Which language constructs could be done as a library if we accept some breaking changes?

[Guillaume Piolat]

On Tuesday, 30 November 2021 at 14:46:59 UTC, Ola Fosheim Grøstad 
wrote:
>
> There should absolutely be an upgrade path

Such as with -preview/-revert then?

> For files ending in ".d" emulate most of the current behaviour. 
> For files ending in ".d3" provide the full cleanup.

Can you give an example of breaking changes that this .d3 trick 
allows, that cannot be covered by -preview/-revert?