Fix: batch registration chain to avoid Roslyn CS8078 on large scans

[waynebrantley]

Summary

GenerateRegistrationsSource always emits one fluent chain per generated method:

return services
    .AddTransient<A, B>()
    .AddTransient<C, D>()
    // ...
    .AddTransient<Y, Z>();

That's a single C# expression whose parse tree depth grows linearly with the registration count. Roslyn's operation-tree walkers hit a recursion cap around ~1000 links and the compilation aborts with:

error CS8078: An expression is too long or complex to compile

Any consumer scanning a large assembly (~1000+ matching types) can't compile the generated code at all. Analyzers walking the same tree also stack-overflow, and dotnet format on the generated code crashes with the same trace.

GenerateCustomHandlingSource already emits statement-per-line output, so the ScanForTypes+Handler path doesn't hit this — it's specific to [GenerateServiceRegistrations].

Observed in production: a ~1170-handler CQRS assembly could not compile after switching from Scrutor to [GenerateServiceRegistrations].

Fix

Batch the chain at MaxRegistrationsPerChain = 500 links per statement. Below the threshold: output is byte-for-byte identical to the pre-fix shape (zero snapshot-test churn — all 93 existing tests still green). Above the threshold: split into N services.Add()...; statements, with the final statement carrying the return when the method returns IServiceCollection.

Test

AddServicesTests.LargeAssemblyEmitsBatchedChains — declares 650 handlers, asserts:

• Exactly 2 chain-statements emitted (500 + 150) rather than one 650-link chain
• All 650 registrations present in the emit
• Compiling the generated code as part of the source produces zero CS8078 diagnostics

Verified: fails on main, passes with the fix.

Notes

Threshold value (500) is conservative — actual CS8078 tripping point varies with stack size (~1000+ in typical CI, less on constrained runners). 500 leaves comfortable headroom. Happy to bump or make it configurable if you prefer.

[Dreamescaper]

I'm thinking if it makes sense to make Roslyn's life easier, and simply make separate statements, like

services.AddTransient<A, B>();
services.AddTransient<C, D>();
...
services.AddTransient<Y, Z>();
return services;

[waynebrantley]

Great call — updated to statement-per-line. Force-pushed 6e4a988.

• Same shape you suggested: services.AddTransient<A, B>(); services.AddTransient<C, D>(); ... return services;
• 45 existing snapshot tests updated via mechanical regex transform + 1 hand-edit for the custom-parameter-name case
• Renamed the regression test to LargeAssemblyCompilesWithoutCs8078 and bumped it to 1200 handlers so it actually exercises the CS8078 tripping point on typical CI runners
• All 94 tests green