Fix object.__ArrayCast error message generation so it works with CTFE

[JinShil]

This is a followup to #2264

It is intended to resolve an error currently being encountered on the DMD side in dlang/dmd#9516

Error: `alloca` cannot be interpreted at compile time, because it has no available source code

[dlang-bot]

Thanks for your pull request and interest in making D better, @JinShil! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the annotated coverage diff directly on GitHub with CodeCov's browser extension
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + druntime#2531"

[thewilsonator]

Otherwise LGTM.

[jacob-carlborg]

alloca cannot be made into a template to have the source code available. Or won’t that help?

[thewilsonator]

Its a compiler intrinsic most of the time, I've no idea why or how its part of druntime.

[jacob-carlborg]

https://github.com/dlang/druntime/blob/master/src/rt/alloca.d

I guess inline assembly doesn’t work at compile time.

[JinShil]

See

druntime/src/rt/alloca.d

Line 47 in fbc76f7

* This is a 'magic' function that needs help from the compiler to

It's a magic function, given special treatment by the compiler. It is unlike any other and requires compiler assistance to work properly.

[JinShil]

Test suite is printing something like this in DMD...

/home/mike/github/JinShil/druntime/import/object.d(4772): Error: `['A', 'n', ' ', 'a', 'r', 'r', 'a', 'y', ' ', 'o', 'f', ' ', 's', 'i', 'z', 'e', ' ', '8', ' ', 'd', 'o', 'e', 's', ' ', 'n', 'o', 't', ' ', 'a', 'l', 'i', 'g', 'n', ' ', 'o', 'n', ' ', 'a', 'n', ' ', 'a', 'r', 'r', 'a', 'y', ' ', 'o', 'f', ' ', 's', 'i', 'z', 'e', ' ', '0', ',', ' ', 's', 'o', ' ', '`', 't', 'y', 'p', 'e', 'o', 'f', '(', 'n', 'u', 'l', 'l', ')', '`', ' ', 'c', 'a', 'n', 'n', 'o', 't', ' ', 'b', 'e', ' ', 'c', 'a', 's', 't', ' ', 't', 'o', ' ', '`', 's', 't', 'r', 'i', 'n', 'g', '`', '\x00', '\xff', '\xff', '\xff',

It seems to work fine at run.dlang.org, though: https://run.dlang.io/is/5a6tje

How do I get it to render as a normal string? Beware, we are already casting an array, when we're in this function, so I don't think cast(string)msg would be wise.

[jacob-carlborg]

I’ve seen the same thing with failing static asserts

[JinShil]

Yep, looks like CTFE string rendering has a problem: https://run.dlang.io/ (Sigh, I guess I need to figure out how to fix that too).

[jacob-carlborg]

Regression?

[JinShil]

Regression?

No, it appears to have been that way since ancient times. I may address it in a new PR, if it ends up blocking the DMD PR.

I think this is ready to go, but I think the DMD side requires some more work. At least it will allow me to continue with the DMD PR. If additional fixes are needed, I can submit them in followup PRs.