Suboptimal layout for nested enums

I tried this code:

pub enum Outer {
    A(Inner),
    B(u32, u8),
}

pub enum Inner {
    A(u32),
    B(u32),
}

fn main() {
  dbg!(std::mem::size_of::<Outer>());
}

I expected size_of::<Outer>() to be 8, indicating that rustc was able to "merge" the two enums:

• Outer::A(Inner::A) gets discriminant 0;
• Outer::B(Inner::B) gets discriminant 1;
• Outer::B gets discriminant 2;

Instead, size_of::<Outer>() is 12, indicating that no layout optimization was done.

Meta

$ rustc +nightly --version
rustc 1.92.0-nightly (fa3155a64 2025-09-30)

Reproduce the example with the stated nightly rustc version and confirm the reported 12-byte layout for Outer. Start by investigating rustc's enum layout and discriminant handling; done means the nested enums receive the described merged layout and size_of::() reports 8, with appropriate compiler tests added.