A workaround is to run with:

CARGO_TARGET_WASM32_WASI_RUNNER="wasmtime --dir=." cargo wasi test

I just hit this, and unless there's a better way to build filenames for test fixtures that I (a very new Rust learner) am unaware of, the solution probably needs to also ensure the CARGO_MANIFEST_DIR env-var is passed into wasmtime.

My currently-working env-var is (PowerShell)

$Env:CARGO_TARGET_WASM32_WASI_RUNNER="wasmtime --dir=${PWD} --env CARGO_MANIFEST_DIR=${PWD}

which makes the following work with just cargo test:

    let manfest_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
    let fixture = path!(
        Path::new(&manfest_dir) / "tests" / "fixtures" / "2024-05-30" / "BlueprintList_info.json"
    );

    let data = fs::read_to_string(fixture).unwrap();

Without the --env CARGO_MANIFEST_DIR value, the call to env::var("CARGO_MANIFEST_DIR").unwrap(); panics with called ``Result::unwrap()`` on an ``Err`` value: NotPresent.

Edit: Playing around a bit,

$Env:CARGO_TARGET_WASM32_WASI_RUNNER="wasmtime --dir=${PWD}::/ --env CARGO_MANIFEST_DIR=/"

seems to work better, as it doesn't get confused by leading backslashes in CLI parameters using cargo wasi run, i.e., this works with the mapping to /, but fails with my earlier version with no guest mapping.

cargo wasi run .\tests\fixtures\2024-05-30\BlueprintList_info.json

This works in both cases, but tab-completion in PowerShell or CMD will convert it to the above.

cargo wasi run ./tests\fixtures\2024-05-30\BlueprintList_info.json

Might be a good time to clarify that I'm using Windows for this, in case that affects more than just the directory-seperator.