How to get a slice as an array in Rust?

You can easily do this with the TryInto trait (which was stabilized in Rust 1.34):

// Before Rust 2021, you need to import the trait:
// use std::convert::TryInto;

fn pop(barry: &[u8]) -> [u8; 3] {
    barry.try_into().expect("slice with incorrect length")
}

But even better: there is no need to clone/copy your elements! It is actually possible to get a &[u8; 3] from a &[u8]:

fn pop(barry: &[u8]) -> &[u8; 3] {
    barry.try_into().expect("slice with incorrect length")
}

As mentioned in the other answers, you probably don't want to panic if the length of barry is not 3, but instead handle this error gracefully.

This works thanks to these impls of the related trait TryFrom (before Rust 1.47, these only existed for arrays up to length 32):

impl<'_, T, const N: usize> TryFrom<&'_ [T]> for [T; N]
where
    T: Copy, 

impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N]

impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N]

Thanks to @malbarbo we can use this helper function:

use std::convert::AsMut;

fn clone_into_array<A, T>(slice: &[T]) -> A
where
    A: Default + AsMut<[T]>,
    T: Clone,
{
    let mut a = A::default();
    <A as AsMut<[T]>>::as_mut(&mut a).clone_from_slice(slice);
    a
}

to get a much neater syntax:

fn main() {
    let original = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    let e = Example {
        a: clone_into_array(&original[0..4]),
        b: clone_into_array(&original[4..10]),
    };

    println!("{:?}", e);
}

as long as T: Default + Clone.

If you know your type implements Copy, you can use this form:

use std::convert::AsMut;

fn copy_into_array<A, T>(slice: &[T]) -> A
where
    A: Default + AsMut<[T]>,
    T: Copy,
{
    let mut a = A::default();
    <A as AsMut<[T]>>::as_mut(&mut a).copy_from_slice(slice);
    a
}

Both variants will panic! if the target array and the passed-in slice do not have the same length.