Enum inside enum optimization allows only one "inner" variant

I tried this code:

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

enum Outer {
    C(u32),
    D(Inner),
    
    // Adding this variant increases size_of from 8 to 12
    E(Inner),
}


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

I expected to see this happen:

size_of::<Outer>() = 8

Instead, this happened:

size_of::<Outer>() = 12

Meta

rustc --version:

1.98.1 (48a229cea 2026-09-01)

1.100.0-nightly (2026-09-12 809936eac66c547a5127)

Making Outer enum 8-byte long would speed up wasmtime:

• Outer https://github.com/bytecodealliance/wasmtime/blob/817c58787f432bcdbbb87679011f72c5bc80dbda/crates/environ/src/types.rs#L461-L471
• Inner https://github.com/bytecodealliance/wasmtime/blob/817c58787f432bcdbbb87679011f72c5bc80dbda/crates/environ/src/types.rs#L344-L357

Start by compiling the minimal enum example and checking its reported size on the stable and nightly versions listed in the issue. Then inspect the referenced Wasmtime enum definitions in crates/environ/src/types.rs at lines 461-471 and 344-357, and trace the compiler's enum layout handling. Done means the reproducer achieves the expected 8-byte size without regressing relevant layout behavior.