In Rust, sometime for-loop is faster than while-loop, but sometime it is not.
First of all, In Rust, the codes below
let a = [1, 2, 3, 4, 5];
let mut i = 0;
while i < 5 {
println!("the value is: {}", a[i]);
i += 1;
}
may be slower because the a[i] is actually a.at(i). And the at() method requires bounds-check, and in Rust bounds-check is one of the few cases which need a runtime check, so it is considered that it may slow down to the same level of most other programming languages with a garbage collector.
In this specific case, to achieve the same performance as C/C++, an unsafe will need to be introduced, so the a[i] will become unsafe { a.get_unchecked(i) }.
For loop will be faster only in cases of iter:
let a = [1, 2, 3, 4, 5];
for element in a {
println!("the value is: {element}");
}
However, this iter will not mutably access the slice, which is required in these cases.
Second, for-loop doesn't always reduce the "variable" and "calculation". In the codes
for i in 0..128 {
notes[i] = Some(Note::new(i));
}
the 0..128 is a Range struct with 128 usize values, similar to a slice. Rust will iter this Range and assign them one-by-one to the i. The i is variable, and the Range also contains many values that waiting to be iterated. This doesn't meet the intention of reducing variable and calculation.