Suggest try if someone uses do catch

Author: scottmcm

src/libsyntax/parse/parser.rs

@@ -2386,6 +2386,11 @@ impl<'a> Parser<'a> {
                         BlockCheckMode::Unsafe(ast::UserProvided),
                         attrs);
                 }
+                if self.is_do_catch_block() {
+                    let mut db = self.fatal("found removed `do catch` syntax");
+                    db.help("Following RFC #2388, the new non-placeholder syntax is `try`");
+                    return Err(db);
+                }
                 if self.is_try_block() {
                     let lo = self.span;
                     assert!(self.eat_keyword(keywords::Try));
@@ -4406,6 +4411,13 @@ impl<'a> Parser<'a> {
         )
     }
 
+    fn is_do_catch_block(&mut self) -> bool {
+        self.token.is_keyword(keywords::Do) &&
+        self.look_ahead(1, |t| t.is_keyword(keywords::Catch)) &&
+        self.look_ahead(2, |t| *t == token::OpenDelim(token::Brace)) &&
+        !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL)
+    }
+
     fn is_try_block(&mut self) -> bool {
         self.token.is_keyword(keywords::Try) &&
         self.look_ahead(1, |t| *t == token::OpenDelim(token::Brace)) &&

src/test/parse-fail/do-catch-suggests-try.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.
+
+// compile-flags: -Z parse-only
+
+fn main() {
+    let _: Option<()> = do catch {};
+    //~^ ERROR found removed `do catch` syntax
+    //~^^ HELP Following RFC #2388, the new non-placeholder syntax is `try`
+}