When creating an embedded no_std application for a CPU architecture that does not have an implementation of the standard library, rust-analyzer may mysteriously complain that it can't find the test crate. This has something to do with how rust-analyzer checks the code against all targets and tries to pull in test, which depends on std.
The fix is to tell VS Code to only invoke rust-analyzer for the architecture you are developing for. You can put this into .vscode/settings.json.
{
"rust-analyzer.checkOnSave.allTargets": false,
"rust-analyzer.checkOnSave.extraArgs": [
"--target",
"thumbv7em-none-eabi"
]
}