new realloc bug reproducer

[ZuseZ4]

Reproducer:

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

#[autodiff(sin_vec, Reverse, Duplicated, Const, Active)]
fn cos_vec(x: &mut Vec<f32>, y: usize) -> f32 {
    // uses enum internally and breaks
    for i in 0..y {
        x.push(i as f32);
    }
    let res = x.into_iter().collect::<Vec<&mut f32>>();

    *res[0]
}

fn main() {
    let mut x = vec![1.0, 1.0, 1.0];
    let mut d_x = vec![0.0; 3];

    sin_vec(&mut x, &mut d_x, 4, 1.0);

    dbg!(&d_x, &x);
}

RUSTFLAGS="-Z autodiff=LooseTypes,Print" cargo +enzyme run

mod.ll.txt

Demangled function names:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=2fdf75b03825f5f63f353ff1122f3e0f

Demangled function names are:

alloc::raw_vec::finish_grow::hce1ff89b620ba31c
alloc::raw_vec::RawVecInner<A>::grow_amortized::h6d11486032b72786
alloc::raw_vec::RawVecInner<A>::reserve::do_reserve_and_handle::hf69592d80ba560b8
alloc::vec::Vec<T,A>::reserve::hfbf4bb7fa5035d73
alloc::vec::Vec<T,A>::extend_trusted::h9fccf647fedf83c0

I'd say reserve is a reasonable function to provide a custom-derivative for @wsmoses , what would we need to register?
Augmented primal is just a reserve for the same additional on the shadow, and do nothing for the reverse?
For context, that's the definition:

pub struct Vec<T, A: Allocator = Global> {
    buf: RawVec<T, A>,
    len: usize,
}

    pub fn reserve(&mut self, additional: usize) {
        self.buf.reserve(self.len, additional);
    }

This example and some playing around in release mode indicates we should also have a custom rule for alloc::raw_vec::RawVecInner<A>::reserve::do_reserve_and_handle
https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=3d81e4ea9dcd7abd8acace3d9e40685c

[ZuseZ4]

A best guess on what to do here:

1. Gett all names for relevant vector reserve calls (here e.g. _ZN5alloc3vec16Vec$LT$T$C$A$GT$7reserve17hfbf4bb7fa5035d73E).
   2A) Call EnzymeRegisterCallHandler (?), using the name from 1)
   2B) Pass a nullptr as CustomFunctionReverse since it shouldn't need anything in the reverse pass
   2C) Pass a Builder and gutils as first and third argument to the CustomAugmentedFunctionForward
   2D) Pass callinst to the reserve() call from 1) as the second arg to CustomAugmentedFunctionForward
   2E) do something (??) with the last three arguments of CustomAugmentedFunctionForward