Rust: the trait `Sum<&T>` is not implemented for `T`

Question

I am trying to perform a sum over a vector, but I keep getting the error,

error[E0277]: the trait bound `T: Sum<&T>` is not satisfied
    --> src\equations\equation.rs:86:45
     |
86   |                 a[i][j][0] = a[i][j].iter().sum();
     |                                             ^^^ the trait `Sum<&T>` is not implemented for `T`
     |
note: required by a bound in `std::iter::Iterator::sum`
    --> C:\Users\Utilizador\.rustup\toolchains\stable-x86_64-pc-windows-gnu\lib/rustlib/src/rust\library\core\src\iter\traits\iterator.rs:2985:12
     |
2985 |         S: Sum<Self::Item>,
     |            ^^^^^^^^^^^^^^^ required by this bound in `std::iter::Iterator::sum`
help: consider further restricting this bound
     |
39   |         T: Float + AddAssign + DivAssign + Sum + std::iter::Sum<&T>

As mentioned in the error, T is a generic type that implements Float from the num-traits crate.

This is part of a larger Finite Volume code I am implementing from scratch, and I don't really know what can be actually relevant to put here, but this operation is performed inside a trait with the following method implementation:

fn compute_final_ap(&mut self) {
    let i0: usize = *self.get_i0();
    let iend: usize = *self.get_iend();
    let j0: usize = *self.get_j0();
    let jend: usize = *self.get_jend();

    let mut a: &mut Vec<Vec<Vec<T>>> = self.get_a_matrix();

    for i in i0..iend {
       for j in j0..jend {
           a[i][j][0] = - a[i][j][0];
           a[i][j][0] = a[i][j].iter().sum();
       }
    }
}

I know this is terrible code, but it's just to speed up the code implementation for now. I want to implement a sparse matrix for a later on.

I also tried to use into_iter().sum() but was not capable of using it since it complains with the error move occurs because value has type Vec<T>, which does not implement the Copy trait.

Hope this information is enough for you to help me with the issue. Thanks.

`Float` does not imply `Sum`. You can use the compiler's suggestion. — Chayim Friedman, Feb 16 '22 at 11:43
`iter().copied().sum()` (which requires adding `T: Copy` to your generic constraints if you don't already have it) — Jmb, Feb 16 '22 at 11:44
Thank you @ChayimFriedman. I have also tried to add Sum<&T> but then I get the error: `explicit lifetime name needed here` and don't know to do that exactly. — Sergio Cavaleiro Costa, Feb 16 '22 at 11:48
@Jmb, Actually I have not tried that, but I would prefer to avoid copying it to avoid time overhead. — Sergio Cavaleiro Costa, Feb 16 '22 at 11:49
@SergioCavaleiroCosta I guess copying will not be costlier, but you can use sum over references with HRTB: `T: Float + for<'a> Sum<&'a T>`. — Chayim Friedman, Feb 16 '22 at 11:55
@ChayimFriedman that seemed to work, can you elaborate on what is happening with that code or give me a link to read about it? I have never seen that! Thank you very much to both of you! — Sergio Cavaleiro Costa, Feb 16 '22 at 12:01
[Higher-Rank Trait Bounds (HRTBs)](https://doc.rust-lang.org/nomicon/hrtb.html), [How does for<> syntax differ from a regular lifetime bound?](https://stackoverflow.com/questions/35592750/how-does-for-syntax-differ-from-a-regular-lifetime-bound). — Chayim Friedman, Feb 16 '22 at 12:08
For `f32` and `f64`, `.copied()` is essentially free (since it amounts to a dereference, which you need to do in order to sum the values anyway). — Jmb, Feb 16 '22 at 12:51

Rust: the trait `Sum<&T>` is not implemented for `T`

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