Replace boilerplate COM interface code gen with macro_rules

[kennykerr]

The windows and windows-sys crates avoid derive macros as much as possible as they tend to be very slow at scale. There is however a lot of boilerplate code that could be avoided with selective use of macros, so I am going to start making conservative use of macro_rules to replace large chunks of boilerplate code. I tested the change in this update and could not measure any difference in build time, so I think this is a good first step in reducing code gen and overall crate size.

The new interface_hierarchy macro stamps out most of the From implementations for COM and WinRT types that can often have large interface hierarchies. For example, ID3D12Device9 derives from 11 other interfaces leading to 33 From implementations that can be generated by the interface_hierarchy macro far more concisely.

windows-rs/crates/libs/windows/src/core/mod.rs

Lines 101 to 123 in 6ed9da4

	macro_rules! interface_hierarchy {
	($child:ty, $parent:ty) => {
	impl ::std::convert::From<$child> for $parent {
	fn from(value: $child) -> Self {
	unsafe { ::std::mem::transmute(value) }
	}
	}
	impl ::std::convert::From<&$child> for &$parent {
	fn from(value: &$child) -> Self {
	unsafe { ::std::mem::transmute(value) }
	}
	}
	impl ::std::convert::From<&$child> for $parent {
	fn from(value: &$child) -> Self {
	unsafe { ::std::mem::transmute(::std::clone::Clone::clone(value)) }
	}
	}
	};
	($child:ty, $first:ty, $($rest:ty),+) => {
	$crate::core::interface_hierarchy!($child, $first);
	$crate::core::interface_hierarchy!($child, $($rest),+);
	};
	}

[rylev]

Looks great! ❤️

[rylev]

I don't believe this is necessary given the #[macro_export].

[rylev]

Despite this being not shown to end users, it could be good to have some documentation here for those working on the internals of windows-rs.

[kennykerr]

Will do - thanks @rylev!