Cached offline use regression since lock file copying was implemented

[Veykril]

With Rust analyzer version 0.3.2509 for VSCode I was unable to use my typical workflow for working offline.

Before #20018 was merged I was able to connect to the internet, run cargo metadata, and as long as my crate versions didn't change I was able to continue using it offline.

I'm on a restricted machine where the main GUI desktop can't connect to the internet, but I can open up a shell that has internet access and I tend to run cargo fetch in that shell to get everything downloaded before using the GUI offline.

The issue now is that rust-analzyer is creating a new folder at target/rust-analyzer/metadata and since I haven't run cargo metadata in that folder ( since it doesn't exist until running the GUI with an internet connection ) the metadata isn't cached and rust-analyzer can't startup.

I realize that this is a little bit of a special case, but it still feels like maybe I should be able to have an option to run rust-analyzer against the normal cargo metadata location.

Originally posted by @zicklag in #12499

[Veykril]

@zicklag could you expand on this a bit. I am confused as to how that PR supposedly breaks your workflow. We should be copying the "refreshed" lockfile, so I am not sure what yo umean with the metadata isn't cached. The lockfile is the only cache there is aside from your local registry (which this same lockfile will be using).

[zicklag]

I normally take these steps:

• With access to the internet I run cargo fetch in a terminal. This downloads and caches all the dependencies I need.
• No I go offline.

With the old RA if I open my project in VSCode it uses the cached cargo metadata and works fine.

In the new RA, it fails trying to get to the internet when running cargo metadata.

If I run cargo metadata in a terminal, in the directory at this point it works fine, because RA is trying to create a new target/rust-analyzer/metadata/[something] directory and run cargo metadata against that, it doesn't work.

I'm not exactly sure why that doesn't work, because hypothetically it should have the same crates in there which should also be cached.

[Veykril]

That is odd ... I guess I'll open a thread on zulip.

If this turns out intentional we have a problem as we now have two features at odds here 🤔

• not updating the lock file
• but also supporting offline setups as best as we can (which is to say, we barely can support it)

[zicklag]

but also supporting offline setups as best as we can (which is to say, we barely can support it)

I think it works offline if you make sure that you open Rust analyzer while online and then go offline.

The caveat in my case was that due to kind of weird specific conditions of mine, I never run the VSCode UI ( and therefore Rust analyzer ) with internet access, only the cargo CLI.

Now that I say that, it gives me an idea that if there was a way to run rust-analyzer on the commandline to get it do it's lockfile thing, then that might be a way to workaround.