Source-generate self-aggregating records from their own declaration (marten#4557)

[jeremydmiller]

Problem

Downstream report JasperFx/marten#4557: a user upgrading 8 → 9 has a self-aggregating immutable record with static Create/Apply, registered via Projections.Snapshot<MyType>(Inline). It throws at DocumentStore.For(...):

InvalidProjectionException: No source-generated dispatcher found for
SingleStreamProjection<MyType, System.Guid>...

Root cause

The generator's candidate predicate treats records differently from classes:

• A class with conventional Apply/Create is discovered by Pipeline 1 from its own declaration and gets a free-standing evolver + [assembly: GeneratedEvolver(...)].
• A record was filtered in IsCandidateClass to Evolve/EvolveAsync only (a fossil of the removed runtime-reflection path), so a self-aggregating record only ever got an evolver when the generator could see a Snapshot<T> / AggregateStream<T> call site (Pipeline 3).

That asymmetry is the root of the cross-assembly gap: a record aggregate defined in a domain library but registered in a separate composition-root assembly never has a [GeneratedEvolver] emitted into its own assembly — which is exactly where the runtime scans (tryUseAssemblyRegisteredEvolver keys on typeof(TDoc).Assembly). It also creates the impression that such a record must be partial, which it must not.

Fix

• AggregateEvolverGenerator.IsCandidateClass: treat records identically to classes. A self-aggregating record with Apply/Create now gets its evolver emitted from its own declaration — no Snapshot<T> call site required, and never requiring partial. ExecuteCombined's existing dedup keeps same-assembly registrations single-emit (Pipeline 1 claims the type; Pipeline 3 skips it), exactly as classes already behave.
• Message accuracy: corrected AggregateApplication.MissingDispatcherMessage and the EvolveAsync backstop — dropped the blanket "must be partial" claim (only projection subclasses need it), pointed at the assembly that defines the aggregate as the analyzer/lookup site, noted the generator ships inside the Marten package, and removed the dead docs/codegen/aot.md link.

Guiding principle: self-aggregating types (class or record) must never have to be partial — only projection subclasses do.

Validation

• JasperFx.Events.SourceGenerator.Tests: 19/19 green, including a new self_aggregating_record_emits_evolver_from_its_own_declaration_without_partial (non-partial record, no Snapshot<T> call site → asserts the evolver + [GeneratedEvolver] attribute are emitted, no errors).
• EventTests compiles clean (0 errors) under the new generator.

Downstream

JasperFx/marten#4557. Marten separately bundles this analyzer in its own NuGet package so package consumers get the generator automatically; once a generator release carries this change, Marten's bundled analyzer picks up the record/cross-assembly fix.

🤖 Generated with Claude Code