CloudEvent Factory Source Generator

[tsutomi]

Emit compile-time IEventConvertible implementations from [Event]-annotated classes, eliminating the runtime reflection path in EventFactory and shifting annotation errors left to the build.

The problem today: EventFactory.CreateEventFromData() resolves [Event] metadata via Type.GetCustomAttribute<>() on every publish call, allocating attribute objects and walking type metadata at runtime even in high-throughput paths. The IEventConvertible escape hatch already exists — the benchmarks show it is measurably faster — but it requires repetitive, hand-written ToCloudEvent() boilerplate on every event class. Annotation mistakes (missing DataVersion, non-public class) surface as ArgumentException at runtime rather than at build time.

What we will build: A Deveel.Events.Generators package — a Roslyn incremental source generator (targeting netstandard2.0 as required by the analyzer SDK) that:

• Detects every partial class decorated with [Event] in the current compilation.
• Emits a generated partial class body that implements IEventConvertible.ToCloudEvent(): all CloudEvents envelope values (Type, DataSchema, DataContentType) are sourced from annotation values captured at compile time; the Data field is populated via System.Text.Json serialisation — zero reflection at call time.
• Emits compile-time diagnostics:
  • DLEVT001 — [Event] is applied to a non-partial class (generator cannot act; reflection path is used as a silent fallback).
  • DLEVT002 — [Event] specifies neither a DataVersion nor an absolute DataSchema URI.
  • DLEVT003 — [Event]-annotated class is not public.

Benefits:

• Zero reflection on the publish hot-path — all attribute lookup is resolved at build time, matching the performance of hand-coded IEventConvertible implementations as measured in the existing benchmarks.
• DLEVT001 / DLEVT002 / DLEVT003 shift errors that today manifest as ArgumentException at runtime into compile-time build failures, giving developers immediate feedback.
• No breaking change — classes not declared partial continue to work via the existing reflection path; adopting the generator is a purely opt-in, incremental migration.
• Works correctly in trimmed and ahead-of-time (AOT) compiled deployments where GetCustomAttribute calls on user types may be stripped.