autodiff: higher-order derivative fails: "No forward mode derivative found"

Reduced from https://github.com/rust-lang/rust/issues/160470#issuecomment-5185373594 (a different bug)
I tried this code:

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

#[autodiff_reverse(d_eval, Const)]
fn eval(_out: &mut [f64]) {}

#[autodiff_forward(hvp_eval, Dual)]
fn grad(out: &mut [f64]) {
    d_eval(out);
}

fn main() {
    let mut out = [0.0];
    let mut out_t = [0.0];
    hvp_eval(&mut out, &mut out_t);
}

RUSTFLAGS="-Z autodiff=Enable,NoTT" cargo +enzyme build

I expected to see this happen: compiles

Instead, this happened:
On nightly:

error: src/main.rs:4:1: in function preprocess__RNvCs5ENfmmVW5Vl_3bar6d_eval void (ptr, i64): Enzyme: No forward mode derivative found for __enzyme_autodiff_RNvCs5ENfmmVW5Vl_3bar6d_eval
 at context:   %3 = call {} (...) @__enzyme_autodiff_RNvCs5ENfmmVW5Vl_3bar6d_eval(ptr @_RNvCs5ENfmmVW5Vl_3bar4eval, ptr @enzyme_const, ptr %0, ptr @enzyme_const, i64 %1) #174, !dbg !75

On head (self-build) we also get:

error: src/main.rs:4:1: in function fwddiffe_RNvCs8wtiwuS546I_3bar6d_eval void (ptr, ptr, i64): Enzyme: Cannot cast __enzyme_autodiff primal argument 3, found ptr %0, type ptr (simplified to ptr %0 )  - to arg 1, i64

error: src/main.rs:4:1: in function fwddiffe_RNvCs8wtiwuS546I_3bar6d_eval void (ptr, ptr, i64): Enzyme: Cannot cast __enzyme_autodiff primal argument 3, found ptr %0, type ptr (simplified to ptr %0 )  - to arg 1, i64

Start with the reproducer in src/main.rs, the nested #[autodiff_reverse] and #[autodiff_forward] entry points, and the cargo +enzyme build command. Investigate why the higher-order derivative lookup or argument handling produces the shown Enzyme errors; done means this example compiles on nightly and self-build configurations.