compiler: Export String_index_expression.

    
    Exports String_index_expression and adds the getter `string` that
    returns the underlying string.  This will be used to handle string
    indexing different from array indexing in escape analysis.
    
    Reviewed-on: https://go-review.googlesource.com/18545

From-SVN: r235602

gcc/go/gofrontend/MERGE

-ba520fdcbea95531ebb9ef3d5be2de405ca90df3
+b17e404f5b8954e008b512741296d238ab7b2ef9
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -10380,66 +10380,7 @@ Expression::make_array_index(Expression* array, Expression* start,
   return new Array_index_expression(array, start, end, cap, location);
 }
 
-// A string index.  This is used for both indexing and slicing.
-
-class String_index_expression : public Expression
-{
- public:
-  String_index_expression(Expression* string, Expression* start,
-			  Expression* end, Location location)
-    : Expression(EXPRESSION_STRING_INDEX, location),
-      string_(string), start_(start), end_(end)
-  { }
-
- protected:
-  int
-  do_traverse(Traverse*);
-
-  Expression*
-  do_flatten(Gogo*, Named_object*, Statement_inserter*);
-
-  Type*
-  do_type();
-
-  void
-  do_determine_type(const Type_context*);
-
-  void
-  do_check_types(Gogo*);
-
-  Expression*
-  do_copy()
-  {
-    return Expression::make_string_index(this->string_->copy(),
-					 this->start_->copy(),
-					 (this->end_ == NULL
-					  ? NULL
-					  : this->end_->copy()),
-					 this->location());
-  }
-
-  bool
-  do_must_eval_subexpressions_in_order(int* skip) const
-  {
-    *skip = 1;
-    return true;
-  }
-
-  Bexpression*
-  do_get_backend(Translate_context*);
-
-  void
-  do_dump_expression(Ast_dump_context*) const;
-
- private:
-  // The string we are getting a value from.
-  Expression* string_;
-  // The start or only index.
-  Expression* start_;
-  // The end index of a slice.  This may be NULL for a single index,
-  // or it may be a nil expression for the length of the string.
-  Expression* end_;
-};
+// Class String_index_expression.
 
 // String index traversal.
 

gcc/go/gofrontend/expressions.h

@@ -44,6 +44,7 @@ class Func_descriptor_expression;
 class Unknown_expression;
 class Index_expression;
 class Array_index_expression;
+class String_index_expression;
 class Map_index_expression;
 class Bound_method_expression;
 class Field_reference_expression;
@@ -675,6 +676,13 @@ class Expression
   array_index_expression()
   { return this->convert<Array_index_expression, EXPRESSION_ARRAY_INDEX>(); }
 
+  // If this is an expression which refers to indexing in a string,
+  // return the String_index_expression structure.  Otherwise, return
+  // NULL.
+  String_index_expression*
+  string_index_expression()
+  { return this->convert<String_index_expression, EXPRESSION_STRING_INDEX>(); }
+
   // If this is an expression which refers to indexing in a map,
   // return the Map_index_expression structure.  Otherwise, return
   // NULL.
@@ -2583,6 +2591,72 @@ class Array_index_expression : public Expression
   Type* type_;
 };
 
+// A string index.  This is used for both indexing and slicing.
+
+class String_index_expression : public Expression
+{
+ public:
+  String_index_expression(Expression* string, Expression* start,
+			  Expression* end, Location location)
+    : Expression(EXPRESSION_STRING_INDEX, location),
+      string_(string), start_(start), end_(end)
+  { }
+
+  // Return the string being indexed.
+  Expression*
+  string() const
+  { return this->string_; }
+
+ protected:
+  int
+  do_traverse(Traverse*);
+
+  Expression*
+  do_flatten(Gogo*, Named_object*, Statement_inserter*);
+
+  Type*
+  do_type();
+
+  void
+  do_determine_type(const Type_context*);
+
+  void
+  do_check_types(Gogo*);
+
+  Expression*
+  do_copy()
+  {
+    return Expression::make_string_index(this->string_->copy(),
+					 this->start_->copy(),
+					 (this->end_ == NULL
+					  ? NULL
+					  : this->end_->copy()),
+					 this->location());
+  }
+
+  bool
+  do_must_eval_subexpressions_in_order(int* skip) const
+  {
+    *skip = 1;
+    return true;
+  }
+
+  Bexpression*
+  do_get_backend(Translate_context*);
+
+  void
+  do_dump_expression(Ast_dump_context*) const;
+
+ private:
+  // The string we are getting a value from.
+  Expression* string_;
+  // The start or only index.
+  Expression* start_;
+  // The end index of a slice.  This may be NULL for a single index,
+  // or it may be a nil expression for the length of the string.
+  Expression* end_;
+};
+
 // An index into a map.
 
 class Map_index_expression : public Expression