Why are immutable array literals heap allocated?

[Patrick Schluter]

On Friday, 5 July 2019 at 23:08:04 UTC, Patrick Schluter wrote:
> On Thursday, 4 July 2019 at 10:56:50 UTC, Nick Treleaven wrote:
>> immutable(int[]) f() @nogc {
>>     return [1,2];
>> }
>>
>> onlineapp.d(2): Error: array literal in `@nogc` function 
>> `onlineapp.f` may cause a GC allocation
>>
>> This makes dynamic array literals unusable with @nogc, and 
>> adds to GC pressure for no reason. What code would break if 
>> dmd used only static data for [1,2]?
>
> int[] in D is not an array but a fat pointer. When one realizes 
> that then it become quite obvious why [1,2] was allocated. 
> There is somewhere in the binary a static array [1,2] but as it 
> is assigned to a pointer to mutable data, the compiler has no 
> choice as to allocate a mutable copy of that immutable array.

and it cannot optimize it away because it doesn't know what the 
caller want to do with it. It might in another module invoke it 
and modify it, the compiler cannot tell. auto a=f(); a[0]++;