Fail fast when a mixed-lifetime IEnumerable<T> singleton mirror is missing (#381)

[jeremydmiller]

Summary

Fixes #381 — the mixed-lifetime IEnumerable<T> codegen routes each singleton element through a keyed "mirror" registration created by AddJasperFxEnumerableSingletonSupport(). If a consumer never calls that method (or registers a mixed-lifetime family after it runs), the mirror doesn't exist and the singleton element was injected as null with no error — the enumerable silently contained a null that surfaced far away as an NRE inside a handler (the footgun behind #380).

This implements option 2 from the issue: a small guard in the generated enumerable code.

What changed

• CreateArrayFrame.GenerateCode now emits a null guard before the new T[]{…} array literal for any singleton element sourced from a keyed mirror. When the mirror is absent, the generated code throws an actionable InvalidOperationException instead of yielding a null element:

  Keyed mirror jasperfx-enumerable-singleton-0 for the singleton element of IEnumerable<T> was not found. Call AddJasperFxEnumerableSingletonSupport() once during DI setup, before BuildServiceProvider().

• EnumerableSingletons gains a KeyPrefix constant, an IsMirrorKey() detector, and a MissingMirrorMessage() builder.

The guard is robust regardless of how the constructor parameter was populated (MS DI's keyed resolution, or JasperFx's QuickBuild optional GetKeyedService path that produced the silent null).

Tests

Added to inline_enumerable_with_mixed_lifetimes:

• missing_mirror_emits_a_failfast_guard_in_the_generated_code — asserts the guard text
• missing_mirror_throws_actionable_error_instead_of_injecting_null — exercises the silent-null path (optional keyed resolution returns null) and asserts the actionable throw
• registered_mirror_does_not_trip_the_guard — no-regression case with support registered

Full CodegenTests (359) and CoreTests (439) pass on net9.0 and net10.0.

Version

Bumps JasperFxVersion 2.1.3 → 2.1.4.

🤖 Generated with Claude Code