Unclear if block arguments must be required to implement `Encode`

[mwcampbell]

I noticed that block2 requires arguments to implement Encode, but it doesn't do anything with that trait on the arguments. Blocks have the same encoding regardless. So does this really need to be a requirement?

[madsmtm]

The ABI that block2 builds upon allows for legacy reasons that blocks don't specify their encoding, though ideally they should (search for signature in that document). Since we can't easily generate them in const/static contexts yet (requires some form of const generics, see #70), we make use of the legacy option and don't provide the block's encoding, yet!

More importantly though, the bound is there to prevent accidentally passing types that are not FFI-safe, such as Vec<T>, or ZSTs like (). So mostly there to help with ensuring soundness.

Do you have a use-case for passing a non-Encode type?

[madsmtm]

See also #442, in which an API using blocks is unusable because we don't yet set the encoding on blocks.

[madsmtm]

I'm going to keep the Encode requirement on blocks because it is a great boon to safety.

If you want to pass a specific non-Encode type, you can do something like the following:

use block2::ConcreteBlock;
use objc2::encode::{Encode, Encoding};

#[repr(transparent)]
struct Helper(Box<u32>); // Or whatever type you need to pass across

unsafe impl Encode for Helper {
    // Or whatever encoding would be most descriptive for your type
    const ENCODING: Encoding = Encoding::Pointer(Encoding::Void);
}

let block = ConcreteBlock::new(|helper: Helper| {
    let my_type = helper.0;
    // Do whatever you need to do with your type
});
let block = block.copy();

let my_type = Box::new(5);
unsafe { block.call((Helper(my_type),)) };