Describe why and how to use a separate build directory for rust-analyzer

[Zalathar]

There is some existing advice about this in the dev guide, but it is terse and incomplete.

https://rust-lang.zulipchat.com/#narrow/channel/122651-general/topic/Setup.20multiple.20editors.20for.20work.20on.20standard.20lib.3F/near/479397923

[jieyouxu]

Thanks!

[lnicola]

My comments on this from the r-a side:

• if you prefer to run ./x.py check from the command line, another option is to disable checkOnSave
• a check started by r-a will block one started from the terminal, but the latter will be fast enough since it will have nothing to do besides checking for freshness; running two checks at once is wasteful but with CPU and disk space

[Zalathar]

The issue is more that r-a running checks in the background will block all command-line bootstrap actions, including building the compiler or running tests, even if the check is relatively unimportant.

And once r-a has started its check, there's no way to stop it without stopping the server, which then implies several minutes of re-indexing when the server is restarted.

And aside from that, there are a few known cases where bootstrap will delete artifacts to avoid incompatibilities between different commands, which means that letting r-a touch the main build directory in the background is a really bad idea (even if everything is protected by locks).

[lnicola]

The issue is more that r-a running checks in the background will block all command-line bootstrap actions, including building the compiler or running tests, even if the check is relatively unimportant.

Yes, but will the equivalent of cargo check & cargo test --no-run; wait be faster than cargo check; cargo test --no-run? I wouldn't be sure.

And once r-a has started its check, there's no way to stop it without stopping the server

In VS Code (and other clients which implement the extension) there is a "Cancel running flycheck" command that does exactly that. Clicking on the status bar item and Cancel might also work.