EX: Stateless Components are Zero-Sized

Consider the following (grasping at constructor parameter captures and value formatting a bit ahead of time):

#![allow(unused)]
fn main() {
use std::{fmt::Debug, mem::size_of};

asteracea::component! {
  MySize<T: Debug>(
    priv value: T,
  )()

  [
    !"I contain {:?}!"(self.value)
    " My size is " <b !(size_of::<Self>())> " bytes."
    !" I'm located at address {:p}."(self)
  ]
}

asteracea::component! {
  Container()()

  [
    <*MySize::<()> *value = {()}> "\n"
    <*MySize::<()> *value = {()}> "\n"
    <*MySize::<u8> *value = {1}> "\n"
    <*MySize::<usize> *value = {2}> "\n"
    "The container instance occupies " <b !(size_of::<Self>())> !" bytes at {:p}."(self)
  ]
}
}

I contain ()! My size is <B>0</B> bytes. I'm located at address 0x55907f620500.
I contain ()! My size is <B>0</B> bytes. I'm located at address 0x55907f620500.
I contain 1! My size is <B>1</B> bytes. I'm located at address 0x55907f620508.
I contain 2! My size is <B>8</B> bytes. I'm located at address 0x55907f620500.
The container instance occupies <B>16</B> bytes at 0x55907f620500.

The layout here is somewhat implementation-defined, but generally what you should see is that the MySize::<()> instances take up no space inside the Container instance and don't displace other children in memory.

This is because Asteracea components contain no hidden instance state, which means they are sized to content (and padding requirements), all the way down to zero. () is Rust's unit type, the most convenient zero sized type. The same applies to components without instance fields, of course.

Zero-sizing is transitive and has many interesting implications, but for our purposes the most notable one is that stateless components are almost¹ function-like at runtime. It's for this reason that Asteracea doesn't provide a special "slim" component syntax.

¹ There is likely a small amount of overhead during instantiation due to the dependency extraction system. The compiler is in theory allowed to optimise it away, but this isn't guaranteed.

If you have an idea how to make this process meaningfully conditional without changing Asteracea's macro syntax, I'd like to hear about it!