`error: #[ctor] is not supported on the current target` when compiling for `wasm32-unknown-emscripten`

[kylebarron]

👋 I'm trying to compile polars to wasm to work with pyodide (ref #2412 and pola-rs/polars#3672 (comment)).

After removing some dependencies, I got all of polars's dependencies to build for wasm32-unknown-emscripten using RUSTUP_TOOLCHAIN=nightly maturin build --release -o dist --target wasm32-unknown-emscripten -i python3.10, but compilation failed on polars itself with a couple hundred #[ctor] is not supported on the current target errors:

2310   │ error: #[ctor] is not supported on the current target
2311   │   --> src/lazy/dataframe.rs:95:1
2312   │    |
2313   │ 95 | #[pymethods]
2314   │    | ^^^^^^^^^^^^
2315   │    |
2316   │    = note: this error originates in the attribute macro `$crate::ctor` (in Nightly builds, run with -Z macro-backtrac
       │ e for more info)

I've seen mmastrac/rust-ctor#14 and #1434. Does this mean there's no way to use a pyo3 class in wasm32-unknown-emscripten? Did the opt-out functionality mentioned here ever get implemented?

[messense]

I wonder if it's possible to get rid of multiple pymethods usage in polars?

[kylebarron]

When compiling from this commit, this (log1.txt) is the output log from

RUSTUP_TOOLCHAIN=nightly maturin build --release -o dist --target wasm32-unknown-emscripten -i python3.10

So for example several of the errors come from here: https://github.com/kylebarron/polars/blob/3e52839c89367e8fb676c483f6c618a83f0460f9/py-polars/src/series.rs#L31-L61

2265   │ error: #[ctor] is not supported on the current target
2266   │   --> src/series.rs:53:1
2267   │    |
2268   │ 53 | init_method!(new_i8, i8);
2269   │    | ^^^^^^^^^^^^^^^^^^^^^^^^
2270   │    |
2271   │    = note: this error originates in the attribute macro `$crate::ctor` (in Nightly builds, run with -Z macro-backtrac
       │ e for more info)

I wonder if it's possible to get rid of multiple pymethods usage in polars?

I'm not sure I understand pyo3 well enough to answer this question 😅 (I also didn't write the polars python bindings; I'm just exploring compiling them to pyodide). It seems like there's a compile error every time #[pymethods] is used...?

[messense]

Only multiple-pymethods feature in pyo3 requires #[ctor], see https://pyo3.rs/v0.16.4/class.html#implementation-details

[kylebarron]

Oh I see! That makes sense now!

It looks like polars uses multiple pymethods to simplify their macros (they use #[pymethods] dozens of times on just the PySeries struct). I'll talk to the polars project and see what they say.

Thanks for your help!

[davidhewitt]

As this is documented and not anything PyO3 can realistically change, I will close this issue.

[messense]

UPDATE: inventory just got wasi and emscripten support in 0.3.16 (when using Rust nightly): https://github.com/dtolnay/inventory/releases/tag/0.3.16