alloc.rs

#![feature(alloc)]
#![feature(heap_api)]

extern crate alloc;

use alloc::heap::{usable_size};
use std::mem::{align_of, size_of};
use std::str::{from_utf8};

fn print_vec_sizes<T>(v: &Vec<T>) {
    let elem_size = size_of::<T>();
    let elem_align = align_of::<T>();
    let cap = v.capacity();

    let actual = elem_size * cap;
    let usable = usable_size(actual, elem_align);
    let possible = usable / elem_size;

    println!("item size/align: {:#?}/{:#?} ; cap/mem: {:#?}/{:#?} ; usable cap/mem {:#?}/{:#?}",
             elem_size, elem_align, cap, actual, possible, usable);
}

fn print_string_sizes(s: &mut String) {
    unsafe {
        print_vec_sizes(s.as_mut_vec());
    }
}

fn main() {
    let mut v: Vec<u32> = Vec::new();

    v.push(1);
    print_vec_sizes(&v);

    v.reserve(1024);
    print_vec_sizes(&v);

    v.extend([1; 1025].iter());
    print_vec_sizes(&v);


    let mut s = String::new();

    s.extend("a".chars());
    print_string_sizes(&mut s);

    s.reserve(1024);
    print_string_sizes(&mut s);

    s.extend(from_utf8(&[97u8; 1025]).unwrap().chars());
    print_string_sizes(&mut s);
}

output

item size/align: 4/4 ; cap/mem: 4/16 ; usable cap/mem 4/16
item size/align: 4/4 ; cap/mem: 1025/4100 ; usable cap/mem 2048/8192
item size/align: 4/4 ; cap/mem: 2050/8200 ; usable cap/mem 3072/12288
item size/align: 1/1 ; cap/mem: 1/1 ; usable cap/mem 8/8
item size/align: 1/1 ; cap/mem: 1025/1025 ; usable cap/mem 1280/1280
item size/align: 1/1 ; cap/mem: 2050/2050 ; usable cap/mem 2560/2560