rust_binary(out_binary=True) is broken for Wasm targets.

[PiotrSikora]

The out_binary = True feature to build Rust binaries for Wasm targets introduced in #312 is broken since #361.

$ cat BUILD

load("@io_bazel_rules_rust//rust:rust.bzl", "rust_binary")

rust_binary(
    name = "test_rust",
    srcs = ["test_rust.rs"],
    edition = "2018",
    visibility = ["//visibility:public"],
    crate_type = "cdylib",
    out_binary = True,
)

$ cat test_rust.rs

#[no_mangle]
extern "C" fn abort() {
    panic!("abort")
}

This works fine when using last commit before #361 (i.e. fda9a1ce6482973adfda022cadbfa6b300e269c3):

$ bazel build --platforms=@io_bazel_rules_rust//rust/platform:wasm //:test_rust

Starting local Bazel server and connecting to it...
INFO: Invocation ID: fdecbf32-5468-4c23-9b20-7cf6c993a24c
INFO: SHA256 (https://static.rust-lang.org/dist/rust-std-1.44.0-wasm32-unknown-unknown.tar.gz) = 8e12796a0c2fb083953042218f832bdeb78da1bfaf67b9dfe3d719920084d755
INFO: Analyzed target //:test_rust (24 packages loaded, 161 targets configured).
INFO: Found 1 target...
Target //:test_rust up-to-date:
  bazel-bin/test_rust.wasm
INFO: Elapsed time: 19.079s, Critical Path: 2.90s
INFO: 5 processes: 1 remote cache hit, 4 processwrapper-sandbox.
INFO: Build completed successfully, 11 total actions

but is broken starting from #361 until master (i.e. fdf9655ba95616e0314b4e0ebab40bb0c5fe005c):

$ bazel build --platforms=@io_bazel_rules_rust//rust/platform:wasm //:test_rust

Starting local Bazel server and connecting to it...
INFO: Invocation ID: 3d32152c-bc89-4e2c-98ef-40caa00823a7
INFO: SHA256 (https://static.rust-lang.org/dist/rust-std-1.44.0-wasm32-unknown-unknown.tar.gz) = 8e12796a0c2fb083953042218f832bdeb78da1bfaf67b9dfe3d719920084d755
ERROR: /tmp/out_binary/BUILD:15:12: in rust_binary rule //:test_rust: 
Traceback (most recent call last):
        File "/tmp/out_binary/BUILD", line 15
                rust_binary(name = 'test_rust')
        File ".../external/io_bazel_rules_rust/rust/private/rust.bzl", line 142, in _rust_binary_impl
                rustc_compile_action(ctx = ctx, <2 more arguments>)
        File ".../external/io_bazel_rules_rust/rust/private/rustc.bzl", line 510, in rustc_compile_action
                establish_cc_info(ctx, <4 more arguments>)
        File ".../external/io_bazel_rules_rust/rust/private/rustc.bzl", line 535, in establish_cc_info
                cc_common.create_library_to_link(<4 more arguments>)
'test_rust.wasm' does not have any of the allowed extensions .so, .dylib or .dll
ERROR: Analysis of target '//:test_rust' failed; build aborted: Analysis of target '//:test_rust' failed
INFO: Elapsed time: 17.614s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (25 packages loaded, 159 targets configured)

[GregBowyer]

My apologies.

I am going to assume right now that it doesnt make the most sense for you to link a wasm binary to further cc_library / cc_binary targets since the wasm support is rather bolted on at a right angle. Is this a use case for you?

[PiotrSikora]

Not linked, no.

But ideally, those .wasm modules could be used as data inputs in cc_{binary,library,test} targets.

Using them in rust_{binary,library,test} is going to be broken until we get dedicated rust_wasm_{binary,library} (see previous discussion in #240 (comment) and #312), since right now we're enabling Wasm target using --platforms=@io_bazel_rules_rust//rust/platform:wasm , which would also convert Rust test runners into Wasm.