Hash Tables in D

[Adrian Matoga via Digitalmars-d-announce]

On Sunday, 3 January 2016 at 19:29:05 UTC, Martin Nowak wrote:
> There is a bug.
>
> You should never do this b/c of iterator/range invalidation.
>
> foreach (key; aa.keys)
>     aa.remove(key);

I've recently hit this when trying to remove some of the elements 
from an AA while iterating over it. It's easy to forget that it 
is not allowed, especially when working on more complex 
algorithms. Accidentally, it worked all fine even with large data 
sets with DMD 2.069, but with GDC mysterious segfaults appeared - 
the first thought in such case was obviously a bug in GDC or in 
the older front-end.

However, this is a very convenient, natural and intuitive syntax 
for something that is needed quite frequently, yet this code 
breaks silently in non-predictable ways.
There are already registered issues concerning this (or similar 
with insertion) problem, including:

https://issues.dlang.org/show_bug.cgi?id=4179
https://issues.dlang.org/show_bug.cgi?id=2255
https://issues.dlang.org/show_bug.cgi?id=10821
https://issues.dlang.org/show_bug.cgi?id=10876
https://issues.dlang.org/show_bug.cgi?id=13903

I've personally encountered segfaults, infinite loops, programs 
producing incorrect results. Some solutions to this were proposed 
in the bug reports - either:
a) explicitly allow removing during iteration, and make sure it 
always works, or
b) explicitly disallow removing during iteration, and always 
throw an Error whenever it is attempted. In some cases it could 
even be detectable in compile time.

And I don't mean including a single sentence about this somewhere 
in the docs. I mean an error reported by the compiler and/or 
runtime. The runtime checks could be disabled for -release, but 
there should be at least an option to detect such errors.
Probably the worst part of it is that you're free to wrap it in 
@safe, while it's not safe at all.