Begin Equals on records with check for equality

[Xenoage]

Currently, the auto-generated Equals method for records look like this:

public virtual bool Equals(MyRecord? other) {
   return (object)other != null
     && EqualityContract == other!.EqualityContract
     && EqualityComparer<T>.Default.Equals(FirstProp, other!.FirstProp)
     && ...
}

Since records will be used a lot for immutable data structures, which encourages to reuse and share objects, it would be very helpful to check for reference equality first. In this case, the Equals method would be speeded up enormously, especially for large object trees. Like this:

public virtual bool Equals(MyRecord? other) {
   if (object.ReferenceEquals(this, other)) // Shortcut: Equal reference always means equal content
     return true;
   return (object)other != null
     && EqualityContract == other!.EqualityContract
     && EqualityComparer<T>.Default.Equals(FirstProp, other!.FirstProp)
     && ...
}

What do you think?

[HaloFour]

Honestly, kind of surprised it doesn't do this already.

[333fred]

We had an internal thread based on benchmarks that @PathogenDavid collected here #39 (comment), and had decided to change the implementation. We apparently forgot to follow up on that desire. I'll bring it up with the team and see if we change the behavior in 16.10.

[alrz]

I think dotnet/roslyn#47741 is related.

[333fred]

Filed dotnet/roslyn#51136 to follow up on this.