# Enum Size Opt MirPass

For all variables & temporaries in the MirPass:

If its type is not an enum, ignore it.

Otherwise, check if there's a "large enough" discrepancy between the enum variant sizes.

If there is, find all `Assign` where the lhs (not the local, but the entire projection in case of `_x.foo[42]`) is of the enum type.

_x.foo = _y.bar // rhs is `Operand`

StorageLive(new_local)
// This is the tricky bit since it has to be runtime memory?
new_local = const &[size_1, size_2, ...];

This constant is a reference to an array of all variants' sizes.

Then find the size of the current variant:

discr = discriminant(_y.bar)
size = new_local[discr]

Then replace `_x.foo = _y.bar` with

_x1 = &_x.foo as *const _
_y1 = &_y.bar as *const _
copy_nonverlapping(_x1, _y1, 1)

Based on [this comment](https://github.com/rust-lang/rust/issues/54360#issuecomment-422924534).