Casting away immutability

[Sergei Degtiarev via Digitalmars-d-learn]

On Thursday, 3 September 2015 at 05:15:35 UTC, Jonathan M Davis 
wrote:
> On Wednesday, September 02, 2015 14:00:07 Sergei Degtiarev via 
> Digitalmars-d-learn wrote:
> Well, that's how mmap works. You're just getting raw bytes as 
> void*, and the D code converts that to void[], which is 
> slightly safer. It doesn't inherently have a type, and often, 
> the data referred to by mmap never did have a type. It's just 
> bytes in a file. So, if you want to use it as a type, you have 
> to tell the compiler what it's type is - and get it right - via 
> a cast. And yes, you can cast to immutable as part of that, 
> because casts let you shoot yourself in the foot, because 
> you're telling the compiler that you know better than it does, 
> but you shouldn't be casting anything you get from C to 
> immutable, because it's not going to be immutable. Most code 
> shouldn't be casting to immutable any more than it should be 
> casting away immutable.
>
> immutable data is treated by the compiler as immutable - it 
> will _never_ change over the course of the program - so if it 
> does change, you're going to have fun bugs, and if it really 
> refers to mutable data (as would be the case with an mmapped 
> file), then it could change. Additionally, immutable is treated 
> as implicitly shared, so the compiler could do different 
> optimizations based on that (though that probably won't affect 
> anything if you only ever have it one a single thread), and if 
> it's not really immutable, you could have fun bugs caused by 
> that.
>
> Don't cast to or from immutable unless you know what you're 
> doing, and in most of those cases, there's a decent chance that 
> that's not the best way to do what you're trying to do anyway.

Agree, however, memory obtained with mmap(..., PROT_READ, ..); is 
essentially read-only and any attempt to write to it will cause 
immediate crash with segmentation violation. It would be very 
desirable in this case to return something that could be cast to 
immutable(type)[] but not type[].
This is what I tried to find out. Thank you for the help.