String concatenations of non negative numbers in Rust.

Cargo.toml

[package]
name = "rustconcat"
version = "0.1.0"
authors = ["Dmitry Kravtsov <idkravitz@gmail.com>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[lib]
name="rustconcat"
crate-type=["cdylib"]

[profile.release]
incremental=false
codegen-units=1
lto=true

[dependencies]
itoa="0.4.5"

[dependencies.pyo3]
version="0.9.0-alpha.1"
features=["extension-module"]

lib.rs

#![feature(vec_into_raw_parts)]

use pyo3::prelude::*;
use pyo3::wrap_pyfunction;
use itoa;
use std::slice;


#[pyfunction]
fn rust_concat(n: usize) -> PyResult<String> {
    let mut result = String::new();
    for i in 0..n {
        result.push_str(&i.to_string());
    }
    Ok(result)
}

#[pyfunction]
fn rust_concat2(n: usize) -> PyResult<String> {
    let mut result = String::with_capacity(calculate_len(n));
    for i in 0..n {
        result.push_str(&i.to_string());
    }
    Ok(result)
}

fn calculate_len(n: usize) -> usize {
    let mut l = 0;
    let mut m = 1;
    let mut b = 1;
    while n > 10 * b {
        l += m * b;
        m += 1;
        b *= 10;
    }
    9 * l + m * (n - b) + 1
}

#[pyfunction]
fn concat_mutable(n: usize) -> PyResult<String> {
    //#![feature(vec_into_raw_parts)] required to be in the beggining of file
    // for following to work. Also may require rustc nightly profile in rustup
    let (ptr, _, cap) = String::with_capacity(calculate_len(n)).into_raw_parts();
    let buff = unsafe { slice::from_raw_parts_mut(ptr, cap) };
    let mut offset = 0;
    for i in 0..n {
        let n_write = itoa::write(&mut buff[offset..], i)?;
        offset += n_write;
    }

    Ok(unsafe { String::from_raw_parts(ptr, cap, cap) })
}

#[pymodule]
fn rustconcat(_py: Python, m: &PyModule) -> PyResult<()> {
    m.add_wrapped(wrap_pyfunction!(rust_concat))?;
    m.add_wrapped(wrap_pyfunction!(rust_concat2))?;
    m.add_wrapped(wrap_pyfunction!(concat_mutable))?;

    Ok(())
}