Temporarily prefix catch block with do keyword

src/libsyntax/parse/parser.rs

@@ -602,12 +602,6 @@ impl<'a> Parser<'a> {
         }
     }
 
-    pub fn error_if_typename_is_catch(&mut self, ident: ast::Ident) {
-        if ident.name == keywords::Catch.name() {
-            self.span_err(self.span, "cannot use `catch` as the name of a type");
-        }
-    }
-
     /// Check if the next token is `tok`, and return `true` if so.
     ///
     /// This method will automatically add `tok` to `expected_tokens` if `tok` is not
@@ -2280,6 +2274,7 @@ impl<'a> Parser<'a> {
                         attrs);
                 }
                 if self.is_catch_expr() {
+                    assert!(self.eat_keyword(keywords::Do));
                     assert!(self.eat_keyword(keywords::Catch));
                     let lo = self.prev_span.lo;
                     return self.parse_catch_expr(lo, attrs);
@@ -3103,7 +3098,7 @@ impl<'a> Parser<'a> {
         Ok(self.mk_expr(span_lo, hi, ExprKind::Loop(body, opt_ident), attrs))
     }
 
-    /// Parse a `catch {...}` expression (`catch` token already eaten)
+    /// Parse a `do catch {...}` expression (`do catch` token already eaten)
     pub fn parse_catch_expr(&mut self, span_lo: BytePos, mut attrs: ThinVec<Attribute>)
         -> PResult<'a, P<Expr>>
     {
@@ -3721,8 +3716,9 @@ impl<'a> Parser<'a> {
     }
 
     fn is_catch_expr(&mut self) -> bool {
-        self.token.is_keyword(keywords::Catch) &&
-        self.look_ahead(1, |t| *t == token::OpenDelim(token::Brace)) &&
+        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)) &&
 
         // prevent `while catch {} {}`, `if catch {} {} else {}`, etc.
         !self.restrictions.contains(Restrictions::RESTRICTION_NO_STRUCT_LITERAL)
@@ -4868,7 +4864,6 @@ impl<'a> Parser<'a> {
     /// Parse struct Foo { ... }
     fn parse_item_struct(&mut self) -> PResult<'a, ItemInfo> {
         let class_name = self.parse_ident()?;
-        self.error_if_typename_is_catch(class_name);
 
         let mut generics = self.parse_generics()?;
 
@@ -4919,7 +4914,6 @@ impl<'a> Parser<'a> {
     /// Parse union Foo { ... }
     fn parse_item_union(&mut self) -> PResult<'a, ItemInfo> {
         let class_name = self.parse_ident()?;
-        self.error_if_typename_is_catch(class_name);
 
         let mut generics = self.parse_generics()?;
 
@@ -5437,7 +5431,6 @@ impl<'a> Parser<'a> {
             let struct_def;
             let mut disr_expr = None;
             let ident = self.parse_ident()?;
-            self.error_if_typename_is_catch(ident);
             if self.check(&token::OpenDelim(token::Brace)) {
                 // Parse a struct variant.
                 all_nullary = false;
@@ -5479,7 +5472,6 @@ impl<'a> Parser<'a> {
     /// Parse an "enum" declaration
     fn parse_item_enum(&mut self) -> PResult<'a, ItemInfo> {
         let id = self.parse_ident()?;
-        self.error_if_typename_is_catch(id);
         let mut generics = self.parse_generics()?;
         generics.where_clause = self.parse_where_clause()?;
         self.expect(&token::OpenDelim(token::Brace))?;

src/libsyntax/parse/token.rs

@@ -86,6 +86,7 @@ fn ident_can_begin_expr(ident: ast::Ident) -> bool {
     !ident_token.is_any_keyword() ||
     ident_token.is_path_segment_keyword() ||
     [
+        keywords::Do.name(),
         keywords::Box.name(),
         keywords::Break.name(),
         keywords::Continue.name(),

src/test/compile-fail/feature-gate-catch_expr.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 pub fn main() {
-    let catch_result = catch { //~ ERROR `catch` expression is experimental
+    let catch_result = do catch { //~ ERROR `catch` expression is experimental
         let x = 5;
         x
     };

src/test/run-pass/catch-expr.rs

@@ -10,8 +10,10 @@
 
 #![feature(catch_expr)]
 
+struct catch {}
+
 pub fn main() {
-    let catch_result = catch {
+    let catch_result = do catch {
         let x = 5;
         x
     };