Support underscore as constant name

Issue: 54912

src/test/ui/feature-gate-underscore_const_names.rs

@@ -0,0 +1,24 @@
+// Copyright 2012-2018 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.
+#![feature(const_let)]
+
+trait Trt {}
+struct Str {}
+
+impl Trt for Str {}
+
+const _ : () = {
+    use std::marker::PhantomData;
+    struct ImplementsTrait<T: Trt>(PhantomData<T>);
+    let _ = ImplementsTrait::<Str>(PhantomData);
+    ()
+};
+
+fn main() {}

src/test/ui/feature-gate-underscore_const_names.stderr

@@ -0,0 +1,16 @@
+error[E0658]: naming constants with `_` is unstable (see issue #54912)
+  --> $DIR/feature-gate-underscore_const_names.rs:17:1
+   |
+LL | / const _ : () = {
+LL | |     use std::marker::PhantomData;
+LL | |     struct ImplementsTrait<T: Trt>(PhantomData<T>);
+LL | |     let _ = ImplementsTrait::<Str>(PhantomData);
+LL | |     ()
+LL | | };
+   | |__^
+   |
+   = help: add #![feature(underscore_const_names)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/underscore_const_names.rs

@@ -0,0 +1,43 @@
+// Copyright 2012-2018 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.
+
+// compile-pass
+
+#![feature(const_let)]
+#![feature(underscore_const_names)]
+
+trait Trt {}
+struct Str {}
+impl Trt for Str {}
+
+macro_rules! check_impl {
+    ($struct:ident,$trait:ident) => {
+        const _ : () = {
+            use std::marker::PhantomData;
+            struct ImplementsTrait<T: $trait>(PhantomData<T>);
+            let _ = ImplementsTrait::<$struct>(PhantomData);
+            ()
+        };
+    }
+}
+
+#[deny(unused)]
+const _ : () = ();
+
+const _ : i32 = 42;
+const _ : Str = Str{};
+
+check_impl!(Str, Trt);
+check_impl!(Str, Trt);
+
+fn main() {
+  check_impl!(Str, Trt);
+  check_impl!(Str, Trt);
+}