Fix fn main() -> impl Trait for non-Termination trait

Fixes #50595.

This bug currently affects stable. Why I think we can go for hard error:

- It will in stable for at most one cycle and there is no legitimate
reason to abuse it, nor any known uses in the wild.

- It only affects `bin` crates (which have a `main`), so there is
little practical difference between a hard error or a deny lint, both
are a one line fix.

The fix was to just unshadow a variable. Thanks @nikomatsakis for the
mentoring!

r? @nikomatsakis

src/test/ui/rfc-1937-termination-trait/termination-trait-impl-trait.rs

@@ -0,0 +1,13 @@
+// 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.
+
+// Tests that an `impl Trait` that is not `impl Termination` will not work.
+fn main() -> impl Copy { }
+//~^ ERROR `main` has invalid return type `impl std::marker::Copy`

src/test/ui/rfc-1937-termination-trait/termination-trait-impl-trait.stderr

@@ -0,0 +1,11 @@
+error[E0277]: `main` has invalid return type `impl std::marker::Copy`
+  --> $DIR/termination-trait-impl-trait.rs:12:14
+   |
+LL | fn main() -> impl Copy { }
+   |              ^^^^^^^^^ `main` can only return types that implement `std::process::Termination`
+   |
+   = help: consider using `()`, or a `Result`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.