Cargo check should not block cargo run

[ConradIrwin]

Problem

I work at Zed and we are heavy users of cargo check, primarily through rust-analyzer.

One problem that we have as a relatively large rust project is that cargo check and cargo run can take a while. We have other ideas to help with this (e.g. rust-lang/compiler-team#790), but it is our reality today.

This causes the common flow of wanting to run my changes after making them to be slower than I think they should be. This is because when I save a file, rust-analyzer starts a cargo check in the background, so by the time I switch to a terminal to type cargo run, it almost always is "Blocking waiting for file lock on build directory" for a long time.

As a (somewhat impatient) user, I would love if my build that I just kicked off happened immediately.

Proposed Solution

I don't know what the lock is protecting, so I can see a few different options here:

• Reduce the granularity of locking to the crate level or module level so that both cargo run and cargo build could run co-operatively.
• Add a mechanism to "prioritize" cargo processes; so a user-kicked-off build would pause the background cargo run while it was running.

Notes

It is possible today to avoid this locking by pointing cargo check to a different target directory when run by rust analyzer.

This works, but has the unfortunate side-effect of using more disk space (Even without this workaround, I have to run cargo clean about once a week because my target directory grows to several hundred gigabytes in that time, which causes builds to get slower still...), and seems to make the overall process slower (I guess there are some shared artifacts that cargo check and cargo build re-use from each other).

[P1n3appl3]

Related issue on rust-analyzer: rust-lang/rust-analyzer#18515
... and on cargo: #3501
... and on rust: rust-lang/rust#65427

[epage]

#4282 is our issue for changing the locking scheme.

I'm going to close in favor of that issue. If there is a reason for us to keep this specific issue open, let us know!

[ConradIrwin]

Thanks! And sorry for missing that.