Auto merge of #50155 - est31:label_expressions, r=petrochenkov

'label can start expressions

```Rust
let foo = 'label: loop { break 'label 42; };
```

is valid Rust code.

src/libsyntax/parse/token.rs

@@ -277,9 +277,10 @@ impl Token {
             DotDot | DotDotDot | DotDotEq     | // range notation
             Lt | BinOp(Shl)                   | // associated path
             ModSep                            | // global path
+            Lifetime(..)                      | // labeled loop
             Pound                             => true, // expression attributes
             Interpolated(ref nt) => match nt.0 {
-                NtIdent(..) | NtExpr(..) | NtBlock(..) | NtPath(..) => true,
+                NtIdent(..) | NtExpr(..) | NtBlock(..) | NtPath(..) | NtLifetime(..) => true,
                 _ => false,
             },
             _ => false,

src/test/ui/lifetime_starts_expressions.rs

@@ -0,0 +1,23 @@
+// 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 foo() -> u32 {
+    return 'label: loop { break 'label 42; };
+}
+
+fn bar() -> u32 {
+    loop { break 'label: loop { break 'label 42; }; }
+    //~^ ERROR expected identifier, found keyword `loop`
+    //~| ERROR expected type, found keyword `loop`
+}
+
+pub fn main() {
+    foo();
+}

src/test/ui/lifetime_starts_expressions.stderr

@@ -0,0 +1,14 @@
+error: expected identifier, found keyword `loop`
+  --> $DIR/lifetime_starts_expressions.rs:16:26
+   |
+LL |     loop { break 'label: loop { break 'label 42; }; }
+   |                          ^^^^ expected identifier, found keyword
+
+error: expected type, found keyword `loop`
+  --> $DIR/lifetime_starts_expressions.rs:16:26
+   |
+LL |     loop { break 'label: loop { break 'label 42; }; }
+   |                          ^^^^ expecting a type here because of type ascription
+
+error: aborting due to 2 previous errors
+