Unsafe fields

// This example shows how to work with a struct that has an unsafe field
// in Rust 1.98.1 (edition 2024). The `unsafe` field modifier is not yet
// available, so we simulate the intended behaviour by manually
// requiring an unsafe block to access the field.

struct Foo {
    // In the future this could be written as `unsafe data: *mut u8`
    // but currently we just document the intended use.
    data: *mut u8,
}

fn main() {
    // Allocate some memory to use as a raw pointer.
    let mut value = 42u8;
    let ptr: *mut u8 = &mut value as *mut u8;

    // Create an instance of Foo. The field is considered unsafe,
    // so we must use an unsafe block to set it.
    let mut foo = unsafe {
        Foo { data: ptr }
    };

    // Reading the unsafe field also requires an unsafe block.
    unsafe {
        // Dereference the raw pointer safely.
        *foo.data = 99;
    }

    // Verify that the value was updated.
    assert_eq!(value, 99);
    println!("Value updated via unsafe field access");
}