Problem

rustdoc (on nightly) exposes an --output-format option to choose the format of the generated documentation: either HTML (the default) or JSON (the new unstable backend).

If you choose the JSON format, the output file of the unit is no longer /target/doc/<crate_name>/index.html; it becomes /target/doc/<crate name>.json.
Since there is no way to specify the output format as an option of cargo rustdoc itself, cargo assumes that the only possible output path for a doc unit is the HTML one. As a consequence, the JSON output is never considered "fresh" and we miss out on caching (see the test case in #12100, which fails on master).

Proposed Solution

Since cargo must remain unaware of the semantics of rustdoc's extra arguments, I suggest we add --output-format as an unstable cargo rustdoc argument, mimicking the semantics of the underlying --output-format option for rustdoc.

This would allow cargo to reason about it and take it into account in the fingerprinting logic.

Notes

No response