D safety! New Feature?

[ag0aep6g via Digitalmars-d]

On 08/03/2016 09:33 PM, Mark J Twain wrote:
> The built in array is mutable and exposes the same interface for the
> immutable copy. The only difference is that the immutable copy is marked
> immutable.

I don't understand. An immutable array does not let you overwrite 
elements. A mutable array does. So they have different interfaces, don't 
they?

> My method changes the interface. An "immutable" type has an immutable
> interface while a mutable type has a mutable interface.
>
> For simple types, and value types, there is obviously no advantage...
> their interfaces are essentially empty/DNE.

Can you show some example code where there is an advantage over built-in 
arrays?

[...]
>> As far as I see, "marking for reuse" is practically the same as
>> freeing here. If Data were static, there could be a difference.
>>
>> For built-in arrays, you can mark an array for reuse by setting the
>> length to 0 and calling assumeSafeAppend, like so:
>
> No again. Data in the example above is a local variable that allocates
> memory that would generally be free'ed at the end of the function call.
> Hence every call to foo results in a malloc/free pair for Data.  By
> reusing the memory, if possible, one can potentially skip the
> malloc/free.  Therefore instead of potentially thrashing the memory
> pool, after a few runs of foo, Data would generally not allocate.
>
[...]
> Your assumeSafeAppend works while the object is in existence. This thing
> works between the existence of the object.

The allocator (e.g. the GC) is free to reuse memory it obtained from the 
operating system. So repeatedly allocating and freeing the same amount 
of memory can already be faster than one might think.

Of course, the compiler is also free to reuse stuff, if it's guaranteed 
that no reference escapes the function. I don't expect dmd to do stuff 
like this. ldc or gdc might.

What do Mutable/ImmutableArray enable beyond this? What do they do that 
built-in arrays don't? Again, example code would help doofuses like me 
make sense of this.