D just makes assert() part of the language:

https://dlang.org/spec/expression.html#assert_expressions

The behavior of it can be set with a compiler switch to one of:

1. Immediately halting via execution of a special CPU instruction

2. Aborting the program

3. Calling the assert failure function in the corresponding C runtime library

4. Throwing the AssertError exception in the D runtime library

So there's no issue with parsing it. The compiler also understands the semantics of assert(), and so things like `assert(0)` can be recognized as being the end of the program.