    let sum = hrtb(|x| x * 2);
+    assert_eq!(sum, 22*2 + 44*2);
+}

src/test/run-pass/impl-trait/universal_hrtb_named.rs

@@ -0,0 +1,20 @@
+// 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.
+
+#![feature(universal_impl_trait)]
+
+fn hrtb(f: impl for<'a> Fn(&'a u32) -> &'a u32) -> u32 {
+    f(&22) + f(&44)
+}
+
+fn main() {
+    let sum = hrtb(|x| x);
+    assert_eq!(sum, 22 + 44);
+}

src/test/run-pass/impl-trait/universal_in_adt_in_parameters.rs

@@ -0,0 +1,31 @@
+// 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.
+
+#![feature(universal_impl_trait)]
+use std::fmt::Display;
+
+fn check_display_eq(iter: &Vec<impl Display>) {
+    let mut collected = String::new();
+    for it in iter {
+        let disp = format!("{} ", it);
+        collected.push_str(&disp);
+    }
+    assert_eq!("0 3 27 823 4891 1 0", collected.trim());
+}
+
+fn main() {
+    let i32_list_vec = vec![0i32, 3, 27, 823, 4891, 1, 0];
+    let u32_list_vec = vec![0u32, 3, 27, 823, 4891, 1, 0];
+    let str_list_vec = vec!["0", "3", "27", "823", "4891", "1", "0"];
+
+    check_display_eq(&i32_list_vec);
+    check_display_eq(&u32_list_vec);
+    check_display_eq(&str_list_vec);
+}

src/test/run-pass/impl-trait/universal_in_trait_defn_parameters.rs

@@ -0,0 +1,28 @@
+// 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.
+
+#![feature(universal_impl_trait)]
+
+use std::fmt::Debug;
+
+trait InTraitDefnParameters {
+    fn in_parameters(_: impl Debug) -> String;
+}
+
+impl InTraitDefnParameters for () {
+    fn in_parameters(v: impl Debug) -> String {
+        format!("() + {:?}", v)
+    }
+}
+
+fn main() {
+    let s = <() as InTraitDefnParameters>::in_parameters(22);
+    assert_eq!(s, "() + 22");
+}