Allocators are the way that programs in Rust obtain memory from the system at runtime. Previously, Rust did not allow changing the way memory is obtained, which prevented some use cases. On some platforms, this meant using jemalloc, on others, the system allocator, but there was no way for users to control this key component. With 1.28.0, the #[global_allocator] attribute is now stable, which allows Rust programs to set their allocator to the system allocator, as well as define new allocators by implementing the GlobalAlloc trait.

The default allocator for Rust programs on some platforms is jemalloc. The standard library now provides a handle to the system allocator, which can be used to switch to the system allocator when desired, by declaring a static and marking it with the #[global_allocator] attribute.

use std::alloc::System;

#[global_allocator]
static GLOBAL: System = System;

fn main() {
    let mut v = Vec::new();
    // This will allocate memory using the system allocator.
    v.push(1);
}

However, sometimes you want to define a custom allocator for a given application domain. This is also relatively easy to do by implementing the GlobalAlloc trait. You can read more about how to do this in the documentation.