How to convert a Rust integer type to its string representation without allocating a String? [duplicate] - io

Question

I want to do something like:
let x = 123;
let mut buf = [0 as u8; 20];
format_to!(x --> buf);
assert_eq!(&buf[..3], &b"123"[..]);
With #![no_std] and without any memory allocator.
As I understand, there is an implementation of core::fmt::Display for u64, and I want to use it if possible.
In other words, I want to do something like format!(...), but without a memory allocator. How can I do this?

Answer 1

Let's start with the standard version:
use std::io::Write;
fn main() {
let x = 123;
let mut buf = [0 as u8; 20];
write!(&mut buf[..], "{}", x).expect("Can't write");
assert_eq!(&buf[0..3], b"123");
}
If we then remove the standard library:
#![feature(lang_items)]
#![no_std]
use core::panic::PanicInfo;
#[lang = "eh_personality"]
extern "C" fn eh_personality() {}
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
loop {}
}
fn main() {
let x = 123;
let mut buf = [0 as u8; 20];
write!(&mut buf[..], "{}", x).expect("Can't write");
assert_eq!(&buf[0..3], b"123");
}
We get the error
error[E0599]: no method named `write_fmt` found for type `&mut [u8]` in the current scope
--> src/main.rs:17:5
|
17 | write!(&mut buf[..], "{}", x).expect("Can't write");
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
write_fmt is implemented in the core library by core::fmt::Write. If we implement it ourselves, we are able to pass that error:
#![feature(lang_items)]
#![feature(start)]
#![no_std]
use core::panic::PanicInfo;
#[lang = "eh_personality"]
extern "C" fn eh_personality() {}
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
loop {}
}
use core::fmt::{self, Write};
struct Wrapper<'a> {
buf: &'a mut [u8],
offset: usize,
}
impl<'a> Wrapper<'a> {
fn new(buf: &'a mut [u8]) -> Self {
Wrapper {
buf: buf,
offset: 0,
}
}
}
impl<'a> fmt::Write for Wrapper<'a> {
fn write_str(&mut self, s: &str) -> fmt::Result {
let bytes = s.as_bytes();
// Skip over already-copied data
let remainder = &mut self.buf[self.offset..];
// Check if there is space remaining (return error instead of panicking)
if remainder.len() < bytes.len() { return Err(core::fmt::Error); }
// Make the two slices the same length
let remainder = &mut remainder[..bytes.len()];
// Copy
remainder.copy_from_slice(bytes);
// Update offset to avoid overwriting
self.offset += bytes.len();
Ok(())
}
}
#[start]
fn start(_argc: isize, _argv: *const *const u8) -> isize {
let x = 123;
let mut buf = [0 as u8; 20];
write!(Wrapper::new(&mut buf), "{}", x).expect("Can't write");
assert_eq!(&buf[0..3], b"123");
0
}
Note that we are duplicating the behavior of io::Cursor into this wrapper. Normally, multiple writes to a &mut [u8] will overwrite each other. This is good for reusing allocation, but not useful when you have consecutive writes of the same data.
Then it's just a matter of writing a macro if you want to.
You should also be able to use a crate like arrayvec, which has written this code for you. This is untested:
#![feature(lang_items)]
#![feature(start)]
#![no_std]
use core::panic::PanicInfo;
#[lang = "eh_personality"]
extern "C" fn eh_personality() {}
#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
loop {}
}
use arrayvec::ArrayString; // 0.4.10
use core::fmt::Write;
#[start]
fn start(_argc: isize, _argv: *const *const u8) -> isize {
let x = 123;
let mut buf = ArrayString::<[u8; 20]>::new();
write!(&mut buf, "{}", x).expect("Can't write");
assert_eq!(&buf, "123");
0
}

Answer 2

With bare_io:
use bare_io::{Cursor, Write};
let mut buf = [0 as u8; 256];
let mut cur = Cursor::new(&mut buf[..]);
write!(&mut cur, "hello world, stack buf, {}\n\0", 234).expect("!write");
unsafe { puts(buf.as_ptr()) };
With bare_io, smallvec and alloc:
use smallvec::{Array, SmallVec};
struct WriteSmallVec<A: Array<Item = u8>>(SmallVec<A>);
impl<A: Array<Item = u8>> Write for WriteSmallVec<A> {
fn write(&mut self, buf: &[u8]) -> bare_io::Result<usize> {
self.0.extend_from_slice(buf);
Ok(buf.len())
}
fn flush(&mut self) -> bare_io::Result<()> {
Ok(())
}
}
let mut sv = WriteSmallVec(SmallVec::<[u8; 256]>::new());
write!(&mut sv, "hello world, SmallVec, prev len: {}\n\0", len).expect("!write");
unsafe { puts(sv.0.as_ptr()) };
With bare_io, patched inlinable_string and alloc:
use core::fmt::Write;
use inlinable_string::{InlinableString, StringExt};
let mut is = InlinableString::new();
write!(&mut is, "hello world, InlinableString, {}\n\0", 345).expect("!write");
unsafe { puts(is.as_ptr()) };
Tested in Linux kernel,
cargo build --release -Z build-std=core,alloc --target=x86_64-linux-kernel
Also did some benchmarks, comparing a simple array with SmallVec and InlinableString: https://gitlab.com/artemciy/lin-socks/-/blob/95d2bb96/bench/stack-string.rs
p.s. bare-io has been yanked though.