Tracking issue for `-Zsave-analysis`

[nrc]

-Z flags are now unstable. However, we plan to use save-analysis info for the RLS and Rustdoc and we want these to work on stable compilers. Therefore, we need a stable flag. Options:

• --save-analysis
• --emit analysis or analysis-info or analysis-data or whatever
• Keep the -Z flag and give tools a loophole to use it on stable

I propose that the existence of the flag is stable, but that we make no guarantees about the output when using the flag. We might consider guaranteeing that the output is valid JSON (but that includes {}). We could promise that the shape of the ouput won't change, but I'd rather not.

[nrc]

Another alternative is that the RLS and Rustdoc could use a custom compiler driver. This would mean we would not need a stable CL flag. This seems fine for the RLS (it would mean a marginally more complex build, but nothing too serious) but would mean some extra work for Rustdoc and mean it would only work if distributed with Rustdoc.

(Motivation for avoiding a stable flag is to prevent other users using the save-analysis data and it becoming de facto stable).

[steveklabnik]

What does "custom compiler driver" mean exactly?

[oli-obk]

It means that you are using rustc as a library. So instead of extending rustc via plugins, you are writing your own rustc binary (but you are reusing all of the boilerplate offered by the rustc_driver that rustc itself uses). You basically end up implementing a small trait, forwarding all methods to rustc's default implementation, and injecting your own code at the appropriate locations.

[nrc]

I've implemented a shim at https://github.com/nrc/rls-rustc I have a couple more features to add, but this should be a drop-in replacement for rustc for the RLS and rustdoc. I'll leave this issue open until we confirm this approach works, but I think we don't need to stabilise -Zsave-analysis any more.