Equivalent Method to Specify `--target=wasm32-unknown-emscripten`

[kwhitehouse]

I'd like to use Bazel to build a Wasm target from a Rust library. With cargo, I'd use the following:

cargo build --release --target=wasm32-unknown-emscripten

What's the equivalent method with rules_rust to specify a particular target architecture?

Trial & Error

I tried mucking around with rustc_flags, per below:

rust_library(
    name = "my_library",
    srcs = ["lib.rs"],
     rustc_flags = [
        "--target=wasm32-unknown-unknown"
    ]
 )

But this resulted in, "error: Option 'target' given more than once."

More details

I want to use Bazel to create a Wasm binary from a C++ binary that relies on a Rust library. I'm making use of wasm_cc_binary as follows.

load("@emsdk//emscripten_toolchain:wasm_rules.bzl", "wasm_cc_binary")
 
wasm_cc_binary(
    name = "my_wasm_binary",
    cc_target = ":main",
)

The cc_binary associated looks as follows:

cc_binary(
    name = "main",
    srcs = ["main.cpp"],
    deps = [
      "//my/rust:mylibrary",
    ]
)

And lastly, the rust_library looks as follows:

rust_library(
    name = "mylibrary",
    srcs = ["lib.rs"],
    deps = all_crate_deps(normal = True),
)

If I run bazel build on the above, I get the following error:

No matching toolchains found for types @@rules_rust~//rust:toolchain_type.
To debug, rerun with --toolchain_resolution_debug='@@rules_rust~//rust:toolchain_type'
If platforms or toolchains are a new concept for you, we'd encourage reading https://bazel.build/concepts/platforms-intro.

A hacky fix

I'm able to successfully build a Wasm target if I do the following.

1. Run cargo build --release --target=wasm32-unknown-emscripten to generate a target/wasm32-unknown-emscripten/release/mylibrary.a output
2. Update my cc_binary above so that it uses this mylibrary.a file as a src:

cc_binary(
    name = "main",
    srcs = [
        "main.cpp"
        "path/to/mylibrary.a"
    ]
)

I hope I didn't miss a simple answer in the documentation!