isbits layouts

The layout of a primitive type in Julia usually matches its analog in Rust: if the DataType is Int8, a Value of this type is a pointer to an i8; if it's Float32 the Value is a pointer to an f32. There are two exceptions: Bool and Char map to types of the same name defined in jlrs, not bool and char.

Composite types are a bit more involved. isbits types are immutable types made up of primitive types and other isbits types. Their layout maps to the obvious repr(C) representation in Rust. For example, the following Rust and Julia types have the same layout in memory

struct InnerBits
    a::Int8
end

struct OuterBits
    inner::InnerBits
    b::UInt8
end

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

#[repr(C)]
struct OuterBits {
    inner: InnerBits,
    b: u8,
}

Because InnerBits and OuterBits in Rust faithfully represent the layout of their corresponding type in Julia they can implement ValidLayout. They can implement ValidField because they're inlined into the composite when used as a field type. The layouts correspond to a single type in Julia, so these types can implement ConstructType to map the Rust type to their Julia counterpart.