Vector Implementation

In the cortex-m-rt docs it recommends PACs create a Vector type like so:

pub union Vector {
    handler: unsafe extern "C" fn(),
    reserved: usize,
}

and this is exactly what PACs do. I'm curious why it is like this? The reserved variant being typed as usize when it should always be 0 and constructed like:

Vector { reserved: 0 }

is very odd to me.

Why not define Vector like this:

pub struct Vector(#[allow(unused)] *const ());

impl Vector {
    /// Create a vector with the provided function pointer.
    pub const fn handler(f: unsafe extern "C" fn()) -> Self {
        Self(f as _)
    }

    /// Create a vector that is reserved.
    pub const fn reserved() -> Self {
        Self(core::ptr::null())
    }
}

and use it like this:

pub static __INTERRUPTS: [Vector; 2] = [
    Vector::handler(Foo),
    Vector::reserved(),
];

and why is this type not provided by cortex-m-rt so PACs don't have to redefine it every time?

I imagine there is some low-level issue with representing the function pointer as a unit pointer that I don't know of.

I am currently using my proposed Vector implementation and it does work, I'm just not sure if it is correct.

Thanks!

Read the cortex-m-rt documentation and the PAC Vector definitions referenced in the issue first. Check whether the proposed pointer representation preserves the required vector-table behavior and whether cortex-m-rt should expose a shared type; done means a documented decision on correctness and API ownership.