Niche optimization makes use of niches, but padding bytes are neglected

[tczajka]

@Sunshine

Can niche optimization be enhanced to make use of padding bytes in the future?

It can't because padding bytes can be overwritten:

Missed layout optimization

Padding bytes cannot contain niches, because: Suppose you have an &mut Foo pointing at the value of an Option<Foo>. It's allowed to assign to this Foo by copying mem::size_of::<Foo>() bytes (4 bytes), which would then overwrite the discriminant if it were stored there. In general, only byte values that cannot be a part of a valid value of Foo can be used as a niche in Foo. Padding bytes are allowed to be anything, so they cannot make the value invalid, so the Option can't rely on the Foo user …