Support standalone code generation

[kennykerr]

https://kennykerr.ca/rust-getting-started/standalone-code-generation.html

Fixes: #1720

[kennykerr]

Note that this PR only changes the bindgen crate. The changes to the windows and windows-sys code gen are merely whitespace differences due to limitations in rustfmt.

[poliorcetics]

I wasn’t one of the person asking for it, but still, it looks absolutely amazing, thanks for that !

To avoid recompiling the bindings all the time when building the crate, we could add a recommandation to move them to a sub-crate only for that, for example parking_lot would introduce « parking_lot_bindings_windows » and simply save the bindings in this new crate’s lib.rs.

Are the bindings always windows-sys’ style or is it possible to have them windows’ style in the future (I’m thinking about COM APIs), with added traits and all (probably not for this MR though) ?

[djc]

This looks great, thanks for working on this! It would be great if, as @poliorcetics suggests, there would be a way to bring in the safe bindings at a similar level of granularity.

[kennykerr]

Yes, I just need to move some of the core types needed to support the windows-style bindings into the windows-core crate so that they can be reused without a dependency on the windows crate itself. That's a bit more work so I'll tackle that in a separate PR.

[djc]

Awesome, looking forward to it!

[MarijnS95]

👍 for having this on windows, I already suggested the same (including the mentioned windows-core split) in #1720 (comment) in hopes of having the best of both worlds: compile-times similar to windows-sys, much smaller code size than both crates, and safe(r) and more complete (COM) apis.