[Issue 24619] Allow pointer slicing for slices of length 0 or 1

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24619

--- Comment #2 from Bolpat <qs.il.paperinik at gmail.com> ---
(In reply to Dennis from comment #1)
> Similar situation to this:
> 
> https://github.com/dlang/dmd/pull/15581#issuecomment-1738649867

It has some vague similarity, but AFAICT, PR 15581 is almost the reverse, going
from slice to pointer to the first element. The `@safe` way to get a pointer to
the first element is `&xs[0]`. If `xs` has compile-time-known length, surely
the compiler does not do a run-time check for index bounds.

On the other hand, going from pointer to 1-element slice, best one can do is
`(() @trusted => ptr[0 .. 1])()`, and that is unnecessary. I don’t know about
the semantics about a slice with a `null` pointer component and a length of 1,
but my bet would be that it should be exactly as any other `null` dereference.
Any index except `0` would throw an `ArrayIndexError` and the index `0` is a
`null` dereference.

> Do you have a use case?

The use case is passing a single value to a function that expects a slice of
such values. While one can create a length-1 array with a copy of the value,
not all values are copyable and not all values are cheap to copy.

I can’t imagine a use case for `[0 .. 0]` though. I just included that one
because it’s obviously safe and if we make an exception for `[0 .. 1]`, it
would be weird to disallow it.

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