bkietz opened a new issue, #35581:
URL: https://github.com/apache/arrow/issues/35581
### Describe the bug, including details regarding any error messages,
version, and platform.
`ArraySpan::FillFromScalar` can store pointers into the structure itself
(specifically, into `ArraySpan::scratch_space`) which produces an ArraySpan
which easily be unsafely moved and copied:
```c++
ArraySpan span;
span.FillFromScalar(scalar);
UseSpan(span); // Fine; the original span is still alive.
return span; // Undefined Behavior; the returned copy views
// a stack variable whose lifetime has ended.
```
The capability to view a Scalar as an ArraySpan can be preserved and made
safer by restricting access to the span to an explicitly delineated scope:
```c++
ArraySpan::FillFromScalar(scalar, [&](ArraySpan span) {
UseSpan(span); // Fine; the viewed scratch space is alive inside this
scope.
});
```
This has the pleasant side effect of reducing the size of ArraySpan by 16
bytes.
### Component(s)
C++
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