Support underscore as constant name

Issue: 54912

src/libsyntax/feature_gate.rs

@@ -499,6 +499,9 @@ declare_features! (
 
     // #[cfg_attr(predicate, multiple, attributes, here)]
     (active, cfg_attr_multi, "1.31.0", Some(54881), None),
+
+    // Allows `const _: TYPE = VALUE`
+    (active, underscore_const_names, "1.31.0", Some(54912), None),
 );
 
 declare_features! (
@@ -1583,6 +1586,13 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 }
             }
 
+            ast::ItemKind::Const(_,_) => {
+                if i.ident.name == "_" {
+                    gate_feature_post!(&self, underscore_const_names, i.span,
+                                        "naming constants with `_` is unstable");
+                }
+            }
+
             ast::ItemKind::ForeignMod(ref foreign_module) => {
                 self.check_abi(foreign_module.abi, i.span);
             }

src/libsyntax/parse/parser.rs

@@ -6346,7 +6346,13 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_item_const(&mut self, m: Option<Mutability>) -> PResult<'a, ItemInfo> {
-        let id = self.parse_ident()?;
+        let id = match self.token {
+                token::Ident(ident, false) if ident.name == keywords::Underscore.name() => {
+                    self.bump(); // `_`
+                    ident.gensym()
+                    },
+                _ => self.parse_ident()?,
+            };
         self.expect(&token::Colon)?;
         let ty = self.parse_ty()?;
         self.expect(&token::Eq)?;

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);
+}