Declare String using char constants in Rust

Question

I'm looking for ways to declare String constants in Rust by concatenating chars.

What I would like to achieve is something like this:

{ Pascal Code }
const ESC = #27;
const RESET_PRINTER = ESC + '@';

Here's where I ended for now after few hours of research:

const ESC: char  = '\u{001b}';

const RESET_PRINTER_ARR: [char; 2] = [ESC, '@'];
const RESET_PRINTER_STR: &str = "\u{001b}@"; // ESC + '@' ?

fn cde_str(cde: &[char], len: usize) -> String {
    let mut s = String::from("");
    for i in 0..len {
        s.push(cde[i]);
    }
    s
}

fn main() {
  let r1 = cde_str(&RESET_PRINTER_ARR, RESET_PRINTER_ARR.len()); 
  println!("{}", r1);
  
  let r2 = String::from(RESET_PRINTER_STR); 
  println!("{}", r2);
}

Playground link

Edit

Per @e-net4-stays-away-from-meta suggestion a String can easily be created from [char] by using String::from_iter() :

use std::iter::FromIterator;

const ESC: char = '\u{001b}';
const RESET_PRINTER_ARR: [char; 2] = [ESC, '@'];

fn main() {
  let r0 = String::from_iter(&RESET_PRINTER_ARR);
  println!("{}", r0);
}

The same principle of this question may apply: create an iterator of chars, then collect it. — E_net4
– E_net4, Commented Aug 12, 2020 at 13:55
I can't really find the question in this post. Could you please be more explicit? Would you like to evaluate the constants at compile time? — Sven Marnach
– Sven Marnach, Commented Aug 12, 2020 at 14:17
You can also iterate &[char] without passing its size. Just for c in cde {...} or even better cde.into_iter().collect() — Alexey S. Larionov
– Alexey S. Larionov, Commented Aug 12, 2020 at 14:19
@SvenMarnach it's exactly that. The goal was to reuse a const char to make a const String , so evaluate a char concatenation for a const. To solve this problem, i use an array and I "collect" it in a String when I need. — aniki02
– aniki02, Commented Aug 12, 2020 at 14:30
@aniki02 There is no such thing as a const String in Rust, since you can't allocate memory on the heap when creating a constant. Would a &str work as well for you? Would a lazily initialized static String fit the bill? — Sven Marnach
– Sven Marnach, Commented Aug 12, 2020 at 14:32

poolie · Accepted Answer · 2020-08-12 18:45:23Z

2

It looks like these are escape sequences to send to a printer, and that kind of printer probably does not naturally work in Unicode. I would make them a byte string.

To me the natural way to write it in Rust is just like this:

const RESET_PRINTER: &[u8] = b"\x1b@";

// other examples
const TERM_BOLD: &[u8] = b"\x1b[1m";
const TERM_UNDERLINE: &[u8] = b"\x1b[4m";

If you're going to generate very many of them, or they are very long, you can potentially write a macro that produces the byte string literals.

But, I would be inclined not to do the macro unless it's clearly needed: within a single crate, simple straight-line code that's a bit repetitive is often more maintainable than something that requires a reader to think about how the macro works.

answered Aug 12, 2020 at 18:45

poolie

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