Accept underscores in unicode escapes

Fixes #43692.

src/test/parse-fail/issue-23620-invalid-escapes.rs

@@ -41,9 +41,8 @@ fn main() {
     //~^^^ ERROR incorrect unicode escape sequence
     //~^^^^ ERROR unicode escape sequences cannot be used as a byte or in a byte string
 
-    let _ = "\u{ffffff} \xf \u";
-    //~^ ERROR invalid unicode character escape
-    //~^^ ERROR invalid character in numeric character escape:
-    //~^^^ ERROR form of character escape may only be used with characters in the range [\x00-\x7f]
-    //~^^^^ ERROR incorrect unicode escape sequence
+    let _ = "\xf \u";
+    //~^ ERROR invalid character in numeric character escape:
+    //~^^ ERROR form of character escape may only be used with characters in the range [\x00-\x7f]
+    //~^^^ ERROR incorrect unicode escape sequence
 }

src/test/parse-fail/issue-43692.rs

@@ -0,0 +1,15 @@
+// Copyright 2017 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-flags: -Z parse-only
+
+fn main() {
+    '\u{_10FFFF}'; //~ ERROR invalid start of unicode escape
+}

src/test/parse-fail/new-unicode-escapes-2.rs

@@ -11,5 +11,5 @@
 // compile-flags: -Z parse-only
 
 pub fn main() {
-    let s = "\u{260311111111}"; //~ ERROR overlong unicode escape (can have at most 6 hex digits)
+    let s = "\u{260311111111}"; //~ ERROR overlong unicode escape (must have at most 6 hex digits)
 }

src/test/parse-fail/new-unicode-escapes-3.rs

@@ -11,5 +11,6 @@
 // compile-flags: -Z parse-only
 
 pub fn main() {
-    let s = "\u{d805}"; //~ ERROR invalid unicode character escape
+    let s1 = "\u{d805}"; //~ ERROR invalid unicode character escape
+    let s2 = "\u{ffffff}"; //~ ERROR invalid unicode character escape
 }

src/test/parse-fail/new-unicode-escapes-4.rs

@@ -13,6 +13,4 @@
 pub fn main() {
     let s = "\u{lol}";
      //~^ ERROR invalid character in unicode escape: l
-     //~^^ ERROR invalid character in unicode escape: o
-     //~^^^ ERROR invalid character in unicode escape: l
 }

src/test/run-pass/issue-43692.rs

@@ -0,0 +1,14 @@
+// Copyright 2017 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() {
+    assert_eq!('\u{10__FFFF}', '\u{10FFFF}');
+    assert_eq!("\u{10_F0FF__}foo\u{1_0_0_0__}", "\u{10F0FF}foo\u{1000}");
+}