Cannot run simple unit test with default Rust template

[radu-matei]

The default Rust template for a Spin application sets the default build target in .cargo/config.toml as wasm32-wasi:

[build]
target = "wasm32-wasi"

This make sense in order to ensure cargo build creates a module that Spin can run, but this means that a simple unit test that purely wants to test some logic in the handler cannot be run:

• with a regular cargo test, this fails because it would try to instantiate a Wasm binary and run the test:

.../target/wasm32-wasi/debug/deps/rust_api-a5d8eced053c53aa.wasm: cannot execute binary file

• with cargo wasi test, this fails because of host imports that are not available in Wasmtime directly (but are in Spin):

wasm32-wasi/debug/deps/rust_api-a5d8eced053c53aa.wasm`
Error: failed to run main module `.../rust-api/target/wasm32-wasi/debug/deps/rust_api-a5d8eced053c53aa.wasm`

Caused by:
    0: failed to instantiate ".../target/wasm32-wasi/debug/deps/rust_api-a5d8eced053c53aa.wasm"
    1: unknown import: `wasi_experimental_http::req` has not been defined

The workaround for this is to remove the default target in .cargo/config.toml and run cargo test — assuming this only tests logic that does not have anything to do with Wasm or Spin functionality.

Because in the default Rust template we have a build command that sets the target to wasm32-wasi, do we want to remove the default target in the Cargo configuration file so testing with cargo test works for simple unit tests?

[FrankYang0529]

This is similar to what I encountered in spinframework/spin-fileserver#15. Probably, we can use cargo test --target=$$(rustc -vV | sed -n 's|host: ||p') as @lann mentioned in spinframework/spin-fileserver#15 (comment).

[radu-matei]

Given that all our Rust templates explicitly build to --target wasm32-wasi, could we remove .cargo/config.toml from the templates?

[vdice]

It sounds like we've converged on #672 (comment) as being the approved fix here? Shall we move into the Backlog?