Rollup merge of rust-lang#41946 - qnighy:disallow-dot-underscore-in-float, r=petrochenkov

Disallow ._ in float literal.

This patch makes lexer stop parsing number literals before `._`, as well as before `.a`. Underscore itself is still allowed like in `4_000_000.000_000_`.

Fixes a half part of rust-lang#41723. The other is `""_`.

Author: Mark-Simulacrum

src/libsyntax/parse/lexer/mod.rs

@@ -754,9 +754,7 @@ impl<'a> StringReader<'a> {
         // integer literal followed by field/method access or a range pattern
         // (`0..2` and `12.foo()`)
         if self.ch_is('.') && !self.nextch_is('.') &&
-           !self.nextch()
-                .unwrap_or('\0')
-                .is_xid_start() {
+           !ident_start(self.nextch()) {
             // might have stuff after the ., and if it does, it needs to start
             // with a number
             self.bump();

src/test/parse-fail/underscore-suffix-for-float.rs

@@ -0,0 +1,13 @@
+// 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() {
+    let a = 42._; //~ ERROR unexpected token: `_`
+}

src/test/run-pass/underscore-method-after-integer.rs

@@ -0,0 +1,19 @@
+// 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.
+
+trait Tr : Sized {
+    fn _method_on_numbers(self) {}
+}
+
+impl Tr for i32 {}
+
+fn main() {
+    42._method_on_numbers();
+}