Unions are used in C APIs so having them is nice for FFI.
Besides, enums aren't always optimal for some things. Let's say that you have an array of 32-bit values whose prime-numbered indexes contain either a signed or unsigned 32-bit integer, depending on a single global boolean tag. You can't encode that invariant into a (non-dependent) type system. If you want to do it safely, using enums, you are going to have a tag for every value, and that's going to cost you. (A bit crazy example, but bear with me.)
Unions are a way to circumvent the type system a bit. They allow you to keep track of the stuff inside memory slots using the way you see the best. But there's a reason they're unsafe - the responsibility is on you!
Rust doesn't let the programmer specify the layout of enums in detail right now, so you can't specify where the compiler should place the discriminator.
The example I care about is a word which is a pointer if the bottom bit(s) are 0, but otherwise contains a bunch of packed fields, where the least significant bits are a nonzero value I can do something useful with.
Besides, enums aren't always optimal for some things. Let's say that you have an array of 32-bit values whose prime-numbered indexes contain either a signed or unsigned 32-bit integer, depending on a single global boolean tag. You can't encode that invariant into a (non-dependent) type system. If you want to do it safely, using enums, you are going to have a tag for every value, and that's going to cost you. (A bit crazy example, but bear with me.)
Unions are a way to circumvent the type system a bit. They allow you to keep track of the stuff inside memory slots using the way you see the best. But there's a reason they're unsafe - the responsibility is on you!