Roadmap for further development

[sourcebox]

I just want to put some notes here for further discussion, nothing set in stone.

• There's hopefully a 0.4 release of usb-device soon that removes the control-buffer-256 feature in favour of passing a dedicated control buffer to UsbDeviceBuilder::new(). That will solve the issues with large config descriptors.
• We do a new release right after usb-device 0.4 is out.
• A solution how to deal with SysEx messages will be evaluated, some ideas e,g. from @x37v already exist.
• Some examples how to interface with other crates need to be put up, especially how cables and SysEx can be processed.
• MIDI 2.0 is something that comes up on the horizon. Should we try to integrate it or keep everything at 1.0 and start a new crate for it? If it's decided to be integrated, what about all the types it requires?
• Since having more than one jack is already supported, it would be nice to have string descriptors to have them labelled on the host.
• Using a builder pattern could possibly be helpful to manage those things like string descriptors.

[sourcebox]

I updated the list because https://github.com/rust-embedded-community/usbd-midi/pull/11 has been closed.

If we recommend the user to use an external types crate, we maybe should decide what to do with the ones that are now in. Possible options:

• Keep all of them in forever for backwards compatibility.
• Keep some of them in forever for backwards compatibility. If so, which ones?
• Mark them as deprecated, remove them in a later release to give users time to switch.
• Remove them as soon as interfacing with other libraries is implemented.

[btrepp]

I was thinking about this, it could be a good use-case for features?. Eg say midi notes, the bare minimum needed is the u7 type. User facing functions could take the type Into or return something that implements From. IIRC u7 is the full domain of a midi note.

This means the library is cut-down, minimal implementation, but the use can opt into a 'midi-types' feature that implements the traits from midi types. Users that wish to use external crates from note management either a) convert to/from in their own code, or b) add in a Pull request for the implementation of a new feature that allows people to 'seemlessly' use the external crate type.

As far as the change, a breaking change where they are dropped seems okay. I imagine this crate isn't used in a heap of things. So the affected parties are small.

Also I guess on types, everything in data is kinda 'if this type existed' we wouldn't need our own. I think the intent was always to have a more usbd-midi-data crate, with just the domain, and no real procedural logic. It just wasn't worth the effort at the time. If the data types in this crate are made into their own crate, other crates can implement the conversions aswell.

[sourcebox]

Regarding the SysEx processing, maybe the suggestion from @x37v mentioned in the original PR could be further investigated.

My initial idea here is to add an event() method to UsbMidiEventPacket that returns something like:

pub enum UsbMidiEvent<'a> {
  Regular(&'a [u8]),
  SysexStart(&'a [u8]),
  SysexContinue(&'a [u8]),
  SysexEnd(&'a [u8])
}

Because the enum variants contain &[u8] slices to the raw bytes, this would require to store them inside the packet as [u8; 4] instead of cable_number and message as it's done now. So it would be a breaking change because both fields are public. They could then be replaced by cable_number() and message() methods that create them on-the-fly.

For writing SysEx to the host, maybe a solution also based on iterators is possible. Not sure about that yet.