Add help message for incorrect pattern syntax

src/libsyntax/parse/parser.rs

@@ -3431,8 +3431,20 @@ impl<'a> Parser<'a> {
         let mut pats = Vec::new();
         loop {
             pats.push(self.parse_pat()?);
-            if self.check(&token::BinOp(token::Or)) { self.bump();}
-            else { return Ok(pats); }
+
+            if self.token == token::OrOr {
+                let mut err = self.struct_span_err(self.span,
+                                                   "unexpected token `||` after pattern");
+                err.span_suggestion(self.span,
+                                    "use a single `|` to specify multiple patterns",
+                                    "|".to_owned());
+                err.emit();
+                self.bump();
+            } else if self.check(&token::BinOp(token::Or)) {
+                self.bump();
+            } else {
+                return Ok(pats);
+            }
         };
     }
 

src/test/ui/did_you_mean/multiple-pattern-typo.rs

@@ -0,0 +1,17 @@
+// 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 main() {
+    let x = 3;
+    match x {
+        1 | 2 || 3 => (), //~ ERROR unexpected token `||` after pattern
+        _ => (),
+    }
+}

src/test/ui/did_you_mean/multiple-pattern-typo.stderr

@@ -0,0 +1,8 @@
+error: unexpected token `||` after pattern
+  --> $DIR/multiple-pattern-typo.rs:14:15
+   |
+14 |         1 | 2 || 3 => (), //~ ERROR unexpected token `||` after pattern
+   |               ^^ help: use a single `|` to specify multiple patterns: `|`
+
+error: aborting due to previous error
+