Add feature flags for Windows SDK versions

[dpaoliello]

Suggestion

It's fairly common for projects to set an older version of Windows as their "minimum supported version". However, this means that they must be careful which Windows APIs they use as not all APIs are available on all versions of Windows.

One way to verify that the APIs your project uses exist on a given version of Windows is to build against the SDK for that Windows version. However, this isn't always practical (projects may not control what software is on their CI machines and installing Windows SDKs is slow), causes other build issues (e.g., using Arm64EC requires the Windows 11 SDK) and doesn't help at all when using raw-dylib (as the Windows SDK import libraries aren't used). Even if a project is doing this, it would be nice to get clear errors during the Rust build rather than obscure linker failures.

Instead, windows-rs could add feature flags that corresponded to the various Windows SDK versions, which would allow projects to select which set of APIs to make available at build time.

[ChrisDenton]

Specifically for Rust's std use case, feature flags wouldn't be the best approach. It would really need to be an option in the windows-bindgen crate (e.g. --api-version 10.0.10240) that errors if a function is used that isn't supported by the target version.

[riverar]

This will require accurate upstream targeting information. SupportedOSPlatform is not currently stable/usable.

[kennykerr]

Yes assuming we can stabilize the version metadata, I'd initially only make this available through windows-bindgen as a filter/validator of sorts.

It does however appear that there's not been much progress in stabilizing this metadata, so this may all be wishful thinking at present:

microsoft/win32metadata#738

[kennykerr]

Closing this issue until there's a resolution on metadata - let's keep the conversation going on the upstream issue: microsoft/win32metadata#738