fix(events-targets): imported sqs queue cannot be used as a rule dlq (#28165)

packages/aws-cdk-lib/aws-events-targets/lib/util.ts

@@ -140,10 +140,18 @@ export function addToDeadLetterQueueResourcePolicy(rule: events.IRule, queue: sq
 /**
  * Whether two string probably contain the same environment dimension (region or account)
  *
- * Used to compare either accounts or regions, and also returns true if both
- * are unresolved (in which case both are expted to be "current region" or "current account").
+ * Used to compare either accounts or regions on a best effort basis. If we cannot tell definitively
+ * that the dimensions are in different environments, we will pass. 
+ * 
+ * For example, returns true if both are unresolved (in which case both are expected to be
+ * "current region" or "current account").
+ *
+ * Also returns true if one is unresolved (in which case we expect the unresolved dimension to match
+ * the resolved dimension, but it is up to the user to ensure this). Returning true here makes sure
+ * that we are not overly aggressive in producing a synth-time error.
+ *
  * @internal
  */
 function sameEnvDimension(dim1: string, dim2: string) {
-  return [TokenComparison.SAME, TokenComparison.BOTH_UNRESOLVED].includes(Token.compareStrings(dim1, dim2));
+  return [TokenComparison.SAME, TokenComparison.ONE_UNRESOLVED, TokenComparison.BOTH_UNRESOLVED].includes(Token.compareStrings(dim1, dim2));
 }

packages/aws-cdk-lib/aws-events-targets/test/sqs/sqs.test.ts

@@ -2,7 +2,8 @@ import { Match, Template } from '../../../assertions';
 import * as events from '../../../aws-events';
 import * as kms from '../../../aws-kms';
 import * as sqs from '../../../aws-sqs';
-import { App, Duration, Stack } from '../../../core';
+import * as ssm from '../../../aws-ssm';
+import { App, CustomResource, Duration, Stack } from '../../../core';
 import * as cxapi from '../../../cx-api';
 import * as targets from '../../lib';
 
@@ -401,3 +402,38 @@ test('specifying retry policy with 0 retryAttempts', () => {
     ],
   });
 });
+
+test('dead letter queue is imported', () => {
+  const stack = new Stack();
+  const queue = new sqs.Queue(stack, 'MyQueue', { fifo: true });
+  const rule = new events.Rule(stack, 'MyRule', {
+    schedule: events.Schedule.rate(Duration.hours(1)),
+  });
+
+  const dlqArn = 'arn:aws:sqs:eu-west-1:444455556666:queue1';
+  const deadLetterQueue = sqs.Queue.fromQueueArn(stack, 'MyDeadLetterQueue', dlqArn);
+
+  // WHEN
+  rule.addTarget(new targets.SqsQueue(queue, {
+    deadLetterQueue,
+  }));
+
+  Template.fromStack(stack).hasResourceProperties('AWS::Events::Rule', {
+    ScheduleExpression: 'rate(1 hour)',
+    State: 'ENABLED',
+    Targets: [
+      {
+        Arn: {
+          'Fn::GetAtt': [
+            'MyQueueE6CA6235',
+            'Arn',
+          ],
+        },
+        Id: 'Target0',
+        DeadLetterConfig: {
+          Arn: dlqArn,
+        },
+      },
+    ],
+  });
+});