Matching a reference to an uninhabited type

help
1

I just hit my nose on the issue, that current stable Rust's type system allows references to uninhabited types to exist.

MRE:

I have the current workaround to panic on the "unreachable" variant, but I actually hate this, since this will break my code during runtime when I implement the actual inner type of SupportedFooImplementors::B. I also don't want to take ownership of self in the trait method.
Is there a way to have my cake and eat it, i.e. check at compile time, that no such variant can exist? Sure, I can remove the B variant for now, but this is also not what I want.

2

The trick for this situation is to dereference the reference so you are matching a B instead of a &B.

match self {
    Self::A(a) => a.foo(),
    Self::B(b) => match *b {},
}

This will compile sucessfully, and be checked for exhaustiveness.

To my mind, this also ought to work since it is just as much of a dereference, but it doesn't:

match *self {
    Self::A(ref a) => a.foo(),
}
3

Thanks. I actually tried this, but was scared off by this clippy warning:

Is this actually "safe" to do? I am not going to construct such references by unsafe code, but the fact that this is considered to be generally UB kinda scares me.

4

This playground has no warnings. Could you share your code?

5

The playground's Clippy does not have nursery enabled.
Lint @kpreid 's code with nursery.

6

Lints in the nursery category are lints that have significant flaws, such as common false positives.

There is no risk in writing match *b {} unless you are writing unsafe code and b is either a raw pointer or a known-invalid reference (which should never be exposed to code not expecting it).

7

The warning's really meant for unsafe code that could construct bogus references. In safe rust you literally can't make an invalid reference to an uninhabited type, so match *b {} is fine for your purposes.