String copying fails when output from assert

[Jonathan M Davis]

On Tuesday, November 21, 2017 12:05:21 Jonathan M Davis via Digitalmars-d-
learn wrote:
> On Tuesday, November 21, 2017 18:49:40 David Zhang via Digitalmars-d-learn
> wrote:
> > Hi,
> >
> > I've been trying to copy a string, then output it from an
> > `assert(false)` statement, but the copy seems to be corrupted
> > somehow.
> >
> > void main()
> > {
> >
> >   string str = "some string";
> >
> >          //initializing `chars` with any value doesn't do anything
> >
> >   char[64] chars;
> >   //char[64] chars = '!';
> >
> >   //memcpy doesn't work either, though the output's different
> >   //import core.stdc.string;
> >   //memcpy(&chars[0], &str[0], str.length);
> >
> >   chars[0..str.length] = str;
> >   assert(false, chars[0..str.length]);
> >
> > }
> >
> > What am I missing here?
>
> 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.

https://issues.dlang.org/show_bug.cgi?id=18002

- Jonathan M Davis