Why are immutable array literals heap allocated?

[Patrick Schluter]

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.