Make the parser accept 'if' as an alternative to 'when' in alt patterns.

Also fix the pretty printer, making it output 'if' instead of 'when'.

Issue #1396

Author: Austin Seipp

src/comp/syntax/parse/parser.rs

@@ -1389,7 +1389,11 @@ 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)); }
+        if eat_word(p, "when") {
+            guard = some(parse_expr(p));
+        } else if eat_word(p, "if") {
+            guard = some(parse_expr(p));
+        }
         let blk = parse_block(p);
         arms += [{pats: pats, guard: guard, body: blk}];
     }

src/comp/syntax/print/pprust.rs

@@ -843,7 +843,7 @@ fn print_expr(s: ps, &&expr: @ast::expr) {
             }
             space(s.s);
             alt arm.guard {
-              some(e) { word_space(s, "when"); print_expr(s, e); space(s.s); }
+              some(e) { word_space(s, "if"); print_expr(s, e); space(s.s); }
               none. { }
             }
             print_possibly_embedded_block(s, arm.body, block_normal,