allow empty templates and template interpolations

grammar.js

@@ -269,22 +269,22 @@ module.exports = grammar({
 
     quoted_template: $ => prec(PREC.quoted_template, seq(
       $._quoted_template_start,
-      repeat(choice(
+      optional(repeat(choice(
         $.template_literal,
         $.template_interpolation,
         $.template_directive,
-      )),
+      ))),
       $._quoted_template_end,
     )),
 
     heredoc_template: $ => seq(
       $.heredoc_start,
       $.heredoc_identifier,
-      repeat(choice(
+      optional(repeat(choice(
         $.template_literal,
         $.template_interpolation,
         $.template_directive,
-      )),
+      ))),
       $.heredoc_identifier,
     ),
 
@@ -299,7 +299,7 @@ module.exports = grammar({
     template_interpolation: $ => seq(
       $._template_interpolation_start,
       optional($.strip_marker),
-      $.expression,
+      optional($.expression),
       optional($.strip_marker),
       $._template_interpolation_end,
     ),

src/grammar.json

@@ -1181,6 +1181,9 @@
             "type": "SYMBOL",
             "name": "_quoted_template_start"
           },
+          {
+            "type": "CHOICE",
+            "members": [
               {
                 "type": "REPEAT",
                 "content": {
@@ -1201,6 +1204,11 @@
                   ]
                 }
               },
+              {
+                "type": "BLANK"
+              }
+            ]
+          },
           {
             "type": "SYMBOL",
             "name": "_quoted_template_end"
@@ -1219,6 +1227,9 @@
           "type": "SYMBOL",
           "name": "heredoc_identifier"
         },
+        {
+          "type": "CHOICE",
+          "members": [
             {
               "type": "REPEAT",
               "content": {
@@ -1239,6 +1250,11 @@
                 ]
               }
             },
+            {
+              "type": "BLANK"
+            }
+          ]
+        },
         {
           "type": "SYMBOL",
           "name": "heredoc_identifier"
@@ -1292,10 +1308,18 @@
             }
           ]
         },
+        {
+          "type": "CHOICE",
+          "members": [
             {
               "type": "SYMBOL",
               "name": "expression"
             },
+            {
+              "type": "BLANK"
+            }
+          ]
+        },
         {
           "type": "CHOICE",
           "members": [

src/node-types.json

@@ -669,7 +669,7 @@
     "fields": {},
     "children": {
       "multiple": true,
-      "required": true,
+      "required": false,
       "types": [
         {
           "type": "expression",

test/corpus/templates.txt

@@ -20,6 +20,23 @@ foo = "${ var.bar }"
                 (get_attr
                   (identifier))))))))))
 
+================================================================================
+empty template
+================================================================================
+
+foo = "${}"
+
+--------------------------------------------------------------------------------
+
+(config_file
+  (body
+    (attribute
+      (identifier)
+      (expression
+        (template_expr
+          (quoted_template
+            (template_interpolation)))))))
+
 ================================================================================
 quoted template expression with escaped strings and fake strip marker
 ================================================================================