Cargo tries to build dynamic libraries for Emscripten targets

[kornelski]

Given a crate with:

[lib]
crate-type = ["cdylib", "lib", "staticlib"]

cargo build --target wasm32-unknown-emscripten

invokes:

rustc … --crate-type cdylib --crate-type lib --crate-type staticlib --emit=dep-info,link --target wasm32-unknown-emscripten …

which causes:

thread 'rustc' panicked at 'src/librustc_codegen_ssa/back/linker.rs:893: building dynamic library is unsupported on Emscripten', src/librustc_middle/util/bug.rs:37:26

This behavior makes it impossible to support WASM and native (not WASM) dynamic libraries in the same crate.

It would be helpful if Cargo knew that wasm32-unknown-emscripten doesn't support cdylib, and didn't try to build it.

Note that unsupported cdylib breaks builds that don't use the crate as a dynamic library, because Cargo seems to unconditionally build cdylib even when the crate is used as a regular dependency linked as .rlib.

[ehuss]

This seems to have been changed in 1.45 (it no longer errors via rust-lang/rust#71804). Which version are you using?

This also sounds like a rustc issue (rust-lang/rust#67782), not a cargo one. Typically rustc ignores unsupported crate types (here)

[kornelski]

Ah, I've had 1.44.1. Thanks for fixing this in negative amount of time!