Description
Currently, there's a reasonably well-known optimization for two-variant enums, where one variant is a single pointer and the other has no fields. In this case, Rust uses its knowledge that 0x0
is never a valid pointer value to optimize the discriminant into the address: 0x0
is taken to mean that the non-pointer variant is the correct one.
There's a missed optimization, however, when the pointer is a fat one (e.g. a slice or a trait object) and the second variant is small (specifically, <= usize
). In this case, the enum could be optimized by having the address field zero still indicate the second variant, with the fields being stored where the size or vtable pointer would be stored on a trait object.
https://play.rust-lang.org/?gist=7398c28c7f05bc76a06a7fb6d4af40fa&version=nightly
Activity
matthiaskrgr commentedon Mar 2, 2018
@alercah hi, is the playground link correct (seems empty)?
alercah commentedon Mar 2, 2018
My bad, the correct link is https://play.rust-lang.org/?gist=7398c28c7f05bc76a06a7fb6d4af40fa&version=nightly (updated the original comment as well).
steveklabnik commentedon May 14, 2020
Triage: no change
adwinwhite commentedon Sep 19, 2024
This case is optimized now, probably by #94075.