How to Use the alloc Crate Without std in Rust

You cannot use the alloc crate without std because alloc depends on std for its implementation in standard Rust builds. To use alloc in a no_std environment, you must enable the alloc feature in your Cargo.toml and ensure your crate is compiled with #![no_std] while providing a custom allocator if needed, but typically alloc is used alongside std or in embedded contexts with specific configurations. For a true no_std setup with alloc, add #![no_std] and #![no_main] to your entry point, then use extern crate alloc; and configure your Cargo.toml with default-features = false for dependencies that might pull in std.

#![no_std]
#![no_main]

extern crate alloc;

use alloc::vec::Vec;

#[panic_handler]
fn panic(_info: &core::panic::PanicInfo) -> ! {
    loop {}
}

#[no_mangle]
pub extern "C" fn _start() -> ! {
    let _v: Vec<u8> = Vec::new();
    loop {}
}

Note: This requires a custom panic handler and a linker script for embedded targets; alloc alone does not provide a full runtime without std or an embedded environment.

The alloc crate lets you use dynamic memory allocation (like vectors) without the full standard library, which is useful for embedded systems or low-level programming. It works by providing a minimal set of memory management tools that don't rely on the operating system. Think of it as a lightweight toolkit for managing memory when you can't use the full Rust standard library.