Warnings don't show up until `cargo clean` or file change in VSCode

[arsdragonfly]

Right now diagnostics depend on the JSON output of cargo check --all-targets --message-format json, which won't yield any warnings for files that have already been built. Only after cargo clean has been run and/or the file has been saved again will warnings pop up.

[kiljacken]

@arsdragonfly This has presumably been fixed upstream at some point, as it seems to work on my end. Can you confirm?

[bjorn3]

Cargo caches warnings since rust-lang/cargo#7450.

[kiljacken]

Then we can probably close this one too.