Inline and non-inline layouts

In the previous section we saw that fields with an isbits type are inlined in a composite type. A field whose type is mutable won't be inlined.

mutable struct Inner
    a::Int8
end

struct Outer
    inner::Inner
    b::UInt8
end

#[repr(C)]
struct Inner {
    a: i8
}

#[repr(C)]
struct Outer<'scope, 'data> {
    inner: Option<ValueRef<'scope, 'data>>,
    b: u8,
}

An unrooted reference is used instead of a managed type to represent non-inlined fields to account for mutability, which can render the field's old value unreachable. Managed types and unrooted references make use of the Option<NonNull> niche optimization to guarantee Option<ValueRef> has the same size as a pointer.¹ We'll say that instances of Outer reference managed data.

Because mutable types aren't inlined, Inner can only implement ValidLayout, not ValidField. Immutable types are normally inlined, so Outer can implement both traits. The layouts identify single types, so both types can implement ConstructType.