Add illegal_floating_point_literal_pattern compat lint

Adds a compatibility lint to disallow floating point literals in
patterns like in match.

See the tracking issue #41620.

src/test/compile-fail/issue-41255.rs

@@ -0,0 +1,51 @@
+// 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.
+
+// Matching against float literals should result in a linter error
+
+#![feature(slice_patterns)]
+#![feature(exclusive_range_pattern)]
+#![allow(unused)]
+#![forbid(illegal_floating_point_literal_pattern)]
+
+fn main() {
+    let x = 42.0;
+    match x {
+        5.0 => {}, //~ ERROR floating-point literals cannot be used
+                   //~| WARNING hard error
+        5.0f32 => {}, //~ ERROR floating-point literals cannot be used
+                      //~| WARNING hard error
+        -5.0 => {}, //~ ERROR floating-point literals cannot be used
+                    //~| WARNING hard error
+        1.0 .. 33.0 => {}, //~ ERROR floating-point literals cannot be used
+                           //~| WARNING hard error
+                           //~| ERROR floating-point literals cannot be used
+                           //~| WARNING hard error
+        39.0 ... 70.0 => {}, //~ ERROR floating-point literals cannot be used
+                             //~| WARNING hard error
+                             //~| ERROR floating-point literals cannot be used
+                             //~| WARNING hard error
+        _ => {},
+    };
+    let y = 5.0;
+    // Same for tuples
+    match (x, 5) {
+        (3.14, 1) => {}, //~ ERROR floating-point literals cannot be used
+                         //~| WARNING hard error
+        _ => {},
+    }
+    // Or structs
+    struct Foo { x: f32 };
+    match (Foo { x }) {
+        Foo { x: 2.0 } => {}, //~ ERROR floating-point literals cannot be used
+                              //~| WARNING hard error
+        _ => {},
+    }
+}