How to assign to a slice range?

I have a mutable slice, and I want to replace its prefix if it equals a certain slice.

I tried this:

let len = prefix.len();

if slice.starts_with(prefix) {
    slice[.. len - 1] = subst;
}

Playground

However,

error[E0277]: the size for values of type `[{integer}]` cannot be known at compilation time
  --> src/main.rs:13:9
   |
13 |         slice[.. len - 1] = *subst;
   |         ^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time

(Let's forget about the other type error in the playground; in my code, I have proper slices instead of arrays.)

I understand this as saying "The slices might have different length, so you cannot do this".

However, how can I do this when I explicitly checked that the lengths are the same?

Solution

You should try split_at_mut(), something like this:

let len = prefix.len();
if slice.starts_with(prefix) {
    let (left, _right) = slice.split_at_mut(len);
    left.copy_from_slice(subst);
}

There is also explanation about this specific problem here: copy_from_slice()

Edit: As Jmb stated in a comment, split_at_mut() is not needed:

let len = prefix.len();
if slice.starts_with(prefix) {
    slice[..len].copy_from_slice(subst);
}