Ref struct caller

[martindevans]

This builds on #213, merge that first

Third time lucky?

Replaced Caller with a ref struct. With the reflection path stripped out and the removal of IStore this was a fairly simple change.

A few notable changes:

• Can't leak Caller, any breakage here is probably uncovering a bug.
• Caller is no longer disposable (we could add an empty Dispose() method and mark it [Obsolete]?).

Also this test:

[Fact]
public void ItCannotBeConstructedFromNullPointer()
{
    var act = () => new Caller(IntPtr.Zero);
    act.Should().Throw<InvalidOperationException>();
}

Did not compile because act is now a CallerAction instead of an Action, which is surprising change of var inference. Fixed by changing var to Action. Never mind, see comment by kpreisser below.

Benchmark

Unlike my previous epoch based approach in #212 (which was 90ns slower) this benchmarks at exactly the same speed as the class Caller approach.

[martindevans]

I thought I'd let the time zone differential work in my favour today ;)

[kpreisser]

Did not compile because act is now a CallerAction instead of an Action

I think this is unrelated to the new CallerAction<...> and CallerFunc<...> types; it's actually an anonymous delegate type that the C# compiler creates when it can't match an existing delegate type (Action/Func):

This is because it would normally infer a Func<Caller> (as the expression new Caller(...) can also be used as return expression), which can't be matched now due to the ref struct Caller, so it generates a new delegate type like delegate Caller X().

[peterhuene]

This looks good; I guess the explicitly defined delegates weren't as bad as I thought they'd be.

Let's merge #213, rebase this PR, and then I'll review it for inclusion in the next release.

[peterhuene]

We should be able to rebase this now.

[martindevans]

Rebased onto main.

[peterhuene]

Awesome! Thanks for this and I'm glad we can now leave the escaping of Caller from the current frame up to the C# compiler 👍.

[peterhuene]

Okay, with everything merged, I'm going to cut releases and call out any potential API incompatibilities.