Fix 23491 - Nonsensical deprecation message when using delegate

[dkorpel]

The error could still be more informative, but that requires more work. This removes the nonsensical parts of the error.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23491	normal	Nonsensical deprecation message when using delegate

Testing this PR locally

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

dub run digger -- build "master + dmd#14649"

[dkorpel]

It's a bit ugly to avoid run-time string concatenation.

[maxhaton]

Is it always evaluated regardless if there's an error?

[dkorpel]

Yes

[maxhaton]

Can this be easily rectified with a delegate? Its better to avoid the whole operation than do a limbo dance avoiding expensive cycles (i.e. allow the code to be simple and avoid the work entirely)

[dkorpel]

Such a delegate would then create a closure on the heap. I wish there was a standard type for error descriptions in DMD, but since we're currently stuck with printf-style statements, that's what setUnsafeDIP1000 is also based on.

[maxhaton]

God forbid a single memory allocation? It's all on the heap already, the GC really struggles with dmd code.

[dkorpel]

In any case, this PR is not about reworking the signature of setUnsafePreview.