This is an issue that currently blocks me from porting a library to WASM:

I have a library called proj5, which I use in a backend server (in a regular Rust program). I wanted to port the library to wasm (the wasm32-unknown-unknown target) that is now available on nightly and found it to be impossible to do it while still being able to use it in the backend:

The only way to get a WASM binary is by setting the crate-type to "cdylib" in the Cargo.toml file. However, if I do this, I can't use the crate in my regular rust program anymore! And if I leave it out, there is not WASM output.

So currently a crate can be compiled either for WASM or for use in a regular Rust program, not both (controlled by the target). I tried the following so far:

1. Setting the crate-type in Cargo.toml

[target.'cfg(target_arch = "wasm32")']
crate-type = ["dylib"]

This doesn't work, the argument goes unused:

warning: unused manifest key: target.cfg(target_arch = "wasm32").crate-type

I also tried:

[target.wasm32-unknown-unknown]
crate-type = ["dylib"]

Same thing, same error message. The crate builds, but doesn't produce a WASM binary.

2. Setting RUSTFLAGS (in the .cargo/config) does not seem to have any effect. I tried:

[target.wasm32-unknown-unknown]
rustflags = [ "--crate-type=cdylib" ]

This just gives a cryptic error, and sets the crate-type twice:

error: unexpected character in cfg `/`, expected parens, a comma, an identifier, or a string

No line number, no column number, nothing. Not sure where it failed - the error message could be heavily improved upon.

I also tried it with crate_type, -- --crate-type, --crate-type dylib. None of which work. I expected --crate-type=dylib to work, because it is documented this way, however I suspect that the documentation is incorrect or out of date.

3. Setting the crate type in lib.rs

So the last thing I tried was to override the crate type via cfg_attr:

// lib.rs
#![cfg_attr(target_arch = "wasm32", crate_type = "cdylib")]

This is simply ignored by cargo. I still get a .rlib file, not a .wasm file.

So right now I'm out of options. Why is is so hard to build a library for both regular Rust use and WASM? Right now I can only choose either-or.

There is a workaround in that I make a second crate (as cdylib), which just exposes the first one (the rlib), but I don't think this is the way to go. This is important for feature-gating crates so that they can be compiled to WASM without any workarounds.