Rollup merge of #39335 - cramertj:cramertj/can_begin_expr_fix, r=petrochenkov

Fix can_begin_expr keyword behavior

Partial fix for #28784.

src/libsyntax/parse/token.rs

@@ -80,6 +80,28 @@ impl Lit {
     }
 }
 
+fn ident_can_begin_expr(ident: ast::Ident) -> bool {
+    let ident_token: Token = Ident(ident);
+
+    !ident_token.is_any_keyword() ||
+    ident_token.is_path_segment_keyword() ||
+    [
+        keywords::Box.name(),
+        keywords::Break.name(),
+        keywords::Continue.name(),
+        keywords::False.name(),
+        keywords::For.name(),
+        keywords::If.name(),
+        keywords::Loop.name(),
+        keywords::Match.name(),
+        keywords::Move.name(),
+        keywords::Return.name(),
+        keywords::True.name(),
+        keywords::Unsafe.name(),
+        keywords::While.name(),
+    ].contains(&ident.name)
+}
+
 #[derive(Clone, RustcEncodable, RustcDecodable, PartialEq, Eq, Hash, Debug)]
 pub enum Token {
     /* Expression-operator symbols. */
@@ -163,7 +185,7 @@ impl Token {
     pub fn can_begin_expr(&self) -> bool {
         match *self {
             OpenDelim(..)               => true,
-            Ident(..)                   => true,
+            Ident(ident)                => ident_can_begin_expr(ident),
             Literal(..)                 => true,
             Not                         => true,
             BinOp(Minus)                => true,

src/test/compile-fail/can-begin-expr-check.rs

@@ -0,0 +1,30 @@
+// Copyright 2014 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.
+
+pub fn main() {
+
+    return;
+    return ();
+    return as ();
+    return return as ();
+    return return return;
+
+    return if true {
+        ()
+    } else {
+        ()
+    };
+
+    loop {
+        return break as ();
+    }
+
+    return enum; //~ ERROR expected one of `.`, `;`, `?`, `}`, or an operator, found `enum`
+}