Optional Dependency Analyzer

[BadMagic100]

From discord

Ok so, I gave the mixin idea I proposed some more thought, and decided that the actual value proposition is being able to detect when you'll load a type not defined in your assembly or your optional dependency's assembly in an if ModHooks.GetMod(modname) is Mod block.

Then I realized this is actually statically analyzeable! So it could be a Roslyn source analyzer distributed as a nuget package (and referenced by default by certain opinionated templates).

Process:

1. Use syntax analysis to match blocks of the form if ModHooks.GetMod("some mod") <any valid presence check>
2. Search your dependency tree for a subclass of Mod which has a matching name. If not, pass analysis.
3. Read the content of the if block and collect 2 things: (a) all referenced types and (b) all method calls
4. Do type analysis. For each discovered class, check properties, fields, and method signatures for additional types. Types in any assembly that does not define a Mod, and the assembly found in step 2 are ignored. Queue types defined in your own assembly for further investigation. If any types not already covered by these cases are found, flag down the offending line(s) and issue a compiler warning. Repeat this step until the type queue is exhausted.
5. Optionally, check the content of the method calls, repeating steps 2-5 until all the called code under the if check is exhausted. This may be super expensive though and the first 4 steps (that only check for n=1) would catch probably well over 99% of bugs