Improving assert-printing in DMD

[Kapps via Digitalmars-d]

On Tuesday, 29 September 2015 at 21:26:00 UTC, John Colvin wrote:
> Not necessarily. It could just be a defensive assert for 
> something that should already have been verified/cleaned/caught 
> earlier.
>
> auto pass = getPassword();
> pass.clean();
> assert(pass == pass.toLower());
> //and on we go ...

There are a few flaws with this:
First, your assert applies only in debug mode. You're likely not 
deploying your service in debug mode, so your attempt at 
defensive programming does nothing to protect you when you 
actually need it.

Second, and more critical, the way assert is apparently intended 
to be, is a guarantee that can be used towards optimization. The 
existence of an assert means that that situation can *never* 
happen. You take your existing code, and then later on you decide 
that an assert isn't enough so you add another check for release 
mode. In which case (as far as I understand it), you run into the 
following situation:

auto pass = getPassword();
pass.clean();
assert(pass == pass.toLower());
// Later on...
enforce(pass == pass.toLower());

Now not only is your assert not triggered because you're in 
release mode, but that assert provides a guarantee to the 
compiler that pass is *always* equal to pass.toLower, causing the 
compiler to optimize out that enforce call as it's redundant. 
Therefore, your assert does nothing, and your enforce now does 
nothing as well, masking a potential vulnerability.