Implement `-check=nullderef=on`? (D v2.113)

[kinke]

This feature was implemented in DMD's glue layer, and would need to be duplicated (or moved to the frontend upstream instead?). In #5113, I've only added ldmd2 driver support for -check=nullderef=off, bailing out when trying to enable it.

[JohanEngelen]

FWIW, the dmd implementation is on machine code level, which is useless in my opinion. The null deref should be a language level check, i.e. in frontend. See here a trivial example where it does not deliver: https://d.godbolt.org/z/sn3eKEzrv .

Edit: no longer sure about frontend/backend (no exactly clear where the division is), but for example this check should happen before devirtualization.

[rikkimax]

The example Johan gave is a bug that I need to fix.
There are a few branches in function calls, and it gets a little bit complicated, and so I missed a case.

As for where it lives, the glue code is correct. It is no different from an array bounds check, which is also glue code.

[MrcSnm]

@JohanEngelen The first where you call a.foo(), itis not actually a null deref since you're not accessing member parameters.
Look at that:

class A {
    int i;

    final void foo() 
{
   import std.stdio;
   writeln = "Hello World";
}

    final int get() { return i;}
}


void main() {
  A a;
  a.foo(); // OK, but already a null deref (logically)
  auto i = a.get(); // triggers null deref exception,
                    // but not in the location where it
                    // is useful (should report line 14, but
                    // reports line 6)
}

[JohanEngelen]

@JohanEngelen The first where you call a.foo(), itis not actually a null deref since you're not accessing member parameters.

I know very well how all this works under the hood. :)
Regardless of implementation of foo, a.foo() is a dereference of object pointer a. This is what I meant with "language level check". Inside member functions, you have the invariant that this is not null. LDC makes use of this fact for optimization (so even adding the silly if (!this) return 0; inside member functions will have no effect).