rust logging allocator

main.rs

// https://doc.rust-lang.org/std/alloc/struct.System.html

use std::alloc::{GlobalAlloc, Layout, System};
use std::io::Write;

static HEX: [[u8; 1]; 16] = [
    [0x30],
    [0x31],
    [0x32],
    [0x33],
    [0x34],
    [0x35],
    [0x36],
    [0x37],
    [0x38],
    [0x39],
    [0x41],
    [0x42],
    [0x43],
    [0x44],
    [0x45],
    [0x46],
];

fn log(msg: &str, ptr: usize) {
    let mut stderr = std::io::stderr();
    stderr.write(msg.as_bytes()).unwrap();
    stderr.write(": 0x".as_bytes()).unwrap();
    stderr.write(&HEX[ptr >> 60 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 56 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 52 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 48 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 44 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 40 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 36 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 32 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 28 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 24 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 20 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 16 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 12 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 8 & 0xF]).unwrap();
    stderr.write(&HEX[ptr >> 4 & 0xF]).unwrap();
    stderr.write(&HEX[ptr & 0xF]).unwrap();
    stderr.write("\n".as_bytes()).unwrap();
}

struct LoggingAllocator;

unsafe impl GlobalAlloc for LoggingAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        let ptr = unsafe { System.alloc(layout) };
        log("  ALLOC", ptr as usize);
        ptr
    }

    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        unsafe { System.dealloc(ptr, layout) };
        log("DEALLOC", ptr as usize);
    }
}

#[global_allocator]
static ALLOCATOR: LoggingAllocator = LoggingAllocator;

fn f(p: *const u8) -> *const u8 {
    println!("f(): {:p}", p);
    p
}

fn g(p: *const u8) {
    println!("g(): {:p}", p);
}

fn main() {
    println!("main(): start");
    g(f(String::from("hello").as_ptr()));
    println!("main(): end");
}