Struct Header 

struct Header {
    major: u8,
    argument: u64,
    width: usize,
}

A major type and the argument that followed it.

Fields

major: u8

The major type, 0 to 7.

argument: u64

The argument, whatever width it was written in.

width: usize

How many bytes the argument was written in, 0 for the inline form.

Kept because major type 7 reads it as a kind rather than a width: two, four and eight bytes are a half, single and double float, and one byte is a simple value in the extended range.