How do I get the size of a DST whose variable-length portion is empty?

[TheButlah]

Suppose i did:

#[repr(C, align(1)]
struct S {
  length_of_subsequent_fields: u8,
  obj_id: [u8; 4],
  access_rule: AccessRule,
}


#[repr(C, align(1)]
struct AccessRule {
  header: [u8; 4],
  extension: [u8],
}

How can I get the length of the fixed size portion of S programatically? I thought maybe I could create an AccessRule with length zero and then use core::mem::size_of_val() for the dst, but I don't see a way to do that in a const context.

[joshlf]

Outside of a const context, we'd recommend KnownLayout::size_for_metadata.

In const, it's trickier. If you're willing to rely on zerocopy's internals, which provide no stability guarantees, you can look at the associated KnownLayout::LAYOUT const.

Alternatively, you could just re-implement machinery like this, although it's pretty involved.

[TheButlah]

an alternative solution would be to create a zeroed value of the struct (with the variable length portion set to size 0) in a const context. I didn't see a way to do it, would that possible eventually?

[joshlf]

I'd have to think harder about it, but my gut tells me that this would just introduce a chicken-and-egg problem – in order to create the zeroed value, you'd need to use KnownLayout's non-const machinery (or dig into its unstable internals like LAYOUT). You can't create a DST by value – you can only create something that is Sized and happens to have the same size and then do a pointer/reference transmute to get a DST pointer/reference which points to the same memory.