rust-lang/rust — pull request #15406 by luqmana (2014-07-04 08:32:59+00:00)
Title: librustc: Apply null pointer optimization to slices, closures & trait objects.

Extend the null ptr optimization to work with slices, closures, procs, & trait objects by using the internal pointers as the discriminant.

This decreases the size of `Option<&[int]>` (and similar) by one word.