Preserve Sprockets context dependency metadata

[OskarEichler]

Summary

Preserve Sprockets context metadata when rendering a Slim asset so depend_on changes invalidate the compiled asset cache. The transformer currently returns only the rendered string and drops dependency metadata accumulated by the context.

The return value now follows Sprockets' processor contract, as its ERB processor does: context.metadata.merge(data: rendered_html).

Reproduction

With Sprockets 4.4.1, put these files on the asset load path:

- depend_on 'payload.txt'
p = File.read('/absolute/path/to/payload.txt')

Compile page.html using this transformer and a Sprockets::Cache::MemoryStore. Change payload.txt from first to second, advancing its mtime, then create a fresh environment sharing that cache and compile again. Before: <p>first</p> remains cached. After: <p>second</p>, changed digest, and payload dependency retained.

Verification

• Unmodified upstream suite: 14 runs / 54 assertions, zero failures/errors/skips on Ruby 3.2.11 and 4.0.6, Rails 8.1.3.1.
• 100 focused checks across 20 independent real Sprockets cache cases on both Rubies: initial compile, dependency metadata, unchanged cache, changed output and digest.
• Standard lint and Ruby syntax pass. Source-only patch; no test files changed.

Compatibility / limitations

No public rendering API break or dependency upgrade intended. The internal Transformer.call return becomes a processor Hash instead of String; direct users of that internal method must account for this. Assets still expose rendered HTML. Sprockets 2/3 paths are unchanged; only Sprockets 4.4.1 was exercised here. The release-based consumer package retains version 4.0.0 and dependency requirements.

[januszm]

Nice, thanks, could you confirm that the return value of self.call is not used anywhere (ie. it is discarded after call)?

[OskarEichler]

Confirmed the return value is consumed by Sprockets as the processor result; it is not otherwise called directly in this repository. Returning context.metadata merged with data follows that processor contract and preserves the dependency metadata instead of discarding it.