Identical metadata in a new AdditionalText instance re-runs the whole generator, because MetadataFile has no value equality

[matt-edmondson]

What's wrong

GeneratorBase projects each AdditionalText into new MetadataFile(...) and then Collects the results (GeneratorBase.cs:60-66). MetadataFile (MetadataFile.cs:18) is a class with no Equals or GetHashCode. Roslyn compares incremental step outputs with default equality, so the check is effectively reference identity. A new AdditionalText with byte-identical contents therefore always produces a "modified" step, and the whole Generate runs again: every file is deserialized and every source re-emitted.

In an IDE a new AdditionalText instance is created in several ordinary cases, such as an edit followed by an undo, or a save with no content change. The incremental pipeline does not save any work in those cases.

GeneratorHarness.ReusesCachedOutputOnRerun does not catch this. It reuses the same AdditionalText instances and changes only the compilation.

Repro

1. Create a driver with trackIncrementalGeneratorSteps: true and the test project's ThingsGenerator, and run it once.
2. Call driver.ReplaceAdditionalText(t1, new MemText(samePath, sameText)).
3. Run it again against the same compilation.

• Observed: the tracked output step's reason is Modified.
• Expected: Cached or Unchanged.

Suggested fix

• Implement IEquatable<MetadataFile> using ordinal comparison of FileName, the path and Text (or SourceText.ContentEquals), with a matching GetHashCode.
• Extend GeneratorHarness with a check that swaps in equal-content AdditionalTexts and asserts the outputs are cached, so a regression is caught.

[matt-edmondson]

Triage

• Category: Improvement (incremental-generator performance)
• Priority: Low. Output is correct. The cost is unnecessary full regeneration in the IDE after edit-and-undo or a no-change save.
• Area / suggested assignee: MetadataFile equality and the GeneratorBase pipeline (GeneratorBase.cs:60-66), plus GeneratorHarness coverage. Owner: @matt-edmondson
• Duplicates / in progress: not a duplicate. No open PR. Coordinate with A metadata model whose constructor throws (e.g. a null guard) crashes the generator with CS8785 and drops every other file's output #25 and PR Report an empty metadata file as unparseable, not missing #24, which also touch MetadataFile.
• Next step: implement IEquatable<MetadataFile> (ordinal path/name, SourceText.ContentEquals for text) with a matching GetHashCode, and add a harness check that swaps in equal-content AdditionalTexts and asserts Cached/Unchanged.

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