Re: [PATCH RFC v2 1/4] rust: usb: add endpoint abstraction

[Daniel Almeida]

Hi Alexandru,

I think the typestate should go away in favor of something simpler:

#[derive(Clone, Copy)]
pub struct Endpoint(u8);  <------- private constructor

Drivers ask the Rust layer for their endpoints at probe time, through a
wrapper around the C helpers, e.g. usb_find_common_endpoints()
or similar

    pub fn find_bulk_endpoints(&self) -> Result<(Endpoint, Endpoint)>

Unlike your current &HostEndpoint, an Endpoint can be stored in the
driver's data, since it’s just a u8.

Also, perhaps the control endpoint should be its own type, i.e.:

/// # Invariants
///
/// The wrapped [`Endpoint`] is a control endpoint of the device.
#[derive(Clone, Copy)]
pub struct ControlEndpoint(Endpoint);

impl Deref for ControlEndpoint {
    type Target = Endpoint;

    fn deref(&self) -> &Endpoint {
        &self.0
    }
}

impl Interface<device::Bound> {
    pub fn control_endpoint(&self) -> ControlEndpoint {
        // INVARIANT: Endpoint 0 is always a control endpoint.
        ControlEndpoint(Endpoint(0))
    }

    pub fn control_message_send(&self, ep: ControlEndpoint, ...) -> Result
    pub fn control_message_receive(&self, ep: ControlEndpoint, ...) -> Result
}

By the way, the alternate setting accessors and making usb::Device
public don't have a user, not even a planned one, so I'd drop them.

-- Daniel