Const guid

[rylev]

Fixes #136

Note that this will push the MSRV to Rust 1.46 which is currently in beta.

[rylev]

@kennykerr this is ready for review. It won't pass CI until I update const-sha1 with the changes necessary to get this to work, but I'd like to have your eyes on this before I push a new version. Once that's done, I'll push a new version of const-sha1 and we can merge this.

One thing we'll need to consider is that this will cause the crate to only work on beta. If we want we can wait until the next release of Rust before merging this.

[kennykerr]

Thanks!

[kennykerr]

Can't wait to do some run time comparative performance testing with this change.

[Boddlnagg]

Note that this is a breaking change for consumers of winrt-rs who have their own impls of winrt::ComInterface. This should be reflected in the next release version.

(It might also be a breaking change because of the MSRV.)

[rylev]

Yes, our next version will be 0.8 for multiple reasons 👍 .

[kennykerr]

Here's the performance impact of const guids. Consider the following sample:

fn main() -> Result<()> {
    let uri = Uri::create_uri("http://kennykerr.ca")?;
    let query = uri.query_parsed()?;

    let start = std::time::Instant::now();

    for _ in 0..1_000_000 {
        let s = query.size()?;
        debug_assert!(s == 0);
    }

    println!("{:5}ms", start.elapsed().as_millis());
    Ok(())
}

The size method is on a required but non-default interface of the WwwFormUrlDecoder class returned by the query_parsed method. That means that every time size is called, QueryInterface must first be called to retrieve the appropriate vtable. This is just one of those details that a language projection hides from the language as a convenience, but such hidden costs need to be as minimal as possible.

Before this commit:

C:\git\rust\scratch>cargo run --release
    Finished release [optimized] target(s) in 0.09s
     Running `target\release\scratch.exe`
  829ms

After this commit:

C:\git\rust\scratch>cargo run --release
    Finished release [optimized] target(s) in 0.09s
     Running `target\release\scratch.exe`
   21ms

That's about 40 times faster and on par with C++/WinRT.