[wasm32-unknown-emscripten] Js and Wasm files have different names.

[ahaoboy]

The Wasm file and the Js file have different names.

I tried this code:

cargo new a-b-c
cargo build 
// a-b-c

cargo build --target=wasm32-unknown-emscripten
// a-b-c.js
// a_b_c.wasm

cargo build --target=wasm32-unknown-unknown
// a-b-c.wasm

I expected to see this happen: explanation
Wasm and Js files have the same name.

a-b-c.js
a-b-c.wasm

or

a_b_c.js
a_b_c.wasm

Instead, this happened: explanation

a-b-c.js
a_b_c.wasm

Meta

rustc --version --verbose:

rustc 1.76.0-nightly (2b603f95a 2023-11-12)
binary: rustc
commit-hash: 2b603f95a48f10f931a61dd208fe3e5ffd64e491
commit-date: 2023-11-12
host: x86_64-unknown-linux-gnu
release: 1.76.0-nightly
LLVM version: 17.0.4```
  

[ThePuzzlemaker]

This issue stems from Cargo, in fact. The linker properly puts the file as target/wasm32-unknown-emscripten/debug/deps/a_b_c.js, but Cargo copies it back to the binary's name, target/wasm32-unknown-emscripten/debug/a-b-c.js.

This stems from this code in Cargo. I think it would be necessary to special-case Emscripten WASM in this case, or to modify the library path code to convert the .wasm into the crate name in rustc_session or cargo.

[ThePuzzlemaker]

@rustbot modify labels: -T-compiler +T-cargo

[weihanglo]

.js is a "bin" crate type from Cargo's (and rustc?) point of view. When Cargo does an uplift for a bin file from deps directory, it preserves the binary name as-is.

cargo/src/cargo/core/compiler/build_context/target_info.rs

Line 378 in b2e1d3b

should_replace_hyphens: crate_type != CrateType::Bin,

Changing the uplift logic for emscripten target might be a breaking change. The alternative solution for this is using the unstable "different-binary-name" feature if you are not going to change the bin target name (bin.name field). You could also change the bin.name to underscore for consistency reason.

Back to the issue and wasm32-unknown-emscripten target, is there anything bug caused by this behavior?

[ahaoboy]

Back to the issue and wasm32-unknown-emscripten target, is there anything bug caused by this behavior?

There are no errors or bugs so far, just two file names that need to be handled when processing the outputs

const jsName = libName  + '.js'
const wasmName = libName.replace('_', '-') + '.wasm'

[weihanglo]

Thanks for the reply. Just note that replacing the pwasm file name wouldn't work, as the wasm file name is embedded in the .js file.

If you would like to have it consistent without post-processing, you could explicitly add a bin target as mentioned earlier:

# in Cargo.toml
[[bin]]
name = "a_b_c" # <- explicitly name your binary
path = "src/main.rs"

[weihanglo]

While the layout inside target directory is not stable and subject to change, to avoid churns to current users of wasm32-unknown-emscripten, I would recommend aforementioned alternatives. Hence propose to the team to close this.

@rustbot label +S-propose-close -S-needs-info