Stream: compiler development

Topic: Rust glue spec mem::transmute?

Sven van Caem (Sep 23 2024 at 13:25):

I've been looking at the current Rust glue spec's output for tag unions. The generated .get_discriminant() function uses mem::transmute to get at the tag union's raw bytes, then uses pointer arithmatic to pull the discriminant out:

pub fn discriminant(&self) -> discriminant_U1 {
       unsafe {
           let bytes = core::mem::transmute::<&Self, &[u8; core::mem::size_of::<Self>()]>(self);


           core::mem::transmute::<u8, discriminant_U1>(*bytes.as_ptr().add(24))
       }
   }

(same goes for set_discriminant()`)

I'm wondering why the decision was made to do this instead of accessing the discriminant field directly?

Sven van Caem (Sep 23 2024 at 13:41):

Notable here too is that the .is_<variant>() functions do access the field directly:

   pub fn is_VariantA(&self) -> bool {
       matches!(self.discriminant, discriminant_U1::VariantA)
   }
   pub fn is_VariantB(&self) -> bool {
       matches!(self.discriminant, discriminant_U1::VariantB)
   }
   pub fn is_VariantC(&self) -> bool {
       matches!(self.discriminant, discriminant_U1::VariantC)
   }

Folkert de Vries (Sep 23 2024 at 16:21):

not sure, maybe because then the same skeleton can sort of be reused? anyway, the field access method is better of course

Last updated: Jul 06 2025 at 12:14 UTC