Auto merge of #46479 - bkchr:termination_trait, r=arielb1

Implements RFC 1937: `?` in `main`

This is the first part of the RFC 1937 that supports new
`Termination` trait in the rust `main` function.

Thanks @nikomatsakis, @arielb1 and all other people in the gitter channel for all your help!

The support for doctest and `#[test]` is still missing, bu as @nikomatsakis said, smaller pull requests are better :)

src/test/compile-fail/feature-gate-termination_trait.rs

@@ -1,4 +1,4 @@
-// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// 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.
 //
@@ -8,4 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn main() -> i32 { 0 } //~ ERROR E0580
+fn main() -> i32 { //~ ERROR main function has wrong type [E0580]
+    0
+}

src/test/compile-fail/main-wrong-type-2.rs

@@ -7,8 +7,9 @@
 // <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(termination_trait)]
 
 fn main() -> char {
-//~^ ERROR: main function has wrong type [E0580]
+//~^ ERROR: the trait bound `char: std::Termination` is not satisfied
     ' '
 }

src/test/compile-fail/termination-trait-not-satisfied.rs

@@ -0,0 +1,17 @@
+// 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(termination_trait)]
+
+struct ReturnType {}
+
+fn main() -> ReturnType { //~ ERROR `ReturnType: std::Termination` is not satisfied
+    ReturnType {}
+}