Make ReusesCachedOutputOnRerun detect a non-incremental generator - #22
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matt-edmondson merged 1 commit intoSep 26, 2026
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The check reran the driver against the same Compilation instance. Roslyn compares incremental inputs by reference, so every step came back Cached and the check passed for any generator, including one that recomputes on every keystroke. The second run now gets a new but equivalent compilation, which is what an edit hands a generator. A generator with no tracked output steps now reports false instead of passing by default. Downstream tests that call this check may now fail. Each such failure is a generator that really was not incremental. Fixes #19 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_019StKk4VdpfAZ5cdW5X685e
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September 26, 2026 11:49
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Fixes #19
What was wrong
GeneratorHarness.ReusesCachedOutputOnRerunran the driver twice against the sameCSharpCompilationinstance. Roslyn compares incremental inputs by reference, so every step,CompilationProviderincluded, came backCached/Unchanged, and the check returnedtruefor any generator. A generator that recomputes on every keystroke passed. That also meantTheGeneratorReusesItsOutputWhenNothingChangedproved nothing.Change
compilation.AddSyntaxTrees(CSharpSyntaxTree.ParseText(string.Empty)). That is a new, semantically equivalentCompilation, which is what an edit hands a generator.falseinstead of passing by default.Design note: the
AdditionalTextinstances are kept, not rebuilt. The issue listed rebuilding them as optional. On a keystroke the IDE keeps the same instances for unchanged additional files, so reusing them models the scenario the check is for. Rebuilding them would be stricter than the IDE. It would also failGeneratorBaseitself, becauseMetadataFilehas reference equality, and that is a separate question from this bug.Release note: downstream tests that call this check may now fail. Each such failure is a generator that really was not incremental, as the triage comment anticipated.
Tests
TheRerunCheckCatchesAGeneratorThatRecomputesOnEveryEdit: theCompilationProvider.Select(_ => new object())generator from the issue must fail the check.TheRerunCheckDoesNotPassAGeneratorWithNoOutputStepsThingsGenerator) still passes under the stricter check, so it now proves something.This PR is independent of #21 (duplicate diagnostic numbers); the two merge cleanly in either order.
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https://claude.ai/code/session_019StKk4VdpfAZ5cdW5X685e
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