Thanks for working on this!

Not sure whether this is intended as just sharing your experiment or whether you actually want to land this.

I feel like we should explore #2626 (comment) a bit before we invest time in this.

I feel like we should explore #2626 (comment) a bit before we invest time in this.

Especially since that reduced list of installed components might be more amenable to a smaller binary format like Postcard compared to the full manifest.

I mostly wanted to see how much code would have to be changed to use the JSON format, seems like not that much. So I don't think it would be so bad to land as-is.

Doing the other approach would likely require larger code changes, but it might be a better solution overall. I would have to first understand how do the manifests work to implement it though.

The structure of the code makes it a bit challenging to change this. Essentially, the code first loads the full manifest, then it goes through all its rust components for a given target, and for each of them it checks that it is installed. We would need to store some smaller submanifests split by target (one file per target), which would only contain the rust package to make rustup load less data.

However, while investigating this, I also noticed something interesting. In the "common case", where you just execute rustc without any rust-toolchain.toml override, the list of components and targets to be checked is actually empty. So in that case we load the manifest for no reason, we parse it, go through all the components, and then we do .all() on an empty iterator.

#4350 should fix this.

I think that this change is no longer worth it now that #4350 has been merged.

Wouldn't this still be worth it for the people who do specify components or targets in rust-toolchain.toml? Those are pretty common in my experience.

It would definitely make the parsing faster for their use-cases. Not sure whether it is worth it to solve that with this specific approach, it seemed like it wasn't very popular with rustup maintainers. I'm of course willing to reopen and finish this if there's interset :)

The structure of the code makes it a bit challenging to change this. Essentially, the code first loads the full manifest, then it goes through all its rust components for a given target, and for each of them it checks that it is installed. We would need to store some smaller submanifests split by target (one file per target), which would only contain the rust package to make rustup load less data.

Maybe it would make sense to store a "mini manifest" only for targets that are installed?

I guess it would be possible, yeah. And if the target file is not on disk, it would be considered to be missing. Note that I'm not sure how "mini" would the manifests be, it looked like the rust component, cat multirust-channel-manifest.toml | grep "pkg.rust\." | wc -l has 5757 entries for the stable toolchain (although many of these are duplicated for multiple targets).