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A general description is here.
The primitive functions ‘drop()’ and ‘unclass()’ now avoid duplicating their data for atomic vectors that are large enough, by returning ‘ALTREP’ wrapper objects with adjusted attributes. R-level assignments to change attributes will also use wrapper objects to avoid duplicating data for larger atomic vectors. R functions like ‘structure()’ and ‘unname()’ will therefore not duplicate data in these settings. Generic vectors as produced by ‘list()’ are not yet covered by this optimization but may be in due course.