Bug 1451450 - Parse VariantExpressions with MessageReferences inside

Change the `Identifier`-typed `id` field of VariantExpression to an
`Expression`-typed `ref` field, so that it can hold a `MessageReference`.

Author: stasm

fluent-syntax/src/ast.js

@@ -142,10 +142,10 @@ export class AttributeExpression extends Expression {
 }
 
 export class VariantExpression extends Expression {
-  constructor(id, key) {
+  constructor(ref, key) {
     super();
     this.type = "VariantExpression";
-    this.id = id;
+    this.ref = ref;
     this.key = key;
   }
 }

fluent-syntax/src/parser.js

@@ -619,7 +619,7 @@ export default class FluentParser {
 
       ps.expectChar("]");
 
-      return new AST.VariantExpression(literal.id, key);
+      return new AST.VariantExpression(literal, key);
     }
 
     if (ch === "(") {

fluent-syntax/src/serializer.js

@@ -269,9 +269,9 @@ function serializeAttributeExpression(expr) {
 
 
 function serializeVariantExpression(expr) {
-  const id = serializeIdentifier(expr.id);
+  const ref = serializeExpression(expr.ref);
   const key = serializeVariantKey(expr.key);
-  return `${id}[${key}]`;
+  return `${ref}[${key}]`;
 }
 
 

fluent-syntax/test/fixtures_structure/term.json

@@ -189,9 +189,17 @@
             "type": "Placeable",
             "expression": {
               "type": "VariantExpression",
-              "id": {
-                "type": "Identifier",
-                "name": "-brand-name",
+              "ref": {
+                "type": "MessageReference",
+                "id": {
+                  "type": "Identifier",
+                  "name": "-brand-name",
+                  "span": {
+                    "type": "Span",
+                    "start": 145,
+                    "end": 156
+                  }
+                },
                 "span": {
                   "type": "Span",
                   "start": 145,