String copying fails when output from assert

[Jonathan M Davis]

On Tuesday, November 21, 2017 19:13:36 David Zhang via Digitalmars-d-learn 
wrote:
> On Tuesday, 21 November 2017 at 19:05:21 UTC, Jonathan M Davis
>
> wrote:
> > Well, the assertion is going to throw an AssertError, which
> > takes a string for its message. It doesn't copy the contents of
> > the string. It's just taking a slice just like whenever you
> > pass a string to any other function. So, it refers to the same
> > memory as what's passed in. So, if what it's passed is a string
> > that refers to memory on the stack, then when it goes to print
> > the message, it's going to be garbage, because the stack was
> > unwound, and the static array is gone.
> >
> > Honestly, I'd argue that it's a bug that it allows you provide
> > a message as a char[] instead of immutable(char)[]. It seems
> > that the compiler is implicitly converting char[] to
> > immutable(char)[] in this case, which is very bad. It would
> > matter less if you were giving the assertion a char[] that
> > referred to memory on the heap, but it still shouldn't be
> > allowed.
> >
> > You really should be giving assertions a value that is an
> > actual string that lives on the heap when providing a message,
> > not a char[], and definitely not a char[] that's a slice of a
> > static array.
> >
> > - Jonathan M Davis
>
> Right. So a literal would work, because it's in the .data
> segment, or inlined. Basically, only strings from the heap, or
> that are compiled into the code are valid for assert messages.

Or any kind of exception message.

> What kind of behavior would an assert from stdc have (id,
> betterC)?

I don't know. It calls the C implementation at that point, so it won't throw
an AssertError, but I don't know exactly how it's implemented. It probably
just prints the message and then kills the program, so it'll probably work
even with a slice of a static array, but I don't know for sure. However,
unless the code is specifically versioned to only be betterC, it would be a
really bad idea to rely on that behavior. And if assert ever gets fixed such
that it properly rejects messages that aren't actually strings, passing it a
char[] wouldn't work anymore anyway.

- Jonathan M Davis