Does flycheck ignore rust-analyzer.check.workspace?

[mkeeter]

I'm working on configuring rust-analyzer for a project with a customized build system (Hubris). Many crates in the workspace are only valid in the context of a particular manifest and task. I've got a wrapper around rust-analyzer that computes appropriate environment variables and features based on a user-selected (manifest, task) tuple.

Here's where stuff is going off the rails: I'm computing a set of feature to enable for the task crate and all downstream crates. Each feature is in the form crate-name/feature, e.g.

task-control-plane-agent/drv-stm32h7-usart task-control-plane-agent/drv-user-leds-api task-control-plane-agent/task-vpd-api task-control-plane-agent/usart1 task-control-plane-agent/vlan task-control-plane-agent/vpd task-net-api/smoltcp task-net-api/use-smoltcp task-net-api/vlan drv-stm32h7-usart/h753 drv-stm32xx-sys-api/family-stm32h7 drv-stm32xx-sys-api/h753 drv-stm32xx-gpio-common/family-stm32h7

These features are passed to rust-analyzer in the rust-analyzer.cargo.features array. The initial rust-analyzer load and indexing pass works fine. However, subsequent Flycheck passes fail (after saving a file).

Here's an example of a failure from the logs:

Flycheck failed to run the following command: CommandHandle {
  program: "/Users/mjk/.rustup/toolchains/nightly-2025-07-20-aarch64-apple-darwin/bin/cargo",
  arguments: ["check", "-p", "userlib", "--message-format=json",
    "--manifest-path", "/Users/mjk/oxide/hubris/Cargo.toml", "--keep-going",
    "--target", "thumbv7em-none-eabihf", "--no-default-features",
    "--features", "task-control-plane-agent/auxflash task-control-plane-agent/baud_rate_3M task-control-plane-agent/compute-sled task-control-plane-agent/cosmo task-control-plane-agent/drv-auxflash-api task-control-plane-agent/drv-cpu-seq-api task-control-plane-agent/drv-hf-api etc etc
...details elided
error=Cargo watcher failed, the command produced no valid metadata (
  exit code: ExitStatus(unix_wait_status(25856))):
  error: the package 'userlib' does not contain these features: counters/default, counters/derive,drv-stm32h7-usart/h753, drv-stm32xx-gpio-common/family-stm32h7,drv-stm32xx-gpio-common/has-af8-thru-af15,etc,etc

The problem is that I've generated a list of features for all crates, but the check call is targeting one crate (-p userlib), so many of the features are invalid.

This surprises me, because I have not changed rust-analyzer.check.workspace from the default of true, so I'd expect the check to use the workspace. The docs for rust-analyzer.check.workspace read:

Whether --workspace should be passed to cargo check. If false, -p <package> will be passed instead if applicable. In case it is not, no check will be performed.

Is this behaving as expected?

[mkeeter]

I may have messed up my config and set workspace = false elsewhere; future readers, beware!