diff -r 886731829abf go/expressions.cc
--- a/go/expressions.cc	Wed Oct 23 16:56:39 2013 -0700
+++ b/go/expressions.cc	Thu Oct 24 12:12:12 2013 -0700
@@ -10235,7 +10235,9 @@
   if (Expression::traverse(&this->left_, traverse) == TRAVERSE_EXIT
       || Expression::traverse(&this->start_, traverse) == TRAVERSE_EXIT
       || (this->end_ != NULL
-	  && Expression::traverse(&this->end_, traverse) == TRAVERSE_EXIT))
+	  && Expression::traverse(&this->end_, traverse) == TRAVERSE_EXIT)
+      || (this->cap_ != NULL
+          && Expression::traverse(&this->cap_, traverse) == TRAVERSE_EXIT))
     return TRAVERSE_EXIT;
   return TRAVERSE_CONTINUE;
 }
@@ -10250,6 +10252,7 @@
   Expression* left = this->left_;
   Expression* start = this->start_;
   Expression* end = this->end_;
+  Expression* cap = this->cap_;
 
   Type* type = left->type();
   if (type->is_error())
@@ -10260,20 +10263,27 @@
       return Expression::make_error(location);
     }
   else if (type->array_type() != NULL)
-    return Expression::make_array_index(left, start, end, location);
+    return Expression::make_array_index(left, start, end, cap, location);
   else if (type->points_to() != NULL
 	   && type->points_to()->array_type() != NULL
 	   && !type->points_to()->is_slice_type())
     {
       Expression* deref = Expression::make_unary(OPERATOR_MULT, left,
 						 location);
-      return Expression::make_array_index(deref, start, end, location);
+      return Expression::make_array_index(deref, start, end, cap, location);
     }
   else if (type->is_string_type())
-    return Expression::make_string_index(left, start, end, location);
+    {
+      if (cap != NULL)
+        {
+          error_at(location, "invalid 3-index slice of string");
+          return Expression::make_error(location);
+        }
+      return Expression::make_string_index(left, start, end, location);
+    }
   else if (type->map_type() != NULL)
     {
-      if (end != NULL)
+      if (end != NULL || cap != NULL)
 	{
 	  error_at(location, "invalid slice of map");
 	  return Expression::make_error(location);
@@ -10292,14 +10302,15 @@
     }
 }
 
-// Write an indexed expression (expr[expr:expr] or expr[expr]) to a
-// dump context
+// Write an indexed expression
+// (expr[expr:expr:expr], expr[expr:expr] or expr[expr]) to a dump context.
 
 void
 Index_expression::dump_index_expression(Ast_dump_context* ast_dump_context, 
 					const Expression* expr, 
 					const Expression* start,
-					const Expression* end)
+					const Expression* end,
+					const Expression* cap)
 {
   expr->dump_expression(ast_dump_context);
   ast_dump_context->ostream() << "[";
@@ -10309,6 +10320,11 @@
       ast_dump_context->ostream() << ":";
       end->dump_expression(ast_dump_context);
     }
+  if (cap != NULL)
+    {
+      ast_dump_context->ostream() << ":";
+      cap->dump_expression(ast_dump_context);
+    }
   ast_dump_context->ostream() << "]";
 }
 
@@ -10319,16 +10335,16 @@
     const
 {
   Index_expression::dump_index_expression(ast_dump_context, this->left_, 
-                                          this->start_, this->end_);
+                                          this->start_, this->end_, this->cap_);
 }
 
 // Make an index expression.
 
 Expression*
 Expression::make_index(Expression* left, Expression* start, Expression* end,
-		       Location location)
-{
-  return new Index_expression(left, start, end, location);
+		       Expression* cap, Location location)
+{
+  return new Index_expression(left, start, end, cap, location);
 }
 
 // An array index.  This is used for both indexing and slicing.
@@ -10337,9 +10353,9 @@
 {
  public:
   Array_index_expression(Expression* array, Expression* start,
-			 Expression* end, Location location)
+			 Expression* end, Expression* cap, Location location)
     : Expression(EXPRESSION_ARRAY_INDEX, location),
-      array_(array), start_(start), end_(end), type_(NULL)
+      array_(array), start_(start), end_(end), cap_(cap), type_(NULL)
   { }
 
  protected:
@@ -10363,6 +10379,9 @@
 					(this->end_ == NULL
 					 ? NULL
 					 : this->end_->copy()),
+					(this->cap_ == NULL
+					 ? NULL
+					 : this->cap_->copy()),
 					this->location());
   }
 
@@ -10394,6 +10413,9 @@
   // The end index of a slice.  This may be NULL for a simple array
   // index, or it may be a nil expression for the length of the array.
   Expression* end_;
+  // The capacity argument of a slice.  This may be NULL for an array index or
+  // slice.
+  Expression* cap_;
   // The type of the expression.
   Type* type_;
 };
@@ -10412,6 +10434,11 @@
       if (Expression::traverse(&this->end_, traverse) == TRAVERSE_EXIT)
 	return TRAVERSE_EXIT;
     }
+  if (this->cap_ != NULL)
+    {
+      if (Expression::traverse(&this->cap_, traverse) == TRAVERSE_EXIT)
+        return TRAVERSE_EXIT;
+    }
   return TRAVERSE_CONTINUE;
 }
 
@@ -10451,6 +10478,8 @@
   this->start_->determine_type_no_context();
   if (this->end_ != NULL)
     this->end_->determine_type_no_context();
+  if (this->cap_ != NULL)
+    this->cap_->determine_type_no_context();
 }
 
 // Check types of an array index.
@@ -10473,6 +10502,14 @@
       && (!this->end_->numeric_constant_value(&nc)
 	  || nc.to_unsigned_long(&v) == Numeric_constant::NC_UL_NOTINT))
     this->report_error(_("slice end must be integer"));
+  if (this->cap_ != NULL
+      && this->cap_->type()->integer_type() == NULL
+      && !this->cap_->type()->is_error()
+      && !this->cap_->is_nil_expression()
+      && !this->cap_->is_error_expression()
+      && (!this->cap_->numeric_constant_value(&nc)
+	  || nc.to_unsigned_long(&v) == Numeric_constant::NC_UL_NOTINT))
+    this->report_error(_("slice capacity must be integer"));
 
   Array_type* array_type = this->array_->type()->array_type();
   if (array_type == NULL)
@@ -10510,8 +10547,10 @@
     {
       Numeric_constant enc;
       mpz_t eval;
+      bool eval_valid = false;
       if (this->end_->numeric_constant_value(&enc) && enc.to_int(&eval))
 	{
+	  eval_valid = true;
 	  if (mpz_sgn(eval) < 0
 	      || mpz_sizeinbase(eval, 2) >= int_bits
 	      || (lval_valid && mpz_cmp(eval, lval) > 0))
@@ -10521,8 +10560,37 @@
 	    }
 	  else if (ival_valid && mpz_cmp(ival, eval) > 0)
 	    this->report_error(_("inverted slice range"));
-	  mpz_clear(eval);
-	}
+	}
+
+      Numeric_constant cnc;
+      mpz_t cval;
+      if (this->cap_ != NULL
+          && this->cap_->numeric_constant_value(&cnc) && cnc.to_int(&cval))
+        {
+          if (mpz_sgn(cval) < 0
+              || mpz_sizeinbase(cval, 2) >= int_bits
+              || (lval_valid && mpz_cmp(cval, lval) > 0))
+            {
+              error_at(this->cap_->location(), "array index out of bounds");
+              this->set_is_error();
+            }
+	  else if (ival_valid && mpz_cmp(ival, cval) > 0)
+	    {
+	      error_at(this->cap_->location(),
+		       "invalid slice index: capacity less than start");
+	      this->set_is_error();
+	    }
+          else if (eval_valid && mpz_cmp(eval, cval) > 0)
+            {
+              error_at(this->cap_->location(),
+                       "invalid slice index: capacity less than length");
+              this->set_is_error();
+            }
+          mpz_clear(cval);
+        }
+
+      if (eval_valid)
+        mpz_clear(eval);
     }
   if (ival_valid)
     mpz_clear(ival);
@@ -10602,9 +10670,17 @@
       capacity_tree = save_expr(capacity_tree);
     }
 
+  tree cap_arg = capacity_tree;
+  if (this->cap_ != NULL)
+    {
+      cap_arg = this->cap_->get_tree(context);
+      if (cap_arg == error_mark_node)
+        return error_mark_node;
+    }
+
   tree length_type = (length_tree != NULL_TREE
 		      ? TREE_TYPE(length_tree)
-		      : TREE_TYPE(capacity_tree));
+		      : TREE_TYPE(cap_arg));
 
   tree bad_index = boolean_false_node;
 
@@ -10676,6 +10752,29 @@
 
   // Array slice.
 
+  if (this->cap_ != NULL)
+    {
+      if (!DECL_P(cap_arg))
+        cap_arg = save_expr(cap_arg);
+      if (!INTEGRAL_TYPE_P(TREE_TYPE(cap_arg)))
+        cap_arg = convert_to_integer(length_type, cap_arg);
+
+      bad_index = Expression::check_bounds(cap_arg, length_type, bad_index,
+                                           loc);
+      cap_arg = fold_convert_loc(loc.gcc_location(), length_type, cap_arg);
+
+      tree bad_cap = fold_build2_loc(loc.gcc_location(), TRUTH_OR_EXPR,
+                                     boolean_type_node,
+                                     fold_build2_loc(loc.gcc_location(),
+                                                     LT_EXPR, boolean_type_node,
+                                                     cap_arg, start_tree),
+                                     fold_build2_loc(loc.gcc_location(),
+                                                     GT_EXPR, boolean_type_node,
+                                                     cap_arg, capacity_tree));
+      bad_index = fold_build2_loc(loc.gcc_location(), TRUTH_OR_EXPR,
+                                  boolean_type_node, bad_index, bad_cap);
+    }
+
   tree end_tree;
   if (this->end_->is_nil_expression())
     end_tree = length_tree;
@@ -10701,11 +10800,12 @@
 						     end_tree, start_tree),
 				     fold_build2_loc(loc.gcc_location(),
                                                      GT_EXPR, boolean_type_node,
-						     end_tree, capacity_tree));
+						     end_tree, cap_arg));
       bad_index = fold_build2_loc(loc.gcc_location(), TRUTH_OR_EXPR,
                                   boolean_type_node, bad_index, bad_end);
     }
 
+
   Type* element_type = array_type->element_type();
   tree element_type_tree = type_to_tree(element_type->get_backend(gogo));
   if (element_type_tree == error_mark_node)
@@ -10729,8 +10829,7 @@
                                             length_type, end_tree, start_tree);
 
   tree result_capacity_tree = fold_build2_loc(loc.gcc_location(), MINUS_EXPR,
-                                              length_type, capacity_tree,
-                                              start_tree);
+                                              length_type, cap_arg, start_tree);
 
   tree struct_tree = type_to_tree(this->type()->get_backend(gogo));
   go_assert(TREE_CODE(struct_tree) == RECORD_TYPE);
@@ -10780,16 +10879,17 @@
     const
 {
   Index_expression::dump_index_expression(ast_dump_context, this->array_, 
-                                          this->start_, this->end_);
-}
-
-// Make an array index expression.  END may be NULL.
+                                          this->start_, this->end_, this->cap_);
+}
+
+// Make an array index expression.  END and CAP may be NULL.
 
 Expression*
 Expression::make_array_index(Expression* array, Expression* start,
-			     Expression* end, Location location)
-{
-  return new Array_index_expression(array, start, end, location);
+                             Expression* end, Expression* cap,
+                             Location location)
+{
+  return new Array_index_expression(array, start, end, cap, location);
 }
 
 // A string index.  This is used for both indexing and slicing.
@@ -11067,8 +11167,8 @@
 String_index_expression::do_dump_expression(Ast_dump_context* ast_dump_context)
     const
 {
-  Index_expression::dump_index_expression(ast_dump_context, this->string_, 
-					  this->start_, this->end_);
+  Index_expression::dump_index_expression(ast_dump_context, this->string_,
+                                          this->start_, this->end_, NULL);
 }
 
 // Make a string index expression.  END may be NULL.
@@ -11295,8 +11395,8 @@
 Map_index_expression::do_dump_expression(Ast_dump_context* ast_dump_context) 
     const
 {
-  Index_expression::dump_index_expression(ast_dump_context, 
-                                          this->map_, this->index_, NULL);
+  Index_expression::dump_index_expression(ast_dump_context, this->map_,
+                                          this->index_, NULL, NULL);
 }
 
 // Make a map index expression.
diff -r 886731829abf go/expressions.h
--- a/go/expressions.h	Wed Oct 23 16:56:39 2013 -0700
+++ b/go/expressions.h	Thu Oct 24 12:12:12 2013 -0700
@@ -232,19 +232,21 @@
 		    Named_object* function, Location);
 
   // Make an index or slice expression.  This is a parser expression
-  // which represents LEFT[START:END].  END may be NULL, meaning an
-  // index rather than a slice.  At parse time we may not know the
-  // type of LEFT.  After parsing this is lowered to an array index, a
-  // string index, or a map index.
+  // which represents LEFT[START:END:CAP].  END may be NULL, meaning an
+  // index rather than a slice.  CAP may be NULL, meaning we use the default
+  // capacity of LEFT. At parse time we may not know the type of LEFT.
+  // After parsing this is lowered to an array index, a string index,
+  // or a map index.
   static Expression*
   make_index(Expression* left, Expression* start, Expression* end,
-	     Location);
+             Expression* cap, Location);
 
   // Make an array index expression.  END may be NULL, in which case
-  // this is an lvalue.
+  // this is an lvalue.  CAP may be NULL, in which case it defaults
+  // to cap(ARRAY).
   static Expression*
   make_array_index(Expression* array, Expression* start, Expression* end,
-		   Location);
+                   Expression* cap, Location);
 
   // Make a string index expression.  END may be NULL.  This is never
   // an lvalue.
@@ -1672,9 +1674,9 @@
 {
  public:
   Index_expression(Expression* left, Expression* start, Expression* end,
-		   Location location)
+                   Expression* cap, Location location)
     : Parser_expression(EXPRESSION_INDEX, location),
-      left_(left), start_(start), end_(end), is_lvalue_(false)
+      left_(left), start_(start), end_(end), cap_(cap), is_lvalue_(false)
   { }
 
   // Record that this expression is an lvalue.
@@ -1683,10 +1685,11 @@
   { this->is_lvalue_ = true; }
 
   // Dump an index expression, i.e. an expression of the form
-  // expr[expr] or expr[expr:expr], to a dump context.
+  // expr[expr], expr[expr:expr], or expr[expr:expr:expr] to a dump context.
   static void
   dump_index_expression(Ast_dump_context*, const Expression* expr, 
-                        const Expression* start, const Expression* end);
+                        const Expression* start, const Expression* end,
+                        const Expression* cap);
 
  protected:
   int
@@ -1702,6 +1705,9 @@
 				(this->end_ == NULL
 				 ? NULL
 				 : this->end_->copy()),
+				(this->cap_ == NULL
+				 ? NULL
+				 : this->cap_->copy()),
 				this->location());
   }
 
@@ -1723,6 +1729,10 @@
   // The second index.  This is NULL for an index, non-NULL for a
   // slice.
   Expression* end_;
+  // The capacity argument.  This is NULL for indices and slices that use the
+  // default capacity, non-NULL for indices and slices that specify the
+  // capacity.
+  Expression* cap_;
   // Whether this is being used as an l-value.  We set this during the
   // parse because map index expressions need to know.
   bool is_lvalue_;
diff -r 886731829abf go/parse.cc
--- a/go/parse.cc	Wed Oct 23 16:56:39 2013 -0700
+++ b/go/parse.cc	Thu Oct 24 12:12:12 2013 -0700
@@ -3152,7 +3152,7 @@
 }
 
 // Index          = "[" Expression "]" .
-// Slice          = "[" Expression ":" [ Expression ] "]" .
+// Slice          = "[" Expression ":" [ Expression ] [ ":" Expression ] "]" .
 
 Expression*
 Parse::index(Expression* expr)
@@ -3178,14 +3178,31 @@
       // We use nil to indicate a missing high expression.
       if (this->advance_token()->is_op(OPERATOR_RSQUARE))
 	end = Expression::make_nil(this->location());
+      else if (this->peek_token()->is_op(OPERATOR_COLON))
+	{
+	  error_at(this->location(), "middle index required in 3-index slice");
+	  end = Expression::make_error(this->location());
+	}
       else
 	end = this->expression(PRECEDENCE_NORMAL, false, true, NULL, NULL);
     }
+
+  Expression* cap = NULL;
+  if (this->peek_token()->is_op(OPERATOR_COLON))
+    {
+      if (this->advance_token()->is_op(OPERATOR_RSQUARE))
+	{
+	  error_at(this->location(), "final index required in 3-index slice");
+	  cap = Expression::make_error(this->location());
+	}
+      else
+        cap = this->expression(PRECEDENCE_NORMAL, false, true, NULL, NULL);
+    }
   if (!this->peek_token()->is_op(OPERATOR_RSQUARE))
     error_at(this->location(), "missing %<]%>");
   else
     this->advance_token();
-  return Expression::make_index(expr, start, end, location);
+  return Expression::make_index(expr, start, end, cap, location);
 }
 
 // Call           = "(" [ ArgumentList [ "," ] ] ")" .
diff -r 886731829abf go/statements.cc
--- a/go/statements.cc	Wed Oct 23 16:56:39 2013 -0700
+++ b/go/statements.cc	Thu Oct 24 12:12:12 2013 -0700
@@ -5540,7 +5540,7 @@
 
       ref = this->make_range_ref(range_object, range_temp, loc);
       Expression* ref2 = Expression::make_temporary_reference(index_temp, loc);
-      Expression* index = Expression::make_index(ref, ref2, NULL, loc);
+      Expression* index = Expression::make_index(ref, ref2, NULL, NULL, loc);
 
       tref = Expression::make_temporary_reference(value_temp, loc);
       tref->set_is_lvalue();
@@ -5641,7 +5641,7 @@
 
       ref = Expression::make_temporary_reference(for_temp, loc);
       Expression* ref2 = Expression::make_temporary_reference(index_temp, loc);
-      Expression* index = Expression::make_index(ref, ref2, NULL, loc);
+      Expression* index = Expression::make_index(ref, ref2, NULL, NULL, loc);
 
       tref = Expression::make_temporary_reference(value_temp, loc);
       tref->set_is_lvalue();
@@ -5849,7 +5849,7 @@
   Expression* zexpr = Expression::make_integer(&zval, NULL, loc);
   mpz_clear(zval);
 
-  Expression* index = Expression::make_index(ref, zexpr, NULL, loc);
+  Expression* index = Expression::make_index(ref, zexpr, NULL, NULL, loc);
 
   Expression* ne = Expression::make_binary(OPERATOR_NOTEQ, index,
 					   Expression::make_nil(loc),
diff -r 886731829abf go/types.cc
--- a/go/types.cc	Wed Oct 23 16:56:39 2013 -0700
+++ b/go/types.cc	Thu Oct 24 12:12:12 2013 -0700
@@ -5645,12 +5645,12 @@
   Expression* e1 = Expression::make_temporary_reference(p1, bloc);
   e1 = Expression::make_unary(OPERATOR_MULT, e1, bloc);
   ref = Expression::make_temporary_reference(index, bloc);
-  e1 = Expression::make_array_index(e1, ref, NULL, bloc);
+  e1 = Expression::make_array_index(e1, ref, NULL, NULL, bloc);
 
   Expression* e2 = Expression::make_temporary_reference(p2, bloc);
   e2 = Expression::make_unary(OPERATOR_MULT, e2, bloc);
   ref = Expression::make_temporary_reference(index, bloc);
-  e2 = Expression::make_array_index(e2, ref, NULL, bloc);
+  e2 = Expression::make_array_index(e2, ref, NULL, NULL, bloc);
 
   Expression* cond = Expression::make_binary(OPERATOR_NOTEQ, e1, e2, bloc);
 
