D-lang attributes can be used like a label, so for example you can put at the top of your file:

   @safe nogc:

   auto myFunc() { // do stuff }   
   struct Blah {}

Whatever attributes you use apply to all code in the lexical scope below the colon.

It's useful for structs/classes

  struct MyThing {
    auto unsafeFunc() {}
    
    @safe:
    auto safeFunc1() {}
  }

No. We had a very long discussion about safe-by-default on the forums but failed to reach a consensus.

As I understand it, for safe-by-default to work, either all C libraries would have @trusted D bindings written, or all of them would have to be assumed to be @trusted. Most of the community wanted explicit @trusted annotations, Walter and others favored assuming them to be @trusted (or was it assumed @safe?) was the way to go. So, no safe-by-default at this time. :/

Kind of, mark `main` `@safe`. The transitivity of safety will ensure that you can only call safe functions, else you will get a compile error.