Creating immutable arrays in @safe code

[ag0aep6g]

On 17.07.21 00:27, H. S. Teoh wrote:
> Hmm, OK. Not sure why .array isn't being inferred as unique... but yeah,
> you probably have to resort to using @trusted with .assumeUnique.

In addition to `pure`, you also need a const/immutable input and a 
mutable output, so that the output cannot be a slice of the input.

For std.array.array it might be possible to carefully apply `Unqual` to 
the element type.

I tried doing that, but `-preview=dip1000` causes trouble. This fails:

----
int[] array(const int[] input) pure nothrow @safe
{
     int[] output;
     foreach (element; input) output ~= element;
     return output;
}
void main() pure nothrow @safe
{
     const int[] c = [1, 2, 3];
     immutable int[] i = array(c);
     /* Without `-preview=dip1000`: works, because the result is unique.
     With `-preview=dip1000`: "Error: cannot implicitly convert". */
}
----

I'm not sure what's going on. `pure` being involved makes me think of 
issue 20150. But it also fails with my fix for that issue. So maybe it's 
another bug.