ICE: autodiff_reverse crashes in Enzyme's invertPointerM on Active reference/pointer parameters

Code

#![feature(autodiff)]
use std::autodiff::autodiff_reverse;

#[autodiff_reverse(df, Active, Active, Active)]
fn f<'a>(x: &'a f32, y: &'a f32) -> f32 {
    *x * *y
}

fn main() {
    let x = 3.0;
    let y = 4.0;
    let (val, dx, dy) = df(&x, &y, 1.0);
    println!("val: {}, dx: {}, dy: {}", val, *dx, *dy);
}

Build command:

rustc -Z autodiff=Enable -C lto=fat test_references.rs

Differentiating a function that takes reference (&f32) or pointer (*mut f32/*const f32) parameters marked Active in reverse mode crashes the compiler during LLVM/Enzyme codegen, rather than producing a diagnostic. The activity model appears to expect Duplicated (a caller-provided shadow pointer) for reference/pointer-typed parameters rather than Active (gradient returned by value) — using Duplicated instead of Active for the same function compiles and runs correctly, producing the right gradient. So this looks like a case where an unsupported-but-plausible activity choice should produce a clean error rather than an ICE.

Meta

rustc --version --verbose:

rustc 1.99.0-nightly (daf2e5e18 2026-07-13)
host: x86_64-unknown-linux-gnu (WSL2 Ubuntu)

Built with rustup component add rustc-dev llvm-tools-preview enzyme on the same nightly, -Z autodiff=Enable -C lto=fat.

Error output

rustc: /checkout/src/tools/enzyme/enzyme/Enzyme/GradientUtils.cpp:6654: Value *GradientUtils::invertPointerM(Value *const, IRBuilder<> &, TypeTree): Assertion `0 && "cannot find deal with ptr that isnt arg"' failed.

oval=ptr %1 icv=0
rustc: /checkout/src/tools/enzyme/enzyme/Enzyme/GradientUtils.cpp:6654: Value *GradientUtils::invertPointerM(Value *const, IRBuilder<> &, TypeTree): Assertion `0 && "cannot find deal with ptr that isnt arg"' failed.

Suggested fix direction

Detect Active activity on reference/pointer-typed parameters in reverse mode before reaching the Enzyme/LLVM codegen boundary, and emit a diagnostic suggesting Duplicated instead (with a shadow-parameter example), rather than asserting inside Enzyme's GradientUtils::invertPointerM.

I wrote a small third-party static checker (ad-safety, MIR-level rustc_private lint) that catches this pattern at compile time with a suggestion to use Duplicated, in case the reproducer or approach is useful: https://github.com/sangmorg1-debug/ad-safety

Start with the supplied Rust reproducer and build command, then trace reverse-mode autodiff activity handling up to the Enzyme/LLVM codegen boundary. Compare the failing Active reference/pointer case with the working Duplicated case and inspect Enzyme's GradientUtils::invertPointerM context. Done means an unsupported Active parameter produces a clear diagnostic suggesting Duplicated instead of an ICE.