Does the compiler check for safe?

[Charles Hixson via Digitalmars-d-announce]

I took some code that was annotated safe, and modified it to use a 
buffer internal to the class, and an offset counter.  Some of the 
routines adjusted the counter.  I forgot to remove the  "safe" annotation.

It compiled without error.  So...
1) The compiler didn't check for safety
2) It counts as safe if you only modify local-to-the-class variables 
(and I misunderstood).
or
3) This error is only detected on execution

But which?

(I'm still writing the code, so I can't yet try to execute it, and I 
took off the safe annotation because after I read the definition in TDPL 
it seemed a mistaken annotation.  But perhaps I misunderstand.)