error during parsing for malformed inclusive range

Now it is impossible for `...` or `a...` to reach HIR lowering
without a rogue syntax extension in play.

src/libsyntax/parse/parser.rs

@@ -2072,8 +2072,15 @@ impl<'a> Parser<'a> {
                     start: Option<P<Expr>>,
                     end: Option<P<Expr>>,
                     limits: RangeLimits)
-                    -> ast::ExprKind {
+                    -> PResult<'a, ast::ExprKind> {
-        ExprKind::Range(start, end, limits)
+        if end.is_none() && limits == RangeLimits::Closed {
+            Err(self.span_fatal_help(self.span,
+                                     "inclusive range with no end",
+                                     "currently, inclusive ranges must be bounded at the end \
+                                      (`...b` or `a...b`) -- see tracking issue #28237"))
+        } else {
+            Ok(ExprKind::Range(start, end, limits))
+        }
     }
 
     pub fn mk_field(&mut self, expr: P<Expr>, ident: ast::SpannedIdent) -> ast::ExprKind {
@@ -2961,12 +2968,12 @@ impl<'a> Parser<'a> {
                 lhs = self.mk_expr(lhs_span.lo, rhs.span.hi,
                                    ExprKind::Type(lhs, rhs), None);
                 continue
-            } else if op == AssocOp::DotDot {
+            } else if op == AssocOp::DotDot || op == AssocOp::DotDotDot {
-                // If we didn’t have to handle `x..`, it would be pretty easy to generalise
+                // If we didn’t have to handle `x..`/`x...`, it would be pretty easy to
-                // it to the Fixity::None code.
+                // generalise it to the Fixity::None code.
                 //
-                // We have 2 alternatives here: `x..y` and `x..` The other two variants are
+                // We have 2 alternatives here: `x..y`/`x...y` and `x..`/`x...` The other
-                // handled with `parse_prefix_range_expr` call above.
+                // two variants are handled with `parse_prefix_range_expr` call above.
                 let rhs = if self.is_at_start_of_range_notation_rhs() {
                     let rhs = self.parse_assoc_expr_with(op.precedence() + 1,
                                                          LhsExpr::NotYetParsed);
@@ -2985,7 +2992,13 @@ impl<'a> Parser<'a> {
                 } else {
                     cur_op_span
                 });
-                let r = self.mk_range(Some(lhs), rhs, RangeLimits::HalfOpen);
+                let limits = if op == AssocOp::DotDot {
+                    RangeLimits::HalfOpen
+                } else {
+                    RangeLimits::Closed
+                };
+
+                let r = try!(self.mk_range(Some(lhs), rhs, limits));
                 lhs = self.mk_expr(lhs_span.lo, rhs_span.hi, r, None);
                 break
             }
@@ -3003,8 +3016,7 @@ impl<'a> Parser<'a> {
                         this.parse_assoc_expr_with(op.precedence() + 1,
                             LhsExpr::NotYetParsed)
                 }),
-                // the only operator handled here is `...` (the other non-associative operators are
+                // no operators are currently handled here
-                // special-cased above)
                 Fixity::None => self.with_res(
                     restrictions - Restrictions::RESTRICTION_STMT_EXPR,
                     |this| {
@@ -3045,13 +3057,8 @@ impl<'a> Parser<'a> {
                     let aopexpr = self.mk_assign_op(codemap::respan(cur_op_span, aop), lhs, rhs);
                     self.mk_expr(lhs_span.lo, rhs_span.hi, aopexpr, None)
                 }
-                AssocOp::DotDotDot => {
+                AssocOp::As | AssocOp::Colon | AssocOp::DotDot | AssocOp::DotDotDot => {
-                    let (lhs_span, rhs_span) = (lhs.span, rhs.span);
+                    self.bug("As, Colon, DotDot or DotDotDot branch reached")
-                    let r = self.mk_range(Some(lhs), Some(rhs), RangeLimits::Closed);
-                    self.mk_expr(lhs_span.lo, rhs_span.hi, r, None)
-                }
-                AssocOp::As | AssocOp::Colon | AssocOp::DotDot => {
-                    self.bug("As, Colon or DotDot branch reached")
                 }
             };
 
@@ -3103,13 +3110,15 @@ impl<'a> Parser<'a> {
          } else {
             None
         };
-        let r = self.mk_range(None,
+        let limits = if tok == token::DotDot {
-                              opt_end,
-                              if tok == token::DotDot {
             RangeLimits::HalfOpen
         } else {
             RangeLimits::Closed
-                              });
+        };
+
+        let r = try!(self.mk_range(None,
+                                   opt_end,
+                                   limits));
         Ok(self.mk_expr(lo, hi, r, attrs))
     }
 

src/test/compile-fail/impossible_range.rs

@@ -19,7 +19,9 @@ pub fn main() {
     0..1;
 
     ...; //~ERROR inclusive range with no end
-    0...; //~ERROR unexpected token
+         //~^HELP 28237
+    0...; //~ERROR inclusive range with no end
+          //~^HELP 28237
     ...1;
     0...1;
 }