Hello, thank you for the contribution.
Could you elaborate on your usecase/motivation for this change?

This allows me to write a module which uses PIO while being generic over the IRQ it uses. See https://github.com/redolution/cubane/blob/e35d1be482b2e81d074fc699efcec44f840ab4f3/src/exi.rs, in particular these two lines:

• https://github.com/redolution/cubane/blob/e35d1be482b2e81d074fc699efcec44f840ab4f3/src/exi.rs#L166
• https://github.com/redolution/cubane/blob/e35d1be482b2e81d074fc699efcec44f840ab4f3/src/exi.rs#L180

I can't use a match block over the const generic here because the two IRQs have different type signatures, type-level selection is the only option as far as I can tell.
I've tried several approaches to make this work while retaining the const generic, but to no avail. I'm not sure it's actually possible without the ability to construct the object myself, so I fixed it at the root.

I admit the commit/PR message is a bit lacking, I was very frustrated about not getting this to work so I didn't really have the energy to write a better one.

I see, thank you :)

Only adding fn irq<const IRQ: usize>(&self) -> Interrupt<'_, P, IRQ> wouldn't be good as it'd allow to create an Interrupt with any value of IRQ where only 0 and 1 are valid.

Yup, hence I implemented the same construct as is already used for things like state machine and DMA channel IDs.

Actually I found a way to check for that that's transparent to the user:

pub fn irq<const IRQ: usize>(&self) -> Interrupt<'_, P, IRQ> {                                                                    
    struct IRQSanity<const X: usize>;                                                                                             
    impl<const IRQ: usize> IRQSanity<IRQ> {                                                                                       
        const CHECK: () = assert!(IRQ == 0 || IRQ == 1, "Irq Index must be either 0 or 1");                                       
    }                                                                                                                             

    let _test = IRQSanity::<IRQ>::CHECK;                                                                                          
    Interrupt {                                                                                                                   
        block: self.pio.deref(),                                                                                                  
        _phantom: core::marker::PhantomData,                                                                                      
    }                                                                                                                             
}

The error message looking like:

error[E0080]: evaluation of `rp2040_hal::pio::PIO::<P>::irq::IRQSanity::<3>::CHECK` failed
   --> /rp-hal/rp2040-hal/src/pio.rs:130:31
    |
130 |             const CHECK: () = assert!(IRQ == 0 || IRQ == 1, "Irq Index must be either 0 or 1");
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'Irq Index must be either 0 or 1', /rp-hal/rp2040-hal/src/pio.rs:130:31

Oh that's neat, although the error message is a bit weird.
Fwiw since it's a usize, IRQ <= 1 should be enough as the condition.

Updated with your suggestion, so this is no longer a breaking change.