failed Send inference with ndarray v0.17 and uom 0.38

[pcmehlitz]

I'm running into a compile error using an Array2uom::si::f64::Length from inside an async function.
Please note this did work with ndarray v0.16.

• rust 1.98.1
• uom v0.38
• ndarray v0.17.2 (NOTE - v0.16 seems to work)

The problem can be isolated into the following minimal example, which I would expect to compile:

use uom::si::f64::Length;
use uom::ConstZero;
use ndarray::Array2;

fn assert_send<T: Send>(_t: T) {}

fn main() {
    assert_send(check());
}

async fn check() {
    let lengths: Array2<Length> = Array2::from_elem((2, 3), Length::ZERO);
    std::future::pending::<()>().await;
    println!("{}", lengths[(0, 0)].get::<uom::si::length::meter>());
}

This produces the following compiler error message:

error[E0308]: mismatched types
 --> src/main.rs:8:5
  |
8 |     assert_send(check());
  |     ^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected struct `Quantity<..., ..., f64>`
             found struct `Quantity<..., ..., f64>`
note: the lifetime requirement is introduced here
 --> src/main.rs:5:19
  |
5 | fn assert_send<T: Send>(_t: T) {}
  |                   ^^^^

Since it is not clear to me if the problem is on the ndarray or the rustc side I also reported it to rust-lang/rust (#162558).
I have a workaround wrapping the Array2 into a newtype with a at(&self,usize,usize)->Length fn and using that to obtain Length quantities inside the async fn. This comes from a bigger project though (https://github.com/ODIN-fire/odin-rs) where such workarounds are clearly suboptimal since it is highly async.

[pcmehlitz]

for the sake of completeness here is the mentioned workaround, which I consider to be just a stop gap:

use uom::si::f64::Length;
use uom::ConstZero;
use ndarray::Array2;

fn assert_send<T: Send>(_t: T) {}

fn main() {
    assert_send(check());
}

async fn check() {
    let lengths: Grid<Length> = Grid(Array2::from_elem((2, 3), Length::ZERO));
    std::future::pending::<()>().await;
    println!("{}", lengths.at(0, 0).get::<uom::si::length::meter>());
}

/// helper newtype that pulls monomorphization of element accessor out of the async code
struct Grid<T>(Array2<T>);

impl <T: Clone> Grid<T> {. // <<<< simple type bounds on T
    fn at (&self, row: usize, col: usize)->T { self.0[(row,col)].clone() }
}