Address review comments.

Also expose IndexableExpression to Expression casting in the VAST C API.

Author: meheff

xls/codegen/vast/vast.cc

@@ -673,6 +673,8 @@ std::string ModulePort::ToString() const {
                          xls::verilog::ToString(direction), name());
 }
 
+// Returns true if the given string looks like an identifier or keyword. Escaped
+// identifiers are not handled.
 static bool IsDigitsOrIdentifier(std::string_view s) {
   if (s.empty()) {
     return false;
@@ -694,7 +696,7 @@ static bool IsDigitsOrIdentifier(std::string_view s) {
     return false;
   }
   for (char ch : s.substr(1)) {
-    if (!absl::ascii_isalnum(ch) && ch != '_') {
+    if (!absl::ascii_isalnum(ch) && ch != '_' && ch != '$') {
       return false;
     }
   }
@@ -706,11 +708,11 @@ std::string WidthCast::Emit(LineInfo* line_info) const {
   LineInfoEnd(line_info, this);
   std::string width_expr = width_->Emit(line_info);
   if (IsDigitsOrIdentifier(width_expr)) {
-    // Parentheses are not needed for digits or identifiers.
+    // Parentheses are not needed for digits or identifiers. This is a purely
+    // cosmetic as parentheses are always correct to emit.
     return absl::StrFormat("%s'(%s)", width_expr, value_->Emit(line_info));
-  } else {
-    return absl::StrFormat("(%s)'(%s)", width_expr, value_->Emit(line_info));
   }
+  return absl::StrFormat("(%s)'(%s)", width_expr, value_->Emit(line_info));
 }
 
 VerilogFunction::VerilogFunction(std::string_view name, DataType* result_type,

xls/public/c_api_symbols.txt

@@ -309,6 +309,7 @@ xls_vast_def_get_data_type
 xls_vast_def_get_name
 xls_vast_index_as_expression
 xls_vast_index_as_indexable_expression
+xls_vast_indexable_expression_as_expression
 xls_vast_literal_as_expression
 xls_vast_localparam_ref_as_expression
 xls_vast_logic_ref_as_expression

xls/public/c_api_vast.cc

@@ -543,6 +543,13 @@ struct xls_vast_expression* xls_vast_localparam_ref_as_expression(
   return reinterpret_cast<xls_vast_expression*>(cpp_expression);
 }
 
+struct xls_vast_expression* xls_vast_indexable_expression_as_expression(
+    struct xls_vast_indexable_expression* v) {
+  auto* cpp_v = reinterpret_cast<xls::verilog::IndexableExpression*>(v);
+  auto* cpp_expression = static_cast<xls::verilog::Expression*>(cpp_v);
+  return reinterpret_cast<xls_vast_expression*>(cpp_expression);
+}
+
 struct xls_vast_indexable_expression*
 xls_vast_logic_ref_as_indexable_expression(
     struct xls_vast_logic_ref* logic_ref) {

xls/public/c_api_vast.h

@@ -353,6 +353,8 @@ struct xls_vast_expression* xls_vast_parameter_ref_as_expression(
     struct xls_vast_parameter_ref* v);
 struct xls_vast_expression* xls_vast_localparam_ref_as_expression(
     struct xls_vast_localparam_ref* v);
+struct xls_vast_expression* xls_vast_indexable_expression_as_expression(
+    struct xls_vast_indexable_expression* v);
 
 struct xls_vast_indexable_expression*
 xls_vast_logic_ref_as_indexable_expression(