> if optional were a general sum type you wouldn't be able to make these optimizations easily without extra information
Rust has these optimizations (called "niche optimizations") for all sum types. If a type has any unused or invalid bit patterns, then those can be used for enum discriminants, e.g.:
- References cannot be null, so the zero value is a niche
- References must be aligned properly for the target type, so a reference to a type with alignment 4 has a niche in the bottom 2 bits
- bool only uses two values of the 256 in a byte, so the other 254 form a niche
There's limitations though, in that you still must be able to create and pass around pointers to values contained within enum, and so the representation of a type cannot change just because it's placed within an enum. So, for example, the following enum is one byte in size:
enum Foo {
A(bool),
B
}
Variant A uses the valid bool values 0 and 1, whereas variant B uses some other bit pattern (maybe 2).
But this enum must be two bytes in size:
enum Foo {
A(bool),
B(bool)
}
...because bool always has bit patterns 0 and 1, so it's not possible for an invalid value for A's fields to hold a valid value for B's fields.
You also can't stuff niches in padding bytes between struct fields, because code that operates on the struct is allowed to clobber the padding.