Why can't I call .chars().count() on a const &str?

help
1 
fn main() {
    const s: &str = "⊗ symbol";
    const n: usize =  s.chars().count();

    println!("{}", n);
}

why i can't compile this. even though s is const. so the n of char is const and so count is const and know at compile time.

2

the chars and count methods are not const fns. See previous thread where there was some code to count chars (in a low level way) for consts.

The number of const fns in the standard library is gradually being expanded.

1 Like
3

const doesn't mean immutable, it means compile time, similar to C++'s constexpr. Normal let bindings are immutable by default, so you can just use those.

1 Like
4

A const fn can't yet have any loops or control flow. This is being worked on but for now anything that requires iteration can't be const.

5

For future reference: that is this thread from 4 days ago with about the same question.

1 Like