Add [SNSEvent] annotation attribute and source generator support

[GarrettBeatty]

Summary

Adds [SNSEvent] annotation attribute support to the Lambda Annotations framework, enabling developers to declaratively configure SNS topic-triggered Lambda functions directly in C# code. The source generator automatically produces the corresponding SAM/CloudFormation template configuration at build time.

User Experience

With this change, developers can write SNS-triggered Lambda functions like this:

[LambdaFunction]
[SNSEvent("@MyTopic")]
public async Task ProcessMessages(SNSEvent evnt)
{
    foreach (var record in evnt.Records)
    {
        // Handle SNS messages
    }
}

The source generator will automatically generate the SAM template entry:

ProcessMessages:
  Type: AWS::Serverless::Function
  Properties:
    Events:
      MyTopic:
        Type: SNS
        Properties:
          Topic: !Ref MyTopic

Attribute Properties

Property	Required	Description	Default
Topic	Yes	SNS topic reference (@TopicName for CloudFormation Ref, or topic ARN)	-
ResourceName	No	CloudFormation event resource name	Derived from topic name
FilterPolicy	No	JSON filter policy for the subscription	Not set
Enabled	No	Whether the event source is enabled	true

The implementation is pretty similar to sqs event

Related: DOTNET-8573

[Copilot]

Pull request overview

Adds first-class SNS event support to the Lambda Annotations framework by introducing a new [SNSEvent] attribute, validating its usage at compile time, and emitting the corresponding SAM/CloudFormation SNS event configuration via the source generator.

Changes:

• Introduces SNSEventAttribute (with Topic/ResourceName/FilterPolicy/Enabled) and updates docs/examples.
• Extends the source generator pipeline (syntax discovery, modeling, validation, template writing) to support SNS events and diagnostics.
• Adds unit tests, snapshot tests, and an integration test to validate generated templates and real SNS subscription configuration.

Reviewed changes

Copilot reviewed 26 out of 26 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
Libraries/test/TestServerlessApp/TestServerlessApp.csproj	Adds SNSEvents project reference for test app compilation.
Libraries/test/TestServerlessApp/SnsMessageProcessing.cs	Adds an SNS-triggered Lambda handler using [SNSEvent].
Libraries/test/TestServerlessApp/SNSEventExamples/ValidSNSEvents.cs.txt	Adds valid SNS examples used as generator test input (not deployed).
Libraries/test/TestServerlessApp.IntegrationTests/SNSEventSubscription.cs	Adds integration test to verify SNS subscription + filter policy.
Libraries/test/TestServerlessApp.IntegrationTests/IntegrationTestContextFixture.cs	Captures SNS Topic ARN from the deployed stack; updates expected Lambda count.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/WriterTests/SNSEventsTests.cs	Adds CloudFormation writer tests for SNS event emission and syncing behavior.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/SourceGeneratorTests.cs	Adds end-to-end snapshot-based generator test for SNS events.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/ServerlessTemplates/snsEvents.template	Adds expected generated serverless template snapshot for SNS events.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/SNS/ValidSNSEvents_ProcessMessages_Generated.g.cs	Adds expected generated handler snapshot (sync).
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/SNS/ValidSNSEvents_ProcessMessagesAsync_Generated.g.cs	Adds expected generated handler snapshot (async).
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/SNSEventAttributeTests.cs	Adds unit tests for SNSEventAttribute defaults and validation.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/CSharpSourceGeneratorVerifier.cs	Adds SNSEvents metadata reference for compilation in generator tests.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Amazon.Lambda.Annotations.SourceGenerators.Tests.csproj	Adds SNSEvents project reference for test project.
Libraries/src/Amazon.Lambda.Annotations/SNS/SNSEventAttribute.cs	Introduces the public [SNSEvent] attribute and internal validation helpers.
Libraries/src/Amazon.Lambda.Annotations/README.md	Documents SNS event usage and adds an SNS example section.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Writers/CloudFormationWriter.cs	Emits Type: SNS event definitions and associated properties into templates.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Validation/LambdaFunctionValidator.cs	Adds SNS signature/return-type/attribute validation and dependency check.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/TypeFullNames.cs	Adds SNS type/attribute full names and registers SNS attribute in known list.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/SyntaxReceiver.cs	Recognizes SNSEventAttribute during syntax collection.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/EventTypeBuilder.cs	Maps SNSEventAttribute to EventType.SNS.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/EventType.cs	Adds the SNS enum value.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/Attributes/SNSEventAttributeBuilder.cs	Builds SNSEventAttribute models from Roslyn attribute data.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/Attributes/AttributeModelBuilder.cs	Routes SNSEventAttribute into the attribute model pipeline.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Diagnostics/DiagnosticDescriptors.cs	Adds a diagnostic descriptor for invalid SNSEventAttribute usage.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Diagnostics/AnalyzerReleases.Unshipped.md	Registers the new SNS diagnostic in analyzer releases.
.autover/changes/add-snsevent-annotation.json	Adds a minor-version changelog entry for the new SNS annotation support.

---

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[GarrettBeatty]

i think this was a bug. default should be true as per the docs. right now it returns false by default

[philasmar]

Call that out in the changelog

[Copilot]

Pull request overview

Adds first-class SNS event support to the Lambda Annotations framework by introducing a new [SNSEvent] attribute and extending the source generator to validate SNS handlers and emit the corresponding SAM Events configuration.

Changes:

• Introduces Amazon.Lambda.Annotations.SNS.SNSEventAttribute with optional ResourceName, FilterPolicy, and Enabled.
• Extends the source generator to recognize/validate [SNSEvent] and write Type: SNS events into generated templates.
• Adds unit, snapshot, writer, and integration tests plus documentation updates for SNS usage.

Reviewed changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Libraries/test/TestServerlessApp/TestServerlessApp.csproj	Adds SNSEvents project reference for test app compilation.
Libraries/test/TestServerlessApp/SnsMessageProcessing.cs	Adds a deployable SNS-triggered handler example for integration testing.
Libraries/test/TestServerlessApp/SNSEventExamples/ValidSNSEvents.cs.txt	Adds source input for SNS-related generator snapshot tests (not deployed).
Libraries/test/TestServerlessApp.IntegrationTests/SNSEventSubscription.cs	Adds integration test verifying SNS subscription + filter policy.
Libraries/test/TestServerlessApp.IntegrationTests/IntegrationTestContextFixture.cs	Captures SNS topic ARN from the deployed stack for tests.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/WriterTests/SNSEventsTests.cs	Adds CloudFormation writer tests for SNS event serialization + syncing.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/SourceGeneratorTests.cs	Adds snapshot-based generator test coverage for valid SNS handlers.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/ServerlessTemplates/snsEvents.template	Adds expected serverless template snapshot for SNS events.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/SNS/ValidSNSEvents_ProcessMessages_Generated.g.cs	Adds expected generated handler snapshot (sync).
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/SNS/ValidSNSEvents_ProcessMessagesAsync_Generated.g.cs	Adds expected generated handler snapshot (async).
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/SNSEventAttributeTests.cs	Adds unit tests for SNSEventAttribute defaults/validation.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/CSharpSourceGeneratorVerifier.cs	Adds SNSEvent assembly reference for Roslyn test compilation.
Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Amazon.Lambda.Annotations.SourceGenerators.Tests.csproj	Adds SNSEvents project reference for test project.
Libraries/src/Amazon.Lambda.Annotations/SQS/SQSEventAttribute.cs	Fixes SQS Enabled default to true.
Libraries/src/Amazon.Lambda.Annotations/SNS/SNSEventAttribute.cs	Adds the new SNSEventAttribute API and validation helpers.
Libraries/src/Amazon.Lambda.Annotations/README.md	Documents SNS event usage and adds SNSEvent to attribute list.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Writers/CloudFormationWriter.cs	Emits SNS SAM event entries and tracks synced properties.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Validation/LambdaFunctionValidator.cs	Adds SNS validation (dependencies, signature, return type, attribute validation).
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/TypeFullNames.cs	Registers SNSEvent and SNSEventAttribute full names.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/SyntaxReceiver.cs	Recognizes SNSEventAttribute as a “secondary” attribute requiring [LambdaFunction].
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/EventTypeBuilder.cs	Maps SNSEventAttribute to EventType.SNS.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/EventType.cs	Adds SNS to the event type enum.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/Attributes/SNSEventAttributeBuilder.cs	Builds SNSEventAttribute instances from Roslyn AttributeData.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/Attributes/AttributeModelBuilder.cs	Routes SNSEventAttribute through the model builder pipeline.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Diagnostics/DiagnosticDescriptors.cs	Adds Invalid SNSEventAttribute diagnostic descriptor.
Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Diagnostics/AnalyzerReleases.Unshipped.md	Registers the new SNS diagnostic in unshipped analyzer releases.
.autover/changes/add-snsevent-annotation.json	Adds autover changelog entry for the new SNSEvent feature.

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[philasmar]

add license header

[philasmar]

add license header

[philasmar]

Call that out in the changelog