How do I write a function that can take in an array of two possible sizes? [duplicate]

Question

I have two functions that do the same thing; the only difference is that they take in an array of different sizes. How can I refactor this code such that I don't get duplicate code?

fn foo(arr: &[[u8; 21]; 21]) -> [[u8; 21]; 21] {
    // some operations that need to iterate through arr
}

fn bar(arr: &[[u8; 25]; 25]) -> [[u8; 25]; 25] {
    // identical operations as foo
}

I tried using generics:

fn foo<T>(arr: &T) -> T {
  // some operations
}

This doesn't work because Rust has no idea that arr is an array, and I'm not able to call things like arr.len().

Another idea is to use a Vec instead, but I worry it comes at a cost of performance and memory.