[ES-449270] Fix duplicate key error in object comprehension (#156)

Fixes #121 

Please advise on checking for size twice vs calling `contains` once per
key, I haven't benchmarked this.

This might also break some existing files which rely on this behavior
without realizing it's out of spec, either with duplicate list entries
getting reduced or more complex replacements where only the last entry
matters. Not sure if we're ok with that or what other options we have.
Breaking these probably beats allowing duplicate entries in the future.

sjsonnet/src/sjsonnet/Evaluator.scala

@@ -589,12 +589,16 @@ class Evaluator(resolver: CachedResolver,
 
         visitExpr(e.key)(s) match {
           case Val.Str(_, k) =>
+            val prev_length = builder.size()
             builder.put(k, new Val.Obj.Member(false, Visibility.Normal) {
               def invoke(self: Val.Obj, sup: Val.Obj, fs: FileScope, ev: EvalScope): Val =
                 visitExpr(e.value)(
                   s.extend(newBindings, self, null)
                 )
             })
+            if (prev_length == builder.size()) {
+              Error.fail(s"Duplicate key ${k} in evaluated object comprehension.", e.pos);
+            }
           case Val.Null(_) => // do nothing
         }
       }

sjsonnet/test/src/sjsonnet/EvaluatorTests.scala

@@ -338,6 +338,19 @@ object EvaluatorTests extends TestSuite{
     test("givenNoDuplicateFieldsInListComprehension2_expectSuccess") {
       eval("""{ ["bar_" + x]: x for x in [5,12]}""") ==> ujson.Obj("bar_5" -> 5, "bar_12" -> 12)
     }
+    test("givenDuplicateFieldsInListComprehension_expectFailure") {
+      evalErr("""{ [x]: x for x in ["A", "A"]}""") ==>
+        """sjsonnet.Error: Duplicate key A in evaluated object comprehension.
+          |at .(:1:3)""".stripMargin
+    }
+    test("givenDuplicateFieldsInIndirectListComprehension_expectFailure") {
+      evalErr(
+        """local y = { a: "A" };
+          |local z = { a: "A" };
+          |{ [x.a]: x for x in [y, z]}""".stripMargin) ==>
+        """sjsonnet.Error: Duplicate key A in evaluated object comprehension.
+          |at .(:3:3)""".stripMargin
+    }
     test("functionEqualsNull") {
       eval("""local f(x)=null; f == null""") ==> ujson.False
       eval("""local f=null; f == null""") ==> ujson.True