Add documentation for troubleshooting flatpak permission issues

[MisterSirCode]

Refer to #2873 (comment) and my issue #20616

This change adds documentation to the troubleshooting FAQ for any future users experiencing cargo file resolving errors with rust-analyzer due to their IDE lacking permission to see the cargo binaries.

[MisterSirCode]

Documentation moved to the vs-code page

Feel free to edit any wording or detail improvements that should be made

[lnicola]

Wait, isn't this the same as the suggestion right above?

If you want to use Flatpak in combination with rustup, the following steps might help:

• using Flatseal, you must add an environment variable called PATH. Set its value to the output from above, appending :~/.cargo/bin, where ~ is the path to your home directory. You must replace ~, as it won’t be expanded otherwise.

[ptomato]

I think probably the suggestion given in VSCodium/vscodium#2027 (comment) is better - it doesn't require using the host's cargo installation.

I have the Codium flatpak in the system installation so I had to do it without --user, here's what worked for me:

flatpak install flathub org.freedesktop.Sdk.Extension.rust-stable//25.08
flatpak override --env=FLATPAK_ENABLE_SDK_EXT=rust-stable com.vscodium.codium

adding the FLATPAK_ENABLE_SDK_EXT variable is also something that could be done in the flatpak manifest.

[lnicola]

@ptomato isn't that also right above in the docs? 😄

If you use a Flatpak SDK for Rust, it must be in your PATH:

• install the SDK extensions with flatpak install org.freedesktop.Sdk.Extension.{llvm15,rust-stable}//23.08
• enable SDK extensions in the editor with the environment variable FLATPAK_ENABLE_SDK_EXT=llvm15,rust-stable (this can be done using flatseal or flatpak override)

But please feel free to update it to a more modern SDK etc.

[ptomato]

My comment applied to the docs change in this PR, which didn't work for me. If my suggestion is already in the docs, great!

[rustbot]

☔ The latest upstream changes (possibly #21804) made this pull request unmergeable. Please resolve the merge conflicts.