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.

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