6 files changed, 453 insertions, 265 deletions

diff --git a/embassy-usb/Cargo.toml b/embassy-usb/Cargo.toml
index 31d1f4cae..54a8f27c7 100644
--- a/embassy-usb/Cargo.toml
+++ b/embassy-usb/Cargo.toml
@@ -13,6 +13,7 @@ target = "thumbv7em-none-eabi"
 [features]
 defmt = ["dep:defmt", "embassy-usb-driver/defmt"]
 usbd-hid = ["dep:usbd-hid", "dep:ssmarshal"]
+msos-descriptor = ["dep:widestring"]
 default = ["usbd-hid"]
 
 [dependencies]
@@ -24,7 +25,7 @@ embassy-net-driver-channel = { version = "0.1.0", path = "../embassy-net-driver-
 defmt = { version = "0.3", optional = true }
 log = { version = "0.4.14", optional = true }
 heapless = "0.7.10"
-widestring = { version = "1.0.2", default-features = false }
+widestring = { version = "1.0.2", default-features = false, optional = true }
 
 # for HID
 usbd-hid = { version = "0.6.0", optional = true }
diff --git a/embassy-usb/src/builder.rs b/embassy-usb/src/builder.rs
index 2c42fd64e..d1cbf674b 100644
--- a/embassy-usb/src/builder.rs
+++ b/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.
diff --git a/embassy-usb/src/lib.rs b/embassy-usb/src/lib.rs
index 948b8d523..aec18524b 100644
--- a/embassy-usb/src/lib.rs
+++ b/embassy-usb/src/lib.rs
@@ -136,8 +136,8 @@ struct Inner<'d, D: Driver<'d>> {
     set_address_pending: bool,
 
     interfaces: Vec<Interface<'d>, MAX_INTERFACE_COUNT>,
-
-    msos_descriptor: Option<crate::msos::MsOsDescriptorSet<'d>>,
+    #[cfg(feature = "msos-descriptor")]
+    msos_descriptor: crate::msos::MsOsDescriptorSet<'d>,
 }
 
 impl<'d, D: Driver<'d>> UsbDevice<'d, D> {
@@ -150,7 +150,7 @@ impl<'d, D: Driver<'d>> UsbDevice<'d, D> {
         bos_descriptor: &'d [u8],
         interfaces: Vec<Interface<'d>, MAX_INTERFACE_COUNT>,
         control_buf: &'d mut [u8],
-        msos_descriptor: Option<crate::msos::MsOsDescriptorSet<'d>>,
+        #[cfg(feature = "msos-descriptor")] msos_descriptor: crate::msos::MsOsDescriptorSet<'d>,
     ) -> UsbDevice<'d, D> {
         // Start the USB bus.
         // This prevent further allocation by consuming the driver.
@@ -174,6 +174,7 @@ impl<'d, D: Driver<'d>> UsbDevice<'d, D> {
                 address: 0,
                 set_address_pending: false,
                 interfaces,
+                #[cfg(feature = "msos-descriptor")]
                 msos_descriptor,
             },
         }
@@ -608,11 +609,12 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                     None => InResponse::Rejected,
                 }
             }
+            #[cfg(feature = "msos-descriptor")]
             (RequestType::Vendor, Recipient::Device) => {
-                if let Some(msos) = &self.msos_descriptor {
-                    if req.request == msos.vendor_code() && req.index == 7 {
+                if !self.msos_descriptor.is_empty() {
+                    if req.request == self.msos_descriptor.vendor_code() && req.index == 7 {
                         // Index 7 retrieves the MS OS Descriptor Set
-                        InResponse::Accepted(msos.descriptor())
+                        InResponse::Accepted(self.msos_descriptor.descriptor())
                     } else {
                         InResponse::Rejected
                     }
diff --git a/embassy-usb/src/msos.rs b/embassy-usb/src/msos.rs
index df8461258..360f80d91 100644
--- a/embassy-usb/src/msos.rs
+++ b/embassy-usb/src/msos.rs
@@ -1,3 +1,5 @@
+#![cfg(feature = "msos-descriptor")]
+
 //! Microsoft OS Descriptors
 //!
 //! <https://docs.microsoft.com/en-us/windows-hardware/drivers/usbcon/microsoft-os-2-0-descriptors-specification>
@@ -5,10 +7,9 @@
 use core::mem::size_of;
 use core::ops::Range;
 
-pub use widestring::{u16cstr, U16CStr};
+pub use widestring::{u16cstr, u16str, U16CStr, U16Str};
 
-use crate::descriptor::{capability_type, BosWriter};
-use crate::types::InterfaceNumber;
+use super::{capability_type, BosWriter};
 
 fn write_u16<T: Into<u16>>(buf: &mut [u8], range: Range<usize>, data: T) {
     (&mut buf[range]).copy_from_slice(data.into().to_le_bytes().as_slice())
@@ -17,13 +18,12 @@ fn write_u16<T: Into<u16>>(buf: &mut [u8], range: Range<usize>, data: T) {
 /// A serialized Microsoft OS 2.0 Descriptor set.
 ///
 /// Create with [`DeviceDescriptorSetBuilder`].
-pub struct MsOsDescriptorSet<'a> {
-    descriptor: &'a [u8],
-    windows_version: u32,
+pub struct MsOsDescriptorSet<'d> {
+    descriptor: &'d [u8],
     vendor_code: u8,
 }
 
-impl<'a> MsOsDescriptorSet<'a> {
+impl<'d> MsOsDescriptorSet<'d> {
     pub fn descriptor(&self) -> &[u8] {
         self.descriptor
     }
@@ -32,9 +32,150 @@ impl<'a> MsOsDescriptorSet<'a> {
         self.vendor_code
     }
 
-    pub fn write_bos_capability(&self, bos: &mut BosWriter) {
-        let windows_version = self.windows_version.to_le_bytes();
-        let len = self.descriptor.len().to_le_bytes();
+    pub fn is_empty(&self) -> bool {
+        self.descriptor.is_empty()
+    }
+}
+
+/// Writes a Microsoft OS 2.0 Descriptor set into a buffer.
+pub struct MsOsDescriptorWriter<'d> {
+    pub buf: &'d mut [u8],
+
+    position: usize,
+    config_mark: Option<usize>,
+    function_mark: Option<usize>,
+    vendor_code: u8,
+}
+
+impl<'d> MsOsDescriptorWriter<'d> {
+    pub(crate) fn new(buf: &'d mut [u8]) -> Self {
+        MsOsDescriptorWriter {
+            buf,
+            position: 0,
+            config_mark: None,
+            function_mark: None,
+            vendor_code: 0,
+        }
+    }
+
+    pub(crate) fn build(mut self, bos: &mut BosWriter) -> MsOsDescriptorSet<'d> {
+        self.end();
+
+        if self.is_empty() {
+            MsOsDescriptorSet {
+                descriptor: &[],
+                vendor_code: 0,
+            }
+        } else {
+            self.write_bos(bos);
+            MsOsDescriptorSet {
+                descriptor: &self.buf[..self.position],
+                vendor_code: self.vendor_code,
+            }
+        }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.position == 0
+    }
+
+    pub fn is_in_config_subset(&self) -> bool {
+        self.config_mark.is_some()
+    }
+
+    pub fn is_in_function_subset(&self) -> bool {
+        self.function_mark.is_some()
+    }
+
+    /// Write the MS OS descriptor set header.
+    ///
+    /// - `windows_version` is an NTDDI version constant that describes a windows version. See the [`windows_version`]
+    /// module.
+    /// - `vendor_code` is the vendor request code used to read the MS OS descriptor set.
+    pub fn header(&mut self, windows_version: u32, vendor_code: u8) {
+        assert!(self.is_empty(), "You can only call MsOsDescriptorWriter::header once");
+        self.write(DescriptorSetHeader::new(windows_version));
+        self.vendor_code = vendor_code;
+    }
+
+    /// Add a device level feature descriptor.
+    ///
+    /// Note that some feature descriptors may only be used at the device level in non-composite devices.
+    /// Those features must be written before the first call to [`Self::configuration`].
+    pub fn device_feature<T: DeviceLevelDescriptor>(&mut self, desc: T) {
+        assert!(
+            !self.is_empty(),
+            "device features may only be added after the header is written"
+        );
+        assert!(
+            self.config_mark.is_none(),
+            "device features must be added before the first configuration subset"
+        );
+        self.write(desc);
+    }
+
+    /// Add a configuration subset.
+    pub fn configuration(&mut self, config: u8) {
+        assert!(
+            !self.is_empty(),
+            "MsOsDescriptorWriter: configuration must be called after header"
+        );
+        Self::end_subset::<ConfigurationSubsetHeader>(self.buf, self.position, &mut self.config_mark);
+        self.config_mark = Some(self.position);
+        self.write(ConfigurationSubsetHeader::new(config));
+    }
+
+    /// Add a function subset.
+    pub fn function(&mut self, first_interface: u8) {
+        assert!(
+            self.config_mark.is_some(),
+            "MsOsDescriptorWriter: function subset requires a configuration subset"
+        );
+        self.end_function();
+        self.function_mark = Some(self.position);
+        self.write(FunctionSubsetHeader::new(first_interface));
+    }
+
+    /// Add a function level feature descriptor.
+    ///
+    /// Note that some features may only be used at the function level. Those features must be written after a call
+    /// to [`Self::function`].
+    pub fn function_feature<T: FunctionLevelDescriptor>(&mut self, desc: T) {
+        assert!(
+            self.function_mark.is_some(),
+            "function features may only be added to a function subset"
+        );
+        self.write(desc);
+    }
+
+    pub fn end_function(&mut self) {
+        Self::end_subset::<FunctionSubsetHeader>(self.buf, self.position, &mut self.function_mark);
+    }
+
+    fn write<T: Descriptor>(&mut self, desc: T) {
+        desc.write_to(&mut self.buf[self.position..]);
+        self.position += desc.size();
+    }
+
+    fn end_subset<T: DescriptorSet>(buf: &mut [u8], position: usize, mark: &mut Option<usize>) {
+        if let Some(mark) = mark.take() {
+            let len = position - mark;
+            let p = mark + T::LENGTH_OFFSET;
+            buf[p..(p + 2)].copy_from_slice(&(len as u16).to_le_bytes());
+        }
+    }
+
+    fn end(&mut self) {
+        if self.position > 0 {
+            Self::end_subset::<FunctionSubsetHeader>(self.buf, self.position, &mut self.function_mark);
+            Self::end_subset::<ConfigurationSubsetHeader>(self.buf, self.position, &mut self.config_mark);
+            Self::end_subset::<DescriptorSetHeader>(self.buf, self.position, &mut Some(0));
+        }
+    }
+
+    fn write_bos(&mut self, bos: &mut BosWriter) {
+        let windows_version = &self.buf[4..8];
+        let len = (self.position as u16).to_le_bytes();
         bos.capability(
             capability_type::PLATFORM,
             &[
@@ -67,30 +208,7 @@ impl<'a> MsOsDescriptorSet<'a> {
                 self.vendor_code,
                 0x0, // Device does not support alternate enumeration
             ],
-        )
-    }
-}
-
-/// A helper struct to implement the different descriptor set builders.
-struct DescriptorSetBuilder<'a> {
-    used: usize,
-    buf: &'a mut [u8],
-}
-
-impl<'a> DescriptorSetBuilder<'a> {
-    pub fn descriptor<T>(&mut self, desc: T)
-    where
-        T: Descriptor + 'a,
-    {
-        let size = desc.size();
-        let start = self.used;
-        let end = start + size;
-        desc.write_to(&mut self.buf[start..end]);
-        self.used += size;
-    }
-
-    pub fn remaining(&mut self) -> &mut [u8] {
-        &mut self.buf[self.used..]
+        );
     }
 }
 
@@ -120,182 +238,27 @@ pub mod windows_version {
     pub const WIN10: u32 = 0x0A000000;
 }
 
-/// Helps build a Microsoft OS 2.0 Descriptor set.
-///
-/// # Example
-/// ```rust
-/// # use embassy_usb::types::InterfaceNumber;
-/// # use embassy_usb::msos::*;
-/// # let cdc_interface = unsafe { core::mem::transmute::<u8, InterfaceNumber>(0) };
-/// # let dfu_interface = unsafe { core::mem::transmute::<u8, InterfaceNumber>(1) };
-/// let mut buf = [0u8; 256];
-/// let mut builder = DeviceDescriptorSetBuilder::new(&mut buf[..], windows_version::WIN8_1);
-/// builder.feature(MinimumRecoveryTimeDescriptor::new(5, 10));
-/// builder.feature(ModelIdDescriptor::new(0xdeadbeef1234u128));
-/// builder.configuration(1, |conf| {
-///     conf.function(cdc_interface, |func| {
-///         func.winusb_device();
-///         func.feature(VendorRevisionDescriptor::new(1));
-///     });
-///     conf.function(dfu_interface, |func| {
-///         func.winusb_device();
-///         func.feature(VendorRevisionDescriptor::new(1));
-///     });
-/// });
-/// ```
-pub struct DeviceDescriptorSetBuilder<'a> {
-    builder: DescriptorSetBuilder<'a>,
-    windows_version: u32,
-    vendor_code: u8,
-}
+mod sealed {
+    use core::mem::size_of;
 
-impl<'a> DeviceDescriptorSetBuilder<'a> {
-    /// Create a device descriptor set builder.
-    ///
-    /// - `windows_version` is an NTDDI version constant that describes a windows version. See the [`windows_version`]
-    /// module.
-    /// - `vendor_code` is the vendor request code used to read the MS OS descriptor set.
-    pub fn new<'b: 'a>(buf: &'b mut [u8], windows_version: u32, vendor_code: u8) -> Self {
-        let mut builder = DescriptorSetBuilder { used: 0, buf };
-        builder.descriptor(DescriptorSetHeader {
-            wLength: (size_of::<DescriptorSetHeader>() as u16).to_le(),
-            wDescriptorType: (DescriptorSetHeader::TYPE as u16).to_le(),
-            dwWindowsVersion: windows_version.to_le(),
-            wTotalLength: 0,
-        });
-        Self {
-            builder,
-            windows_version,
-            vendor_code,
-        }
-    }
+    /// A trait for descriptors
+    pub trait Descriptor: Sized {
+        const TYPE: super::DescriptorType;
 
-    /// Add a device-level feature descriptor.
-    ///
-    /// Note that some feature descriptors may only be used at the device level in non-composite devices.
-    pub fn feature<T>(&mut self, desc: T)
-    where
-        T: Descriptor + DeviceLevelDescriptor + 'a,
-    {
-        self.builder.descriptor(desc)
-    }
-
-    /// Add a configuration subset.
-    pub fn configuration(&mut self, configuration: u8, build_conf: impl FnOnce(&mut ConfigurationSubsetBuilder<'_>)) {
-        let mut cb = ConfigurationSubsetBuilder::new(self.builder.remaining(), configuration);
-        build_conf(&mut cb);
-        self.builder.used += cb.finalize();
-    }
-
-    /// Finishes writing the data.
-    pub fn finalize(self) -> MsOsDescriptorSet<'a> {
-        let used = self.builder.used;
-        let buf = self.builder.buf;
-        // Update length in header with final length
-        let total_len = &mut buf[8..10];
-        total_len.copy_from_slice((used as u16).to_le_bytes().as_slice());
-
-        MsOsDescriptorSet {
-            descriptor: &buf[..used],
-            windows_version: self.windows_version,
-            vendor_code: self.vendor_code,
+        /// The size of the descriptor's header.
+        fn size(&self) -> usize {
+            size_of::<Self>()
         }
-    }
-}
-
-pub struct ConfigurationSubsetBuilder<'a> {
-    builder: DescriptorSetBuilder<'a>,
-}
-
-impl<'a> ConfigurationSubsetBuilder<'a> {
-    pub fn new<'b: 'a>(buf: &'b mut [u8], configuration: u8) -> Self {
-        let mut builder = DescriptorSetBuilder { used: 0, buf };
-        builder.descriptor(ConfigurationSubsetHeader {
-            wLength: (size_of::<ConfigurationSubsetHeader>() as u16).to_le(),
-            wDescriptorType: (ConfigurationSubsetHeader::TYPE as u16).to_le(),
-            bConfigurationValue: configuration,
-            bReserved: 0,
-            wTotalLength: 0,
-        });
-        Self { builder }
-    }
-
-    /// Add a function subset.
-    pub fn function(&mut self, interface: InterfaceNumber, build_func: impl FnOnce(&mut FunctionSubsetBuilder<'_>)) {
-        let mut fb = FunctionSubsetBuilder::new(self.builder.remaining(), interface);
-        build_func(&mut fb);
-        self.builder.used += fb.finalize();
-    }
-
-    /// Finishes writing the data. Returns the total number of bytes used by the descriptor set.
-    pub fn finalize(self) -> usize {
-        let used = self.builder.used;
-        let buf = self.builder.buf;
-        // Update length in header with final length
-        let total_len = &mut buf[6..8];
-        total_len.copy_from_slice((used as u16).to_le_bytes().as_slice());
-        used
-    }
-}
-
-pub struct FunctionSubsetBuilder<'a> {
-    builder: DescriptorSetBuilder<'a>,
-}
-
-impl<'a> FunctionSubsetBuilder<'a> {
-    pub fn new<'b: 'a>(buf: &'b mut [u8], interface: InterfaceNumber) -> Self {
-        let mut builder = DescriptorSetBuilder { used: 0, buf };
-        builder.descriptor(FunctionSubsetHeader {
-            wLength: (size_of::<FunctionSubsetHeader>() as u16).to_le(),
-            wDescriptorType: (FunctionSubsetHeader::TYPE as u16).to_le(),
-            bFirstInterface: interface.0,
-            bReserved: 0,
-            wSubsetLength: 0,
-        });
-        Self { builder }
-    }
 
-    /// Add a function-level descriptor.
-    ///
-    /// Note that many descriptors can only be used at function-level in a composite device.
-    pub fn feature<T>(&mut self, desc: T)
-    where
-        T: Descriptor + FunctionLevelDescriptor + 'a,
-    {
-        self.builder.descriptor(desc)
+        fn write_to(&self, buf: &mut [u8]);
     }
 
-    /// Adds the feature descriptors to configure this function to use the WinUSB driver.
-    ///
-    /// Adds a compatible id descriptor "WINUSB" and a registry descriptor that sets the DeviceInterfaceGUID to the
-    /// USB_DEVICE GUID.
-    pub fn winusb_device(&mut self) {
-        self.feature(CompatibleIdFeatureDescriptor::new_winusb());
-        self.feature(RegistryPropertyFeatureDescriptor::new_usb_deviceinterfaceguid());
-    }
-
-    /// Finishes writing the data. Returns the total number of bytes used by the descriptor set.
-    pub fn finalize(self) -> usize {
-        let used = self.builder.used;
-        let buf = self.builder.buf;
-        // Update length in header with final length
-        let total_len = &mut buf[6..8];
-        total_len.copy_from_slice((used as u16).to_le_bytes().as_slice());
-        used
+    pub trait DescriptorSet: Descriptor {
+        const LENGTH_OFFSET: usize;
     }
 }
 
-/// A trait for descriptors
-pub trait Descriptor: Sized {
-    const TYPE: DescriptorType;
-
-    /// The size of the descriptor's header.
-    fn size(&self) -> usize {
-        size_of::<Self>()
-    }
-
-    fn write_to(&self, buf: &mut [u8]);
-}
+use sealed::*;
 
 /// Copies the data of `t` into `buf`.
 ///
@@ -303,7 +266,7 @@ pub trait Descriptor: Sized {
 /// The type `T` must be able to be safely cast to `&[u8]`. (e.g. it is a `#[repr(packed)]` struct)
 unsafe fn transmute_write_to<T: Sized>(t: &T, buf: &mut [u8]) {
     let bytes = core::slice::from_raw_parts((t as *const T) as *const u8, size_of::<T>());
-    assert!(buf.len() >= bytes.len());
+    assert!(buf.len() >= bytes.len(), "MSOS descriptor buffer full");
     (&mut buf[..bytes.len()]).copy_from_slice(bytes);
 }
 
@@ -354,6 +317,17 @@ pub struct DescriptorSetHeader {
     wTotalLength: u16,
 }
 
+impl DescriptorSetHeader {
+    pub fn new(windows_version: u32) -> Self {
+        DescriptorSetHeader {
+            wLength: (size_of::<Self>() as u16).to_le(),
+            wDescriptorType: (Self::TYPE as u16).to_le(),
+            dwWindowsVersion: windows_version.to_le(),
+            wTotalLength: 0,
+        }
+    }
+}
+
 impl Descriptor for DescriptorSetHeader {
     const TYPE: DescriptorType = DescriptorType::SetHeaderDescriptor;
     fn write_to(&self, buf: &mut [u8]) {
@@ -361,6 +335,10 @@ impl Descriptor for DescriptorSetHeader {
     }
 }
 
+impl DescriptorSet for DescriptorSetHeader {
+    const LENGTH_OFFSET: usize = 8;
+}
+
 /// Table 11. Configuration subset header.
 #[allow(non_snake_case)]
 #[repr(C, packed(1))]
@@ -372,6 +350,18 @@ pub struct ConfigurationSubsetHeader {
     wTotalLength: u16,
 }
 
+impl ConfigurationSubsetHeader {
+    pub fn new(config: u8) -> Self {
+        ConfigurationSubsetHeader {
+            wLength: (size_of::<Self>() as u16).to_le(),
+            wDescriptorType: (Self::TYPE as u16).to_le(),
+            bConfigurationValue: config,
+            bReserved: 0,
+            wTotalLength: 0,
+        }
+    }
+}
+
 impl Descriptor for ConfigurationSubsetHeader {
     const TYPE: DescriptorType = DescriptorType::SubsetHeaderConfiguration;
     fn write_to(&self, buf: &mut [u8]) {
@@ -379,6 +369,10 @@ impl Descriptor for ConfigurationSubsetHeader {
     }
 }
 
+impl DescriptorSet for ConfigurationSubsetHeader {
+    const LENGTH_OFFSET: usize = 6;
+}
+
 /// Table 12. Function subset header.
 #[allow(non_snake_case)]
 #[repr(C, packed(1))]
@@ -390,6 +384,18 @@ pub struct FunctionSubsetHeader {
     wSubsetLength: u16,
 }
 
+impl FunctionSubsetHeader {
+    pub fn new(first_interface: u8) -> Self {
+        FunctionSubsetHeader {
+            wLength: (size_of::<Self>() as u16).to_le(),
+            wDescriptorType: (Self::TYPE as u16).to_le(),
+            bFirstInterface: first_interface,
+            bReserved: 0,
+            wSubsetLength: 0,
+        }
+    }
+}
+
 impl Descriptor for FunctionSubsetHeader {
     const TYPE: DescriptorType = DescriptorType::SubsetHeaderFunction;
     fn write_to(&self, buf: &mut [u8]) {
@@ -397,16 +403,17 @@ impl Descriptor for FunctionSubsetHeader {
     }
 }
 
+impl DescriptorSet for FunctionSubsetHeader {
+    const LENGTH_OFFSET: usize = 6;
+}
+
 // Feature Descriptors
 
 /// A marker trait for feature descriptors that are valid at the device level.
-pub trait DeviceLevelDescriptor {}
+pub trait DeviceLevelDescriptor: Descriptor {}
 
 /// A marker trait for feature descriptors that are valid at the function level.
-pub trait FunctionLevelDescriptor {
-    /// `true` when the feature descriptor may only be used at the function level in composite devices.
-    const COMPOSITE_ONLY: bool = false;
-}
+pub trait FunctionLevelDescriptor: Descriptor {}
 
 /// Table 13. Microsoft OS 2.0 compatible ID descriptor.
 #[allow(non_snake_case)]
@@ -419,9 +426,7 @@ pub struct CompatibleIdFeatureDescriptor {
 }
 
 impl DeviceLevelDescriptor for CompatibleIdFeatureDescriptor {}
-impl FunctionLevelDescriptor for CompatibleIdFeatureDescriptor {
-    const COMPOSITE_ONLY: bool = true;
-}
+impl FunctionLevelDescriptor for CompatibleIdFeatureDescriptor {}
 
 impl Descriptor for CompatibleIdFeatureDescriptor {
     const TYPE: DescriptorType = DescriptorType::FeatureCompatibleId;
@@ -462,16 +467,12 @@ pub struct RegistryPropertyFeatureDescriptor<'a> {
     wLength: u16,
     wDescriptorType: u16,
     wPropertyDataType: u16,
-    wPropertyNameLength: u16,
     PropertyName: &'a [u8],
-    wPropertyDataLength: u16,
     PropertyData: &'a [u8],
 }
 
 impl<'a> DeviceLevelDescriptor for RegistryPropertyFeatureDescriptor<'a> {}
-impl<'a> FunctionLevelDescriptor for RegistryPropertyFeatureDescriptor<'a> {
-    const COMPOSITE_ONLY: bool = true;
-}
+impl<'a> FunctionLevelDescriptor for RegistryPropertyFeatureDescriptor<'a> {}
 
 impl<'a> Descriptor for RegistryPropertyFeatureDescriptor<'a> {
     const TYPE: DescriptorType = DescriptorType::FeatureRegProperty;
@@ -479,34 +480,44 @@ impl<'a> Descriptor for RegistryPropertyFeatureDescriptor<'a> {
         10 + self.PropertyName.len() + self.PropertyData.len()
     }
     fn write_to(&self, buf: &mut [u8]) {
-        assert!(buf.len() >= self.size());
-        assert!(self.wPropertyNameLength as usize == self.PropertyName.len());
-        assert!(self.wPropertyDataLength as usize == self.PropertyData.len());
+        assert!(buf.len() >= self.size(), "MSOS descriptor buffer full");
         write_u16(buf, 0..2, self.wLength);
         write_u16(buf, 2..4, self.wDescriptorType);
         write_u16(buf, 4..6, self.wPropertyDataType);
-        write_u16(buf, 6..8, self.wPropertyNameLength);
+        write_u16(buf, 6..8, (self.PropertyName.len() as u16).to_le());
         let pne = 8 + self.PropertyName.len();
         (&mut buf[8..pne]).copy_from_slice(self.PropertyName);
         let pds = pne + 2;
         let pde = pds + self.PropertyData.len();
-        write_u16(buf, pne..pds, self.wPropertyDataLength);
+        write_u16(buf, pne..pds, (self.PropertyData.len() as u16).to_le());
         (&mut buf[pds..pde]).copy_from_slice(self.PropertyData);
     }
 }
 
 impl<'a> RegistryPropertyFeatureDescriptor<'a> {
+    pub const DEVICE_INTERFACE_GUIDS_NAME: &U16CStr = {
+        // Can't use defmt::panic in constant expressions (inside u16cstr!)
+        macro_rules! panic {
+            ($($x:tt)*) => {
+                {
+                    ::core::panic!($($x)*);
+                }
+            };
+        }
+
+        u16cstr!("DeviceInterfaceGUIDs")
+    };
+
     /// A registry property.
     ///
     /// `name` should be a NUL-terminated 16-bit Unicode string.
     pub fn new_raw<'n: 'a, 'd: 'a>(name: &'a [u8], data: &'d [u8], data_type: PropertyDataType) -> Self {
+        assert!(name.len() < usize::from(u16::MAX) && data.len() < usize::from(u16::MAX));
         Self {
             wLength: ((10 + name.len() + data.len()) as u16).to_le(),
             wDescriptorType: (Self::TYPE as u16).to_le(),
             wPropertyDataType: (data_type as u16).to_le(),
-            wPropertyNameLength: (name.len() as u16).to_le(),
             PropertyName: name,
-            wPropertyDataLength: (data.len() as u16).to_le(),
             PropertyData: data,
         }
     }
@@ -515,24 +526,6 @@ impl<'a> RegistryPropertyFeatureDescriptor<'a> {
         unsafe { core::slice::from_raw_parts(s.as_ptr() as *const u8, (s.len() + 1) * 2) }
     }
 
-    /// A registry property that sets the DeviceInterfaceGUIDs to the device interface class for USB devices which are
-    /// attached to a USB hub.
-    pub fn new_usb_deviceinterfaceguid() -> Self {
-        // Can't use defmt::panic in constant expressions (inside u16cstr!)
-        macro_rules! panic {
-            ($($x:tt)*) => {
-                {
-                    ::core::panic!($($x)*);
-                }
-            };
-        }
-
-        Self::new_multi_string(
-            u16cstr!("DeviceInterfaceGUIDs"),
-            u16cstr!("{A5DCBF10-6530-11D2-901F-00C04FB951ED}").as_slice_with_nul(),
-        )
-    }
-
     /// A registry property containing a NUL-terminated 16-bit Unicode string.
     pub fn new_string<'n: 'a, 'd: 'a>(name: &'n U16CStr, data: &'d U16CStr) -> Self {
         Self::new_raw(Self::u16str_bytes(name), Self::u16str_bytes(data), PropertyDataType::Sz)
@@ -558,6 +551,10 @@ impl<'a> RegistryPropertyFeatureDescriptor<'a> {
 
     /// A registry property containing multiple NUL-terminated 16-bit Unicode strings.
     pub fn new_multi_string<'n: 'a, 'd: 'a>(name: &'n U16CStr, data: &'d [u16]) -> Self {
+        assert!(
+            data.len() >= 2 && data[data.len() - 1] == 0 && data[data.len() - 2] == 0,
+            "multi-strings must end in double nul terminators"
+        );
         Self::new_raw(
             Self::u16str_bytes(name),
             unsafe { core::slice::from_raw_parts(data.as_ptr() as *const u8, data.len() * 2) },
diff --git a/examples/nrf52840/Cargo.toml b/examples/nrf52840/Cargo.toml
index 95d939873..cfdda076e 100644
--- a/examples/nrf52840/Cargo.toml
+++ b/examples/nrf52840/Cargo.toml
@@ -6,6 +6,7 @@ license = "MIT OR Apache-2.0"
 
 [features]
 default = ["nightly"]
+msos-descriptor = ["embassy-usb/msos-descriptor"]
 nightly = ["embassy-executor/nightly", "embassy-nrf/nightly", "embassy-net/nightly", "embassy-nrf/unstable-traits", "embassy-usb", "embedded-io/async", "embassy-net",
            "embassy-lora", "lorawan-device", "lorawan"]
 
@@ -34,4 +35,8 @@ futures = { version = "0.3.17", default-features = false, features = ["async-awa
 rand = { version = "0.8.4", default-features = false }
 embedded-storage = "0.3.0"
 usbd-hid = "0.6.0"
-serde = { version = "1.0.136", default-features = false }
\ No newline at end of file
+serde = { version = "1.0.136", default-features = false }
+
+[[bin]]
+name = "usb_serial_winusb"
+required-features = ["msos-descriptor"]
diff --git a/examples/nrf52840/src/bin/usb_serial_winusb.rs b/examples/nrf52840/src/bin/usb_serial_winusb.rs
new file mode 100644
index 000000000..443379a07
--- /dev/null
+++ b/examples/nrf52840/src/bin/usb_serial_winusb.rs
@@ -0,0 +1,139 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use core::mem;
+
+use defmt::{info, panic};
+use embassy_executor::Spawner;
+use embassy_futures::join::join;
+use embassy_nrf::usb::{Driver, Instance, PowerUsb, UsbSupply};
+use embassy_nrf::{interrupt, pac};
+use embassy_usb::class::cdc_acm::{CdcAcmClass, State};
+use embassy_usb::driver::EndpointError;
+use embassy_usb::msos::{self, windows_version};
+use embassy_usb::{Builder, Config};
+use {defmt_rtt as _, panic_probe as _};
+
+const DEVICE_INTERFACE_GUIDS: &[u16] = {
+    // Can't use defmt::panic in constant expressions (inside u16str!)
+    macro_rules! panic {
+            ($($x:tt)*) => {
+                {
+                    ::core::panic!($($x)*);
+                }
+            };
+        }
+    msos::u16str!("{EAA9A5DC-30BA-44BC-9232-606CDC875321}\0\0").as_slice()
+};
+
+#[embassy_executor::main]
+async fn main(_spawner: Spawner) {
+    let p = embassy_nrf::init(Default::default());
+    let clock: pac::CLOCK = unsafe { mem::transmute(()) };
+
+    info!("Enabling ext hfosc...");
+    clock.tasks_hfclkstart.write(|w| unsafe { w.bits(1) });
+    while clock.events_hfclkstarted.read().bits() != 1 {}
+
+    // Create the driver, from the HAL.
+    let irq = interrupt::take!(USBD);
+    let power_irq = interrupt::take!(POWER_CLOCK);
+    let driver = Driver::new(p.USBD, irq, PowerUsb::new(power_irq));
+
+    // Create embassy-usb Config
+    let mut config = Config::new(0xc0de, 0xcafe);
+    config.manufacturer = Some("Embassy");
+    config.product = Some("USB-serial example");
+    config.serial_number = Some("12345678");
+    config.max_power = 100;
+    config.max_packet_size_0 = 64;
+
+    // Required for windows compatiblity.
+    // https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.9.1/kconfig/CONFIG_CDC_ACM_IAD.html#help
+    config.device_class = 0xEF;
+    config.device_sub_class = 0x02;
+    config.device_protocol = 0x01;
+    config.composite_with_iads = true;
+
+    // Create embassy-usb DeviceBuilder using the driver and config.
+    // It needs some buffers for building the descriptors.
+    let mut device_descriptor = [0; 256];
+    let mut config_descriptor = [0; 256];
+    let mut bos_descriptor = [0; 256];
+    let mut msos_descriptor = [0; 256];
+    let mut control_buf = [0; 64];
+
+    let mut state = State::new();
+
+    let mut builder = Builder::new(
+        driver,
+        config,
+        &mut device_descriptor,
+        &mut config_descriptor,
+        &mut bos_descriptor,
+        &mut msos_descriptor,
+        &mut control_buf,
+        None,
+    );
+
+    builder.msos_descriptor(windows_version::WIN8_1, 2);
+
+    // Create classes on the builder.
+    let mut class = CdcAcmClass::new(&mut builder, &mut state, 64);
+
+    // Since we want to create MS OS feature descriptors that apply to a function that has already been added to the
+    // builder, need to get the MsOsDescriptorWriter from the builder and manually add those descriptors.
+    // Inside a class constructor, you would just need to call `FunctionBuilder::msos_feature` instead.
+    let msos_writer = builder.msos_writer();
+    msos_writer.configuration(0);
+    msos_writer.function(0);
+    msos_writer.function_feature(msos::CompatibleIdFeatureDescriptor::new("WINUSB", ""));
+    msos_writer.function_feature(msos::RegistryPropertyFeatureDescriptor::new_multi_string(
+        msos::RegistryPropertyFeatureDescriptor::DEVICE_INTERFACE_GUIDS_NAME,
+        DEVICE_INTERFACE_GUIDS,
+    ));
+
+    // Build the builder.
+    let mut usb = builder.build();
+
+    // Run the USB device.
+    let usb_fut = usb.run();
+
+    // Do stuff with the class!
+    let echo_fut = async {
+        loop {
+            class.wait_connection().await;
+            info!("Connected");
+            let _ = echo(&mut class).await;
+            info!("Disconnected");
+        }
+    };
+
+    // Run everything concurrently.
+    // If we had made everything `'static` above instead, we could do this using separate tasks instead.
+    join(usb_fut, echo_fut).await;
+}
+
+struct Disconnected {}
+
+impl From<EndpointError> for Disconnected {
+    fn from(val: EndpointError) -> Self {
+        match val {
+            EndpointError::BufferOverflow => panic!("Buffer overflow"),
+            EndpointError::Disabled => Disconnected {},
+        }
+    }
+}
+
+async fn echo<'d, T: Instance + 'd, P: UsbSupply + 'd>(
+    class: &mut CdcAcmClass<'d, Driver<'d, T, P>>,
+) -> Result<(), Disconnected> {
+    let mut buf = [0; 64];
+    loop {
+        let n = class.read_packet(&mut buf).await?;
+        let data = &buf[..n];
+        info!("data: {:x}", data);
+        class.write_packet(data).await?;
+    }
+}