D mentioned on Rust discussions site

[Joseph Rushton Wakeling]

On Tuesday, 19 May 2020 at 11:38:48 UTC, Russel Winder wrote:
> https://users.rust-lang.org/t/dlang-adds-a-borrowchecker-called-the-ob-system-for-ownership-borrowing/42872

There is a technical question I'd like to ask here.

One of the participants in that thread posted what I considered 
to be a rather contrived example where `malloc`, `free`, and 
casts from pointer to value types and back (!) were combined to 
produce a use-after-free effect.

Of course it's trivial to find ways to get the compiler to catch 
the casts as unsafe.  But in trying this out I settled on a much 
simpler formulation that still doesn't get caught as a violation 
of `@live` rules:

```
@live int* test1()
{
     import core.stdc.stdlib : free, malloc;
     import std.stdio : writefln;

     scope p = cast(int*) malloc(int.sizeof);

     free(p);

     writefln!"%d"(*p);

     return p;
}
```

Now, I'm not making any judgement here -- this is a provisional 
feature with many limitations, that is clearly intended as a 
first step to something bigger, and which is clearly advertised 
as having a _lot_ of bugs right now.  But I would like to 
understand, for my own sake, why the above example is not 
detecting that it's invalid to dereference `p` for the `writeln`, 
or indeed to return `p` from the function.

(Tried with a freshly built dmd 2.092.0.)