cabi_realloc is referenced but never linked on wasm32-unknown-emscripten #1751

crates/guest-rust/build.rs (still the case in v0.63.0) decides whether to link the cabi_realloc shim with:

let target_family = env::var("CARGO_CFG_TARGET_FAMILY").unwrap_or(String::new());
if target_family != "wasm" {
    return;
}

Cargo joins a cfg with several values with commas. wasm32-unknown-emscripten is the one wasm target with two families, so CARGO_CFG_TARGET_FAMILY is unix,wasm there, the comparison fails, and the build script returns before linking anything.

The crate's own Rust cfgs disagree with it. In src/rt/mod.rs, maybe_link_cabi_realloc references cabi_realloc under #[cfg(all(target_family = "wasm", any(target_env = "p1", target_env = "")))], and cfg(target_family = "wasm") is true on Emscripten, whose target_env is "". So the symbol is referenced but never provided. Any static library built on wit-bindgen for Emscripten then fails its final emcc link.

Reproduction (wit-bindgen 0.63.0, Rust 1.98.0, Emscripten 6.0.12):

# Cargo.toml
[lib]
crate-type = ["staticlib"]
[dependencies]
wit-bindgen = "=0.63.0"

// src/lib.rs, with wit/w.wit = `package repro:w; world w { export greet: func(name: string) -> string; }`
wit_bindgen::generate!({ world: "w", path: "wit" });
struct C;
impl Guest for C { fn greet(name: String) -> String { format!("hi {name}") } }
export!(C);
#[unsafe(no_mangle)]
pub extern "C" fn answer() -> i32 { 42 }

cargo build --release --target wasm32-unknown-emscripten
emcc main.c target/wasm32-unknown-emscripten/release/libwbrepro.a -o m.js
# wasm-ld: error: …wit_bindgen….rcgu.o: undefined symbol: cabi_realloc

(main.c calls answer().)

Fix: read the family as a list.

if !target_family.split(',').any(|f| f == "wasm") {
    return;
}

With this patched in through [patch.crates-io], the reproduction links and runs under node. Builds for wasm32-unknown-unknown, wasm32-wasip2 and x86_64-unknown-linux-gnu are unaffected; Emscripten is the only target whose family list differs.

The check dates from #851, where it reads as "any wasm target", with a panic for wasm architectures other than wasm32. I found nothing suggesting Emscripten was meant to be excluded. If it was, the cfgs in rt/mod.rs would need to exclude it too, so as it stands the two halves disagree either way.

Context: we hit this building a C ABI whose types come from wit-bindgen-generated bindings, for hosts that compile to the browser with Emscripten. We work around it with a #[no_mangle] extern "C" fn cabi_realloc on Emscripten that forwards to wit_bindgen::rt::cabi_realloc.