[Andrew Arnott (AArnott)]

VSTHRD200 can incorrectly reject the Async suffix for derived awaitable types, while libraries have no way to identify custom types that should require the suffix.

This change makes awaitable detection inheritance-aware and adds vs-threading.AsyncSuffixRequiredTypes.txt configuration for opt-in async-focused return types. The shared awaitable detection improvement also covers VSTHRD110. Documentation and regression coverage are included.

Fixes #419
Fixes #1023

[Copilot]

Pull request overview

Improves analyzer handling of custom awaitables by making awaitable detection inheritance-aware (fixing false positives for derived awaitables) and by adding an opt-in additional-file configuration to require the Async suffix for library-specific async-focused return types.

Changes:

• Extend CommonInterest.AwaitableTypeTester to recognize awaitable types via base classes and implemented interfaces (covers VSTHRD200 and VSTHRD110 usage).
• Add vs-threading.AsyncSuffixRequiredTypes.txt support to VSTHRD200 so libraries can opt-in custom return types that must use the Async suffix.
• Add regression tests and documentation for the new behavior and configuration.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD200UseAsyncNamingConventionAnalyzerTests.cs	Adds coverage for derived-awaitable returns and for the new async-suffix-required type configuration.
test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD110ObserveResultOfAsyncCallsAnalyzerTests.cs	Adds a regression test ensuring derived custom awaitables still produce diagnostics as expected.
test/Microsoft.VisualStudio.Threading.Analyzers.Tests/AdditionalFiles/vs-threading.AsyncSuffixRequiredTypes.mocks.txt	Provides a mocked additional-file entry to exercise the new VSTHRD200 configuration path in tests.
src/Microsoft.VisualStudio.Threading.Analyzers/VSTHRD200UseAsyncNamingConventionAnalyzer.cs	Reads configured async-suffix-required types and incorporates them into “async-focused return type” detection.
src/Microsoft.VisualStudio.Threading.Analyzers/CommonInterest.cs	Adds the new additional-file name pattern and updates awaitable detection to be inheritance-aware.
docfx/analyzers/VSTHRD200.md	Documents that custom async-focused return types can be configured to require the suffix.
docfx/analyzers/configuration.md	Adds documentation for vs-threading.AsyncSuffixRequiredTypes.txt format and usage.

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