Emphasis on 'builtin' :) For example, Zig's slice type is a 'builtin struct' which contains a pointer and size, the builtin error union type is made of an erro

Emphasis on 'builtin' :) For example, Zig's slice type is a 'builtin struct' which contains a pointer and size, the builtin error union type is made of an error code and the actual result type. These things are not provided by the Zig standard libary, but instead built into the language. In contrast, C's builtin types are all 'primitives' (pointers, integers and floats) - with the arguable exception of VLAs, which have been made non-mandatory in C11.