`c_ptrTo()` accepts array slices that may not be contiguous

[bradcray]

Summary of Problem

Description:

As a result of some of the discussion on #27266 starting around this comment, I found myself wondering whether c_ptrTo(), when applied to an array view whose logical elements were not contiguous, would return a pointer or not. My sense is that it probably should not, much like how we don't support it on other arrays whose data is not contiguous, like a block-distributed array. Here's an example that I think should not work:

use CTypes;
config const n = 100;
var A: [1..n, 1..n] real;
var p = c_ptrTo(A[2..n-1, 2..n-1]);
writeln(p);

As in the discussion referenced above, there's a question as to whether this should be a static check ("This is an array view, so it's not generally possible to apply c_ptrTo() to it because the data may not be contiguous") or a dynamic one (e.g., if n==3 above, the slice would be a single element, so would be contiguous).

Is this issue currently blocking your progress?

no

Configuration Information

• Output of chpl --version: 2.2.0

[bradcray]

If we decided to do the static check, I think we could outlaw this fairly easily by checking for chpl__isArrayView() in the c_ptrTo() implementation. For a dynamic check, we'd need some sort of isContiguous() check which… I'm not immediately recognizing as being something we already support.

[mstrout]

@brandonneth , would such a check (either static or dynamic) have helped with the CWorthy work?