Advanced @Contract capabilities

[burdoto]

I was thinking about possible extra possibilities for the Contract annotation.
I have this example already; I will add more as I stumble upon them:
Consider this method:

    @Contract("CacheReference -> param1; null -> fail; !null -> new")
    protected abstract CacheReference<K, V> advanceIntoCacheRef(Reference<V> reference);

The method should do the following things; as described by the contract:

• Throw an NPE, if the parameter is null
• Wrap the parameter Reference (if not instanceof CacheReference, else see next) into a CacheReference
• Return first parameter if the first parameter is instanceof CacheReference (which is already imported due to return type; return types should be validated by paramN)

Now, obviously this contract won't work, because it doesn't know about more things than nulls; really. Given other capabilities of IntelliJ, it should be fairly easy coming up with a simple, standalone concept for advanced Contract syntax.

[burdoto]

Here's another potential use case for more advanced Contracts:
Simplifying the side effect of a method call, maybe like this:

That would remove the warning from the #pid() call here:

[burdoto]

One more use case of simplifying side effects could be this;

    @Contract(sideEffect = "true -> mod.getPlayers().get(playerId) != null")
    public boolean isOnline(UUID playerId) {...}

where after a method calls isOnline() and checks whether it returns true (e.g. by throwing when its false), the analyzer would know that mod.getPlayers().get(playerId) will not return null when invoked with the same playerId parameter.