Roslyn analyzer + code fix: convert WebApplicationFactory<T> to TestWebApplicationFactory<T>

[thomhurst]

Problem

Users who inherit from Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory<T> directly silently lose:

• Trace propagation (no traceparent/baggage on outgoing requests).
• Per-test logging correlation.
• TestContext.Current resolution inside request handlers.

There is no compile-time signal. Discussion #5479 is one of multiple reports rooted in this exact mistake.

Proposal

Extend the existing WebApplicationFactoryAccessAnalyzer (or add a sibling rule) in TUnit.AspNetCore.Analyzers that flags:

public class MyFactory : WebApplicationFactory<Program> // TUNITxxxx
{
}

with severity Warning (not Error — some users may have legitimate reasons), and a code fix that converts to:

public class MyFactory : TestWebApplicationFactory<Program>
{
}

The code fix should also offer the alternative wrapping pattern when the inheritance can't be changed (e.g. third-party base class):

var factory = new TracedWebApplicationFactory<Program>(existingFactory);

Considerations

• Scope to test projects. Detect via IsTestProject-style heuristics: Microsoft.Testing.Platform reference, or namespace contains .Tests. Avoid false positives in production code.
• Opt-out. Standard #pragma warning disable plus .editorconfig rule severity.
• Documentation link in diagnostic. helpLinkUri pointing to the new Distributed Tracing page.
• Diagnostic ID. Allocate next free TUNITxxxx in the analyzer table.

Related

• Discussion HTML report generation failed #5479.
• Existing analyzer: TUnit.AspNetCore.Analyzers/WebApplicationFactoryAccessAnalyzer.cs.

Documentation

When this lands, update:

• docs/docs/examples/aspnet.md — update the existing "Use TestWebApplicationFactory<T>" warning callout to mention the new analyzer and its diagnostic ID, so users understand the warning they'll see in the IDE.
• docs/docs/guides/distributed-tracing.md — link the diagnostic ID from the "WebApplicationFactory without TUnit's wrapper" limitation entry.
• Analyzer rules documentation (if a per-rule reference exists) — add an entry for the new TUNITxxxx ID.