How to concatenate two ranges into a Vec

How can I make a Vec containing [0,1,2,3,4,3,2,1,0] using ranges?
I tried this:

let mut a = vec![0..len];
let  b = vec![len..=0];
a.extend(&b);

But it raises an error:

type mismatch resolving `<&Vec<RangeInclusive<usize>> as IntoIterator>::Item == std::ops::Range<usize>`

>Solution :

There’s no direct syntax that I know of to create a Vec directly from a range. A range is an first-class object and implements Iterator, so you can collect a range into a Vec.

Ranges must also be from low to high. To get a reversed range create a regular forward range and call rev to create an iterator that iterates in reverse.

let mut a = (0..len).collect::<Vec<_>>();
let b = (0..=len).rev();
a.extend(&b);

You’ll notice that collect requires ::<>, the so-called turbofish operator. That’s because it’s a generic method that can convert an iterator into any type that implements FromIterator. There are many collection types that implement FromIterator, so we must tell it we want a Vec in particular.

Declaring a‘s type would have also worked:

let mut a: Vec<_> = (0..len).collect();
let b = (0..=len).rev();
a.extend(&b);

It’s also possible to construct the vector in place by chaining the two iterators together and then collecting the result. This way a doesn’t need to be mut.

let a = (0..len).chain((0..=len).rev())
    .collect::<Vec<_>>();