When encountering invalid token after unsafe, mention {

src/libsyntax/parse/parser.rs

@@ -6469,6 +6469,8 @@ impl<'a> Parser<'a> {
  && self.look_ahead(1, |t| *t != token::OpenDelim(token::Brace)) {
  // UNSAFE FUNCTION ITEM
  self.bump(); // `unsafe`
+ // `{` is also expected after `unsafe`, in case of error, include it in the diagnostic
+ self.check(&token::OpenDelim(token::Brace));
  let abi = if self.eat_keyword(keywords::Extern) {
  self.parse_opt_abi()?.unwrap_or(Abi::C)
  } else {

src/test/ui/unsafe-block-without-braces.rs

@@ -0,0 +1,16 @@
+// 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.
+
+fn main() {
+ unsafe //{
+ std::mem::transmute::<f32, u32>(1.0);
+ //}
+}
+//~^^^ ERROR expected one of `extern`, `fn`, or `{`, found `std`

src/test/ui/unsafe-block-without-braces.stderr

@@ -0,0 +1,10 @@
+error: expected one of `extern`, `fn`, or `{`, found `std`
+ --> $DIR/unsafe-block-without-braces.rs:13:9
+ |
+12 | unsafe //{
+ | - expected one of `extern`, `fn`, or `{` here
+13 | std::mem::transmute::<f32, u32>(1.0);
+ | ^^^ unexpected token
+
+error: aborting due to previous error
+