Better handle nested interpolation (fixes amplify-education#173)

(breaks some reconstruction tests, tackling that next)

hcl2/hcl2.lark

@@ -1,7 +1,7 @@
 start : body
 body : (new_line_or_comment? (attribute | block))* new_line_or_comment?
 attribute : identifier EQ expression
-block : identifier (identifier | STRING_LIT)* new_line_or_comment? "{" body "}"
+block : identifier (identifier | STRING_LIT | string_with_interpolation)* new_line_or_comment? "{" body "}"
 new_line_and_or_comma: new_line_or_comment | "," | "," new_line_or_comment
 new_line_or_comment: ( NL_OR_COMMENT )+
 NL_OR_COMMENT: /\n[ \t]*/ | /#.*\n/ | /\/\/.*\n/ | /\/\*(.|\n)*?(\*\/)/
@@ -28,6 +28,7 @@ expr_term : "(" new_line_or_comment? expression new_line_or_comment? ")"
             | float_lit
             | int_lit
             | STRING_LIT
+            | string_with_interpolation
             | tuple
             | object
             | function_call
@@ -42,11 +43,10 @@ expr_term : "(" new_line_or_comment? expression new_line_or_comment? ")"
             | for_tuple_expr
             | for_object_expr
 
-
-STRING_LIT : "\"" (STRING_CHARS | INTERPOLATION)* "\""
-STRING_CHARS : /(?:(?!\${)([^"\\]|\\.))+/+ // any character except '"" unless inside a interpolation string
-NESTED_INTERPOLATION : "${" /[^}]+/ "}"
-INTERPOLATION : "${" (/(?:(?!\${)([^}]))+/ | NESTED_INTERPOLATION)+ "}"
+STRING_LIT : "\"" (STRING_CHARS)* "\""
+STRING_CHARS : /(?:(?!\${)([^"\\]|\\.))+/+ // any character except '"' unless inside a interpolation string
+string_with_interpolation: "\"" (STRING_CHARS)* interpolation_maybe_nested (STRING_CHARS | interpolation_maybe_nested)* "\""
+interpolation_maybe_nested: "${" expression "}"
 
 int_lit : DECIMAL+
 !float_lit: DECIMAL+ "." DECIMAL+ (EXP_MARK DECIMAL+)?

hcl2/transformer.py

@@ -277,6 +277,13 @@ def for_object_expr(self, args: List) -> str:
         # e.g. f"{2 + 2} {{2 + 2}}" == "4 {2 + 2}"
         return f"{{{for_expr}}}"
 
+    def string_with_interpolation(self, args: List) -> str:
+        return '"' + ("".join(args)) + '"'
+
+    def interpolation_maybe_nested(self, args: List) -> str:
+        # return "".join(args)
+        return "${" + ("".join(args)) + "}"
+
     def strip_new_line_tokens(self, args: List) -> List:
         """
         Remove new line and Discard tokens.
@@ -287,6 +294,10 @@ def strip_new_line_tokens(self, args: List) -> List:
     def to_string_dollar(self, value: Any) -> Any:
         """Wrap a string in ${ and }"""
         if isinstance(value, str):
+            # if it's already wrapped, pass it unmodified
+            if value.startswith("${") and value.endswith("}"):
+                return value
+
             if value.startswith('"') and value.endswith('"'):
                 value = str(value)[1:-1]
                 return self.process_escape_sequences(value)

test/helpers/terraform-config-json/multi_level_interpolation.json

@@ -0,0 +1,10 @@
+{
+    "block": [
+        {
+            "a": "${\"${\"${\"a\"}\"}\"}"
+        },
+        {
+            "b": "${var.b}"
+        }
+    ]
+}

test/helpers/terraform-config/multi_level_interpolation.tf

@@ -0,0 +1,7 @@
+block {
+  a = "${"${"${"a"}"}"}"
+}
+
+block {
+  b = "${var.b}"
+}