Change selector to match if selector is a mutable reference
I want to change the enum variant based on some properties of the current enumeration variant in Iterator::next
. I have two attempts, none of which compile:
enum Test {
A(Vec<usize>),
B,
}
impl<'a> Iterator for Test {
type Item = usize;
fn next(&mut self) -> Option<Self::Item> {
// attempt 1
if let Test::A(ref a) = *self {
if a.len() == 0 {
*self = Test::B; // doesn't work because a is borrowed
};
}
// attempt 2
*self = match *self {
Test::A(ref a) if a.len() == 0 => Test::B,
_ => *self, // cannot move out of borrowed context
};
None
}
}
fn main() {}
My second attempt works if I don't work with links in the selector:
let mut a = Test::A(vec![]);
a = match a {
Test::A(ref a) if a.len() == 0 => Test::B,
_ => a,
};
This question is related to Is there a way to use match () in rust when the selector changes? but the suggested solution is not general: it only works if the same function is executed in both branches.
What is Rustias' way to achieve my goal?
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Since the condition is not very readable when inserted into a if let
/ block match
, I would just use a helper function to check:
impl Test {
fn is_empty_a(&self) -> bool {
if let Test::A(ref a) = *self {
a.len() == 0
} else {
false
}
}
}
And then there should be no problem with borrowing:
impl<'a> Iterator for Test {
type Item = usize;
fn next(&mut self) -> Option<Self::Item> {
if self.is_empty_a() {
*self = Test::B;
}
None
}
}
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