How do I concatenate two slices in Rust?

Ok, first of all, your initial sequence definition is wrong. You say you want 1, 2, 3, 8, 9, 10 as output, so it should look like:

    let v = (1u64 .. 11).collect::<Vec<_>>();

Next, you say you want to concatenate slices, so let's actually use slices:

    let head = &v[..3];
    let tail = &v[l-3..];

At this point, it's really down to which approach you like the most. You can turn those slices into iterators, chain, then collect...

    let v2: Vec<_> = head.iter().chain(tail.iter()).collect();

...or make a vec and extend it with the slices directly...

    let mut v3 = vec![];
    v3.extend_from_slice(head);
    v3.extend_from_slice(tail);

...or extend using more general iterators (which will become equivalent in the future with specialisation, but I don't believe it's as efficient just yet)...

    let mut v4: Vec<u64> = vec![];
    v4.extend(head);
    v4.extend(tail);

...or you could use Vec::with_capacity and push in a loop, or do the chained iterator thing, but using extend... but I have to stop at some point.

Full example code:

fn main() {
    let v = (1u64 .. 11).collect::<Vec<_>>();
    let l = v.len();

    let head = &v[..3];
    let tail = &v[l-3..];

    println!("head: {:?}", head);
    println!("tail: {:?}", tail);

    let v2: Vec<_> = head.iter().chain(tail.iter()).collect();
    println!("v2: {:?}", v2);

    let mut v3 = vec![];
    v3.extend_from_slice(head);
    v3.extend_from_slice(tail);
    println!("v3: {:?}", v3);

    // Explicit type to help inference.
    let mut v4: Vec<u64> = vec![];
    v4.extend(head);
    v4.extend(tail);
    println!("v4: {:?}", v4);
}

You should collect() the results of the take() and extend() them with the collect()ed results of skip():

let mut p1 = v.iter().take(3).collect::<Vec<_>>();
let p2 = v.iter().skip(l-3);

p1.extend(p2);

println!("{:?}", p1);

Edit: as Neikos said, you don't even need to collect the result of skip(), since extend() accepts arguments implementing IntoIterator (which Skip does, as it is an Iterator).

Edit 2: your numbers are a bit off, though; in order to get 1, 2, 3, 8, 9, 10 you should declare v as follows:

let v = (1u64 .. 11).collect::<Vec<_>>();

Since the Range is left-closed and right-open.

Just use .concat() on a slice of slices:

fn main() {
    let v = (0u64 .. 10).collect::<Vec<_>>();
    let l = v.len();
    let first_and_last = [&v[..3], &v[l - 3..]].concat();
    println!("{:?}", first_and_last);
    // The output is `[0, 1, 2, 7, 8, 9]`
}

This creates a new vector, and it works with arbitrary number of slices.

(Playground link)