perf: SyncVar hook invocations no longer instantiate a new action delegate on every call

[cshunsinger]

This commit makes a couple of small modification to the weaving around syncvars.

I identified 2 cases in which we weave code to invoke the hook for setting a syncvar's value:

1. When the setter is called on the weaved Network property
2. When deserializing messages where the message contains a new value for the syncvar from the remote source

As stated in the linked issue, when the syncvar hook is invoked (if there even is a syncvar hook), an instantiation occurs:
new Action<T, T>(HookMethod)

The modifications to the weaving do the following:

• For each syncvar that contains a hook, a new Action<T, T> instance field is added to the class being weaved, where T is the field type of the syncvar field.
• During constructor injection weaving, all of the Action<T, T> instance fields are instantiated to new Action<T, T>(HookMethod) for each of the syncvar's hooks
• In both cases where the hook is invoked, instead of pushing new Action<T, T>(HookMethod) to the stack, push this.hookActionDelegate to the stack.

ILSpy showing the weaving results: generated delegate fields, no more instantiations, and setting up the delegates in the constructor

resolves #3594

[miwarnec]

interesting, we'll take a look soon after high priority stuff

[James-Frowen]

This is the commit that caused this performance regression: 5369b8f

the new action adds up fast, In a few projects I've seen it causes 100kb+ of allocations per frame...

[Liam2349]

Merged this in, no more GC. Thanks! Hopefully I didn't break anything.

[miwarnec]

we are still discussing this.
the original motivation was to move more and more IL code into C# to reduce weaving headaches.
and now it looks like Unity is going to deprecate weaving entirely: #3630

[miwarnec]

@cshunsinger merged!
Not many people understand Weaver & IL code, so nice work!

[cshunsinger]

@cshunsinger merged! Not many people understand Weaver & IL code, so nice work!

Let me know if you have any other weaving needs. I love navigating the low-level stuff whether its IL/bytecode, or other funky stuff.

With Unity deprecating weaving, that may get tricky. I wonder if the older versions of Unity all support Roslyn because if not then this project will end up being forced to support both Roslyn and IL weaving depending on which version of Unity is used O.O

[James-Frowen]

With Unity deprecating weaving, that may get tricky. I wonder if the older versions of Unity all support Roslyn because if not then this project will end up being forced to support both Roslyn and IL weaving depending on which version of Unity is used O.O

Unity is deprecating the AssemblyBuilder class (which compiles .cs files), not IL weaving (which edits dlls).

IL weaving is done to the dll using mono.cecil. Currently unity has ILPostProcessor as an undocumented feature that lets us hook in and run it easily. but even if unity removes or replaces the current ILPostProcessor we can just go back to how Weaver worked in unity 2019 before ILPostProcessor existed.