rustc: Stop passing --allow-undefined on wasm targets by alexcrichton · Pull Request #149868 · rust-lang/rust (original) (raw)

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This commit updates how the linker is invoked on WebAssembly targets (all of them) to avoid passing the --allow-undefined flag to the linker. Historically, if I remember this correctly, when wasm-ld was first integrated this was practically required because at the time it was otherwise impossible to import a function from the host into a wasm binary. Or, at least, I'm pretty sure that was why this was added.

At the time, as the documentation around this option indicates, it was known that this was going to be a hazard. This doesn't match behavior on native, for example, and can easily paper over what should be a linker error with some sort of other obscure runtime error. An example is that this program currently compiles and links, it just prints null:

unsafe extern "C" {
    static nonexistent: u8;
}

fn main() {
    println!("{:?}", &raw const nonexistent);
}

This can easily lead to mistakes like rust-lang/libc/4880 and defer what should be a compile-time link error to weird or unusual behavior at link time. Additionally, in the intervening time since wasm-ld was first introduced here, lots has changed and notably this program works as expected:

#[link(wasm_import_module = "host")]
unsafe extern "C" {
    fn foo();
}

fn main() {
    unsafe {
        foo();
    }
}

Notably this continues to compile without error and the final wasm binary indeed has an imported function from the host. What this change means, however, is that this program:

unsafe extern "C" {
    fn foo();
}

fn main() {
    unsafe {
        foo();
    }
}

this currently compiles successfully and emits an import from the env module. After this change, however, this will fail to compile with a link error stating that the foo symbol is not defined.

rustbot added A-run-make

Area: port run-make Makefiles to rmake.rs

S-waiting-on-review

Status: Awaiting review from the assignee but also interested parties.

T-compiler

Relevant to the compiler team, which will review and decide on the PR/issue.

labels

Dec 10, 2025

rust-bors bot added a commit that referenced this pull request

Dec 11, 2025

rustc: Stop passing --allow-undefined on wasm targets

try-job: dist-x86_64-linux try-job: dist-various-*

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