zig/doc/codegen.md

Code Generation

Data Representation

Every type has a "handle". If a type is a simple primitive type such as i32 or f64, the handle is "by value", meaning that we pass around the value itself when we refer to a value of that type.

If a type is a container, error union, optional type, slice, or array, then its handle is a pointer, and everywhere we refer to a value of this type we refer to a pointer.

Parameters and return values are always passed as handles.

Error union types are represented as:

struct {
    error: u32,
    payload: T,
}

Optional types are represented as:

struct {
    payload: T,
    is_non_null: u1,
}

Data Optimizations

Optional pointer types are special: the 0x0 pointer value is used to represent a null pointer. Thus, instead of the struct above, optional pointer types are represented as a usize in codegen and the handle is by value.