let mut variable: Type;
loop {
variable = value;
}
or
loop {
let variable: Type = value;
}
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Side note, don’t fall into the trap of premature optimization. You’ll more than likely end up shooting yourself in the foot for something that usually doesn’t matter in most cases.
Functionally equivalent, the compiler will optimize lots for you. As the programmer, your focus should be on whether that variable is relevant to the loop’s scope exclusively or not (i.e. if you need to access the variable after the loop has modified it). If not, keep it in the loop so that its easier to read your code.