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Add "future-changes" flag for CSS compatibility changes #3960
Description
Following a discussion on sass/migrator, opening a feature request for a --future-changes or --future-deprecations flag that will let consuming projects test upcoming changes to how CSS is output, such as the Mixed Declarations deprecation.
Reacted by i'DLisT, Jonas Hvid and Kieran Nichols
Activity
nex3 commentedon Oct 11, 2024
We already have a
--future-deprecationflag for opting into upcoming deprecations. The--future-changeflag would be analogous to that but for opting into upcoming breaking changes. This would specifically be limited to CSS-compatibility breaking changes where the new behavior will reinterpret older valid code to mean something else, to give users a controlled way to verify what what will change.stof commentedon Sep 17, 2025
The existing
--future-deprecationsflag turns deprecations into errors.This works fine for cases where the future behavior of a deprecation is to become an error. But it does not produce the future behavior when the future behavior is to produce a different (non-error) result.
However, the maintenance cost of
--future-changeswould be much higher, as it means that each place triggering a deprecation should add conditional code to switch to the future behavior (which is not even implemented in the codebase today), which might have lots of impact.For reference,
--future-deprecationsis mostly free in term of maintenance cost, as it is implemented in the logger instead (making the logger throw an exception when logging a deprecation configured through--future-deprecations).nex3 commentedon Sep 18, 2025
Given that we've already landed the declaration ordering change, I think this is low-priority for now. If we end up having another major semantic change for existing code, we can revisit this.