Borrowing mutable twice while using the same variable - rust

Question

Suppose that I'm going through a vector (not necessarily linearly, so I can't just use map) and I need to change an element when it satisfies some condition. I would think to use some variable to keep track of where I am, for example, something like a current variable
let mut v = vec![1, 2, 3, 4];
let mut current = &mut v[0];
and then check current to for some condition to see if it needs to be changed. However, when I do
current = &mut v[1];
It gives me the cannot borrow v as mutable more than once at a time.
I feel like this should be allowed, since I've only used one variable, and I can't access the old borrow any more.
Is there some way I can let rust know that I'm giving the first borrow back, so I'm not borrowing twice? Or have I been thinking about this wrong, and there is a different rust idiom I should use? I've solved this problem by using the indeces for the vector instead of a mutable reference, but I think this problem of "traversing using a current and then changing it" goes beyond just vectors. What if the data structure I had didn't have indeces?

Answer 1

The mutable reference exists until the variable goes out of scope, so you can have several mutable borrows in sequence by putting blocks around each one:
fn do_stuff(n: &mut usize) {
*n += 1;
}
fn main() {
let mut v = vec![1, 2, 3, 4];
{
let current = &mut v[1];
do_stuff(current);
}
{
let current = &mut v[0];
do_stuff(current);
}
println!("{:?}", v);
}
This is unnecessary with non-lexical lifetimes, which are currently only available on the nightly version:
#![feature(nll)]
fn do_stuff(n: &mut usize) {
*n += 1;
}
fn main() {
let mut v = vec![1, 2, 3, 4];
let mut current = &mut v[1];
do_stuff(current);
current = &mut v[0];
do_stuff(current);
println!("{:?}", v);
}

Answer 2

If you want to access more than one, you have to create a scope.
fn main() {
let mut vec = vec![2, 3, 5];
{
let mut current1 = vec.get_mut(1);
println!("{:?}", current1);
}
{
let mut current2 = vec.get_mut(2);
println!("{:?}", current2);
}
for i in 0..3 {
let mut current = vec.get_mut(i);
println!("{:?}", current);
}
}