Szathmáry László honlapja @ DEIK

(1) assert

import std.stdio;

int add(const int a, const int b) pure
{
assert(a > 0);
// assert(a > 0, "`a` must be positive");
return a + b;
}

void main()
{
writeln(add(-2, 3));
}

Compile and run:

$ dmd main.d

$ ./main
core.exception.AssertError@main.d(7): Assertion failure
...

Disable assertions:

$ dmd main.d -release

$ ./main
1

Notes:

In release mode, assertions are ignored. The compiler simply drops those lines.
However, there is one exception. assert(0) (and assert(false), they're equivalent) is NOT ignored in release mode! Why? Because it indicates a part of the program that execution should never reach. If execution reaches that part, then it's a problem that mustn't be ignored.
Assertions shouldn't have any side effects, so they can be used in pure functions.