Bug: Interface cast crashes on Windows 10 Builds < 20H2

[qdot]

Which crate is this about?

windows

Crate version

0.28

Summary

First off: This is in 0.28, because I'm still waiting for some fixes in 0.31. If this is invalid for the new library, feel free to close and yell at me to upgrade. :)

We've been getting reports of .unwrap()'s panicking on Windows 10 builds older than 20H2 (which I realize are past lifetime, though we've got someone on an LTS complaining now too).

Specifically, this line in the generated code.

            let this =
              &::windows::Interface::cast::<IBluetoothLEAdvertisementReceivedEventArgs2>(self)
                .unwrap();

Expected behavior

Either no crashes for users on Windows 10 < 20H2, or else some sort of warning/error versus a full panic.

Actual behavior

Crashes on 20H2

Additional comments

No response

[kennykerr]

Whether the cast succeeds depends on whether the object implements the requested interface. The windows crate has nothing to do with that. That's up to the particular OS you're running on.

Whether it crashes is also up to you - you can handle it more gracefully by avoiding the unwrap, which is designed to panic, and instead query the Result, perhaps with something like if let Ok(value) = object.cast::<IBlaBla>() { ... }.

[qdot]

The code block above isn't our code. That's from the out/windows.rs file created by the windows-rs crate, hence the inability to avoid the .unwrap(). That's why I was saying this might also be invalid, because I know the code gen stuff changed in v0.29.

[qdot]

Here's the full context, from the generated windows.rs file:

          pub fn BluetoothAddressType(&self) -> ::windows::Result<super::BluetoothAddressType> {
            let this =
              &::windows::Interface::cast::<IBluetoothLEAdvertisementReceivedEventArgs2>(self)
                .unwrap();
            unsafe {
              let mut result__: <super::BluetoothAddressType as ::windows::Abi>::Abi =
                ::std::mem::zeroed();
              (::windows::Interface::vtable(this).6)(::windows::Abi::abi(this), &mut result__)
                .from_abi::<super::BluetoothAddressType>(result__)
            }
          }

[kennykerr]

Ah! 😄

Yes, that has been fixed. You should already see the fix if you switch from the build macro to the pre-generated APIs using the Devices_Bluetooth_Advertisement crate feature in the current 0.30 version.

Notably, the build macro will no longer be available in 0.31.

Sorry for the confusion.

[kennykerr]

Here's what that looks like in the pre-generated bindings:

windows-rs/crates/libs/windows/src/Windows/Devices/Bluetooth/Advertisement/mod.rs

Lines 1110 to 1117 in f06b0bb

	#[doc = "*Required features: 'Devices_Bluetooth_Advertisement'*"]
	pub fn BluetoothAddressType(&self) -> ::windows::core::Result<super::BluetoothAddressType> {
	let this = &::windows::core::Interface::cast::<IBluetoothLEAdvertisementReceivedEventArgs2>(self)?;
	unsafe {
	let mut result__: super::BluetoothAddressType = ::core::mem::zeroed();
	(::windows::core::Interface::vtable(this).BluetoothAddressType)(::core::mem::transmute_copy(this), &mut result__).from_abi::<super::BluetoothAddressType>(result__)
	}
	}

[qdot]

Awesome, thanks! I'll go ahead and close this out then, and will get to testing against master while waiting for 0.31 ('cause I need #1438).

Thanks for answering questions this late. :)

[107295472]

windows::core::Interface::cast

windows::core::Interface::cast Cast can't be found

[robmikh]

You can find it in the ComInterface trait now.