failed Send inference with ndarray v0.17 and uom 0.38

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
rust
Domain
data

Research direction

Reproduce the minimal example in src/main.rs with Rust 1.98.1, uom 0.38, and ndarray 0.17.2, then compare the result with ndarray 0.16. Read the reported rust-lang/rust issue #162558 alongside ndarray's async and Send-related behavior; done means identifying whether the regression belongs to ndarray or rustc and documenting or fixing the confirmed cause.

Written by the indexing model from the issue text.

Description

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.

Dominant language
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