when riding a bike and the fork is very rusted, can it possibly snap?

sure

It probably would not snap if you mean snap like a twig into two seperate parts. Steel will usally crack before anything bad happens and the fork would most likely just bend completely out of shape which could still cause a bad fall depending on the situation you are in when it goes. Post a pic. Are there any holes in the fork?

I'm going to go with, "this is a troll" for 200, Alex.

As a general rule rust is not a good thing. A little rust isn't bad but a lot can be fatal. How rusty is it and where is it rusty; the fork legs or the steerer tube or both?

Troll? Maybe.

Jaa, exas has a long history in his short time on BF starting troll threads.....

Is it rusty on the inside, or rusty on the outside?

inside, so much that some lines of rust can be seen under the paint

Really, how did you manage to see rust inside a fork? That statement alone makes me think this is a troll.

How long is a piece of string? It is like me saying - I have a spot of rust on my bike frame - is is going to fail? No it is just surface rust on my bike, nothing major for me.

Why don't you just remove the paint - check to see if it is just surface rust or runs a little deeper. If you don't know what you are looking for - then a rust hole is a bad thing to start with - major rust at joins is also bad I would assume. Take it to a frame builder if you are concerned.

If it is not too bad, search other threads for rust treatment.

If it is a troll, then it is a pretty boring attempt.