[Advanced Rust] 1.14. Memory Types Pt.2 - Dynamically Sized Types and Wide Pointers, Packed Layouts, Larger Alignment for Speci…

Full title: [Advanced Rust] 1.14. Memory Types Pt.2 - Dynamically Sized Types and Wide Pointers, Packed Layouts, Larger Alignment for Specific Fields or Types, Memory Representation of Complex Types, and Repr Rust

1.14.1. repr(Rust)

Remember the example in the previous article? That example used repr(C), and the limitation of the C representation is that all fields must be placed in the same order as they are defined in the original struct.

repr(Rust) is the default representation. It intentionally provides fewer layout guarantees than repr(C): the compiler may reorder fields, and two types with the same fields in the same order are still not guaranteed to share a layout.

Because the compiler may reorder fields (for example, placing larger fields first), padding can often be reduced. In the Foo example from the previous article, one possible optimized layout needs no padding.