Remove support for 'when' in alt-patterns from parser

Issue #1396

Author: Austin Seipp

src/comp/syntax/parse/parser.rs

@@ -1389,11 +1389,7 @@ fn parse_alt_expr(p: parser) -> @ast::expr {
     while p.peek() != token::RBRACE {
         let pats = parse_pats(p);
         let guard = none;
-        if eat_word(p, "when") {
-            guard = some(parse_expr(p));
-        } else if eat_word(p, "if") {
-            guard = some(parse_expr(p));
-        }
+        if eat_word(p, "if") { guard = some(parse_expr(p)); }
         let blk = parse_block(p);
         arms += [{pats: pats, guard: guard, body: blk}];
     }