Disallow &a..=b and box a..=b in pattern.

They are disallowed because they have different precedence than
expressions. I assume parenthesis in pattern will be soon stabilized and
thus write that as suggestion directly.

src/test/ui/range-inclusive-pattern-precedence.rs

@@ -0,0 +1,38 @@
+// 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.
+
+// In expression, `&a..=b` is treated as `(&a)..=(b)` and `box a..=b` is
+// `(box a)..=(b)`. In a pattern, however, `&a..=b` means `&(a..=b)`. This may
+// lead to confusion.
+//
+// We are going to disallow `&a..=b` and `box a..=b` in a pattern. However, the
+// older ... syntax is still allowed as a stability guarantee.
+
+#![feature(box_patterns)]
+
+pub fn main() {
+    match &12 {
+        &0...9 => {}
+        &10..=15 => {}
+        //~^ ERROR the range pattern here has ambiguous interpretation
+        //~^^ HELP add parentheses to clarify the precedence
+        &(16..=20) => {}
+        _ => {}
+    }
+
+    match Box::new(12) {
+        box 0...9 => {}
+        box 10..=15 => {}
+        //~^ ERROR the range pattern here has ambiguous interpretation
+        //~^^ HELP add parentheses to clarify the precedence
+        box (16..=20) => {}
+        _ => {}
+    }
+}

src/test/ui/range-inclusive-pattern-precedence.stderr

@@ -0,0 +1,14 @@
+error: the range pattern here has ambiguous interpretation
+  --> $DIR/range-inclusive-pattern-precedence.rs:23:10
+   |
+LL |         &10..=15 => {}
+   |          ^^^^^^^ help: add parentheses to clarify the precedence: `(10 ..=15)`
+
+error: the range pattern here has ambiguous interpretation
+  --> $DIR/range-inclusive-pattern-precedence.rs:32:13
+   |
+LL |         box 10..=15 => {}
+   |             ^^^^^^^ help: add parentheses to clarify the precedence: `(10 ..=15)`
+
+error: aborting due to 2 previous errors
+