Mutate vector within filter - rust

Question

So, I have the following code successfully performing filter in vector:
let mut v1 : Vec<i32> = vec!(1,2,3);
let v2 : Vec<&mut i32> = v1.iter_mut().filter(|x| {**x == 2}).collect();
println!("{:?}", v2);
Since the type signature of the predicate in the filter function is
FnMut(&Self::Item) -> bool, I was assuming that that mutation inside
the closure will work:
let mut v1 : Vec<i32> = vec!(1,2,3);
let v2 : Vec<&mut i32> = v1.iter_mut().filter(|x| {**x = 3; **x == 2}).collect();
println!("{:?}", v2);
But the above code results in a compile error. How to fix that ? Note
that I'm playing with rust to get a better understanding, so the abpve
example doesn't make sense (usually, nobody will try to mutate
things inside filter).

Answer 1

You are confusing two concepts: FnMut means that a function can change its captured variables, like:
fn main() {
let v1 = vec![1, 2, 3];
let mut i = 0usize;
let v2: Vec<_> = v1
.into_iter()
.filter(|x| {
i = i + 1;
*x == 2
})
.collect();
println!("We iterate {} times and produce {:?}", i, v2);
}
This doesn't mean that every parameter of a function will be mutable.
In your code, filter() takes a &Self::Item, which is very different from the map() one that takes Self::Item. Because the real type will translate to Map<Item=&mut i32> and Filter<Item=&&mut i32>. Rust forbids you from mutating a reference if it's behind a non mutable reference:
fn test(a: &&mut i32) {
**a = 5;
}
error[E0594]: cannot assign to `**a` which is behind a `&` reference
This is because Rust follows the the-rules-of-references:
At any given time, you can have either one mutable reference or any number of immutable references.
References must always be valid.
This means you can have more than one &&mut but only one &mut &mut. If Rust didn't stop you, you could mutate a &&mut and that would poison any other &&mut.
Unfortunately the full error description of E0594 is still not available, see #61137.
Note: Avoid side effects when you use the iterator API, I think it's OK to mutate your FnMut state but not the item, you should do this in a for loop, like:
fn main() {
let mut v1 = vec![1, 2, 3];
for x in v1.iter_mut().filter(|x| **x == 2) {
*x = 1;
}
println!("{:?}", v1);
}