Formatter: prefer_splitting_right_hand_side_of_assignments preview style

[MichaReiser]

Implement Black's improved linebreaks preview style. The style is gated behind the prefer_splitting_right_hand_side_of_assignments preview flag.

It seems that the right side now always gets parenthesized regardless if it makes the right fit or not (no longer has the optimisation to omit parentheses if the content still doesn't fit after adding the parentheses).

https://play.ruff.rs/36e390bc-b3d2-41b2-80d9-87608337727a

We may be able to get away by simply not returning BestFit if the expression is an assignment value.

I expect this change to improve the performance because there are fewer cases where we'll need to fall back to use Best Fitting.

Black (and our) stable style:

def f():
    """Black's `Preview.prefer_splitting_right_hand_side_of_assignments`"""
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[
        bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    ] = cccccccc.ccccccccccccc.cccccccc

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[
        bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    ] = cccccccc.ccccccccccccc().cccccccc

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[
        bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    ] = cccccccc.ccccccccccccc(d).cccccccc

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
            cccccccc.ccccccccccccc(d).cccccccc + e
    )

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
            cccccccc.ccccccccccccc.cccccccc + e
    )
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
            cccccccc.ccccccccccccc.cccccccc
            + eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
    )

    self._cache: dict[
        DependencyCacheKey, list[list[DependencyPackage]]
    ] = collections.defaultdict(list)
    self._cached_dependencies_by_level: dict[
        int, list[DependencyCacheKey]
    ] = collections.defaultdict(list)

Preview style, which consistently breaks the right side first:

def f():
    """Black's `Preview.prefer_splitting_right_hand_side_of_assignments`"""
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
        cccccccc.ccccccccccccc.cccccccc
    )

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
        cccccccc.ccccccccccccc().cccccccc
    )

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
        cccccccc.ccccccccccccc(d).cccccccc
    )

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
        cccccccc.ccccccccccccc(d).cccccccc + e
    )

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
        cccccccc.ccccccccccccc.cccccccc + e
    )
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
        cccccccc.ccccccccccccc.cccccccc
        + eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
    )

    self._cache: dict[DependencyCacheKey, list[list[DependencyPackage]]] = (
        collections.defaultdict(list)
    )
    self._cached_dependencies_by_level: dict[int, list[DependencyCacheKey]] = (
        collections.defaultdict(list)
    )

[MichaReiser]

We'll need to make best_fits_parenthesize break the right before the left when using preview style, similar to regular groups. This can be accomplished by measuring the parenthesized content instead of the unparenthesized content inside of fits_element. See #8182 (comment)

We should add a mode to BestFitsParenthesize to avoid exposing the preview flag to the printer (preview is a language specific concept)

[MichaReiser]

Some more details. psf/black#4006

Overall, the assignment will be the odd one that uses a different parenthesizing layout (not applying the logic to omit parentheses if it exceeds the line width). This will be different from clause headers and await, return, etc where expressions should only be parenthesized if necessary. This can be implemented by returning Multline for the NeedsParentheses implementation that currently return BestFit if the parent is an assignment