feat: Implement/fix unary minus operator -pl.col(...)

[stinodego]

Closes #6192

Changes

• Implement the Neg operator in Rust, dispatch to it from Python
• Turn __pos__ into a no-op

Point of discussion: for unsigned integers, should we raise (like now) or should we cast it to some supertype, e.g. cast UInt8 to Int16?

[orlp]

One thing I would like to note is that overflow on negation isn't unique to the unsigned types. E.g. for Int8 you also find that -(-128) 128 (overflow) == -128.

>>> pl.Series([-1, -128], dtype=pl.Int8)
shape: (2,)
Series: '' [i8]
[
        -1
        -128
]
>>> -_
shape: (2,)
Series: '' [i8]
[
        1
        -128
]

[stinodego]

One thing I would like to note is that overflow on negation isn't unique to the unsigned types.

I'm aware of this. In this PR it panicks with the message "attempt to negate with overflow" - there's a test for this. I think that behavior is fine as it is only the lower bound value that causes problems.

[ritchie46]

Great addition @stinodego. This primitive was long overdue.

[ritchie46]

One rebase away....

[stinodego]

Rebased 👍 I just want to note that this does break any workflow that relied on doing -expr on expressions of unsigned data types. It used to auto-cast to a supertype, now it raises. Could be considered a fix, could be considered breaking. I'll let you be the judge.

[ritchie46]

I consider it a fix. The previous behavior was unintentional.