Autodiff often breaks with ZST arguments

[SabrinaJewson]

I tried this code:

#![feature(autodiff)]

#[core::autodiff::autodiff_forward(fd_inner, Const, Dual)]
fn f(_zst: (), _x: &mut f64) {}

#[unsafe(no_mangle)]
pub extern "C" fn fd(x: &mut f64, xd: &mut f64) {
    fd_inner((), x, xd);
}

with this Cargo.toml:

[package]
name = "x"
edition = "2024"
[profile.dev]
lto = "fat"
panic = "abort"
[lib]
crate-type = ["staticlib"]

and RUSTFLAGS="-Zautodiff=Enable" cargo +enzyme build. I got the attached ICE.

rustc-ice-2025-07-16T13_26_31-14135.txt

The error goes away if I use a non-ZST, like u8, as the argument. I can produce various different ICEs (e.g. failing the assert on line 174) by varying the conditions, but my (uneducated) guess is that the root cause is the same, that the autodiff code fails to take into account how ZSTs are translated into LLVM.

Meta

rustc --version --verbose:

rustc 1.90.0-nightly (c8780fff6 2025-07-11)
binary: rustc
commit-hash: c8780fff6a1f3bda416231342fae0dde986ada39
commit-date: 2025-07-11
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7