Convert MS OS descriptor builder to a writer API

This brings it inline with the other embassy-usb descriptor APIs and allows it to integrate well with the Builder to allow class constructors to add MS OS descriptors.

Also adds a `usb_serial_winusb` example to demonstrate how to use the API.

embassy-usb/src/builder.rs

@@ -3,6 +3,8 @@ use heapless::Vec;
 use crate::control::ControlHandler;
 use crate::descriptor::{BosWriter, DescriptorWriter};
 use crate::driver::{Driver, Endpoint, EndpointType};
+#[cfg(feature = "msos-descriptor")]
+use crate::msos::{DeviceLevelDescriptor, FunctionLevelDescriptor, MsOsDescriptorWriter};
 use crate::types::*;
 use crate::{DeviceStateHandler, Interface, UsbDevice, MAX_INTERFACE_COUNT, STRING_INDEX_CUSTOM_START};
 
@@ -130,7 +132,9 @@ pub struct Builder<'d, D: Driver<'d>> {
     device_descriptor: DescriptorWriter<'d>,
     config_descriptor: DescriptorWriter<'d>,
     bos_descriptor: BosWriter<'d>,
-    msos_descriptor: Option<crate::msos::MsOsDescriptorSet<'d>>,
+
+    #[cfg(feature = "msos-descriptor")]
+    msos_descriptor: MsOsDescriptorWriter<'d>,
 }
 
 impl<'d, D: Driver<'d>> Builder<'d, D> {
@@ -145,6 +149,7 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
         device_descriptor_buf: &'d mut [u8],
         config_descriptor_buf: &'d mut [u8],
         bos_descriptor_buf: &'d mut [u8],
+        #[cfg(feature = "msos-descriptor")] msos_descriptor_buf: &'d mut [u8],
         control_buf: &'d mut [u8],
         handler: Option<&'d dyn DeviceStateHandler>,
     ) -> Self {
@@ -183,12 +188,17 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
             device_descriptor,
             config_descriptor,
             bos_descriptor,
-            msos_descriptor: None,
+
+            #[cfg(feature = "msos-descriptor")]
+            msos_descriptor: MsOsDescriptorWriter::new(msos_descriptor_buf),
         }
     }
 
     /// Creates the [`UsbDevice`] instance with the configuration in this builder.
     pub fn build(mut self) -> UsbDevice<'d, D> {
+        #[cfg(feature = "msos-descriptor")]
+        let msos_descriptor = self.msos_descriptor.build(&mut self.bos_descriptor);
+
         self.config_descriptor.end_configuration();
         self.bos_descriptor.end_bos();
 
@@ -201,7 +211,8 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
             self.bos_descriptor.writer.into_buf(),
             self.interfaces,
             self.control_buf,
-            self.msos_descriptor,
+            #[cfg(feature = "msos-descriptor")]
+            msos_descriptor,
         )
     }
 
@@ -218,14 +229,10 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
     ///
     /// If it's not set, no IAD descriptor is added.
     pub fn function(&mut self, class: u8, subclass: u8, protocol: u8) -> FunctionBuilder<'_, 'd, D> {
+        let first_interface = InterfaceNumber::new(self.interfaces.len() as u8);
         let iface_count_index = if self.config.composite_with_iads {
-            self.config_descriptor.iad(
-                InterfaceNumber::new(self.interfaces.len() as _),
-                0,
-                class,
-                subclass,
-                protocol,
-            );
+            self.config_descriptor
+                .iad(first_interface, 0, class, subclass, protocol);
 
             Some(self.config_descriptor.position() - 5)
         } else {
@@ -235,19 +242,31 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
         FunctionBuilder {
             builder: self,
             iface_count_index,
+
+            #[cfg(feature = "msos-descriptor")]
+            first_interface,
         }
     }
 
+    #[cfg(feature = "msos-descriptor")]
     /// Add an MS OS 2.0 Descriptor Set.
     ///
     /// Panics if called more than once.
-    pub fn msos_descriptor(&mut self, msos_descriptor: crate::msos::MsOsDescriptorSet<'d>) {
-        if self.msos_descriptor.is_some() {
-            panic!("msos_descriptor already set");
-        }
-        self.msos_descriptor
-            .insert(msos_descriptor)
-            .write_bos_capability(&mut self.bos_descriptor);
+    pub fn msos_descriptor(&mut self, windows_version: u32, vendor_code: u8) {
+        self.msos_descriptor.header(windows_version, vendor_code);
+    }
+
+    #[cfg(feature = "msos-descriptor")]
+    /// Add an MS OS 2.0 Device Level Feature Descriptor.
+    pub fn msos_feature<T: DeviceLevelDescriptor>(&mut self, desc: T) {
+        self.msos_descriptor.device_feature(desc);
+    }
+
+    #[cfg(feature = "msos-descriptor")]
+    /// Gets the underlying [`MsOsDescriptorWriter`] to allow adding subsets and features for classes that
+    /// do not add their own.
+    pub fn msos_writer(&mut self) -> &mut MsOsDescriptorWriter<'d> {
+        &mut self.msos_descriptor
     }
 }
 
@@ -259,6 +278,16 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
 pub struct FunctionBuilder<'a, 'd, D: Driver<'d>> {
     builder: &'a mut Builder<'d, D>,
     iface_count_index: Option<usize>,
+
+    #[cfg(feature = "msos-descriptor")]
+    first_interface: InterfaceNumber,
+}
+
+impl<'a, 'd, D: Driver<'d>> Drop for FunctionBuilder<'a, 'd, D> {
+    fn drop(&mut self) {
+        #[cfg(feature = "msos-descriptor")]
+        self.builder.msos_descriptor.end_function();
+    }
 }
 
 impl<'a, 'd, D: Driver<'d>> FunctionBuilder<'a, 'd, D> {
@@ -288,6 +317,21 @@ impl<'a, 'd, D: Driver<'d>> FunctionBuilder<'a, 'd, D> {
             next_alt_setting_number: 0,
         }
     }
+
+    #[cfg(feature = "msos-descriptor")]
+    /// Add an MS OS 2.0 Function Level Feature Descriptor.
+    pub fn msos_feature<T: FunctionLevelDescriptor>(&mut self, desc: T) {
+        if !self.builder.msos_descriptor.is_in_config_subset() {
+            self.builder.msos_descriptor.configuration(0);
+        }
+
+        if !self.builder.msos_descriptor.is_in_function_subset() {
+            self.builder.msos_descriptor.function(self.first_interface.0);
+        }
+
+        #[cfg(feature = "msos-descriptor")]
+        self.builder.msos_descriptor.function_feature(desc);
+    }
 }
 
 /// Interface builder.