Rust allows you to specify alternative data layout strategies from the default.

This is the most important repr. It has fairly simple intent: do what C does. The order, size, and alignment of fields is exactly what you would expect from C or C++. Any type you expect to pass through an FFI boundary should have repr(C), as C is the lingua-franca of the programming world. This is also necessary to soundly do more elaborate tricks with data layout such as reinterpreting values as a different type.

However, the interaction with Rust's more exotic data layout features must be kept in mind. Due to its dual purpose as "for FFI" and "for layout control", repr(C) can be applied to types that will be nonsensical or problematic if passed through the FFI boundary.

• ZSTs are still zero-sized, even though this is not a standard behavior in C, and is explicitly contrary to the behavior of an empty type in C++, which still consumes a byte of space.

• DST pointers (fat pointers), tuples, and enums with fields are not a concept in C, and as such are never FFI-safe.

• As an exception to the rule on enum with data, Option<&T> is FFI-safe if *const T is FFI-safe, and they have the same representation, using the null-pointer optimization: Some<&T> is represented as the pointer, and None is represented as a null pointer. (This rule applies to any enum defined the same way as libcore's Option type, and using the default repr.)

• Tuple structs are like structs with regards to repr(C), as the only difference from a struct is that the fields aren’t named.

• This is equivalent to one of repr(u*) (see the next section) for enums. The chosen size is the default enum size for the target platform's C application binary interface (ABI). Note that enum representation in C is implementation defined, so this is really a "best guess". In particular, this may be incorrect when the C code of interest is compiled with certain flags.

• Field-less enums with repr(C) or repr(u*) still may not be set to an integer value without a corresponding variant, even though this is permitted behavior in C or C++. It is undefined behavior to (unsafely) construct an instance of an enum that does not match one of its variants. (This allows exhaustive matches to continue to be written and compiled as normal.)

These specify the size to make a field-less enum. If the discriminant overflows the integer it has to fit in, it will produce a compile-time error. You can manually ask Rust to allow this by setting the overflowing element to explicitly be 0. However Rust will not allow you to create an enum where two variants have the same discriminant.

The term "field-less enum" only means that the enum doesn't have data in any of its variants. A field-less enum without a repr(u*) or repr(C) is still a Rust native type, and does not have a stable ABI representation. Adding a repr causes it to be treated exactly like the specified integer size for ABI purposes.

Any enum with fields is a Rust type with no guaranteed ABI (even if the only data is PhantomData or something else with zero size).

Adding an explicit repr to an enum suppresses the null-pointer optimization.

These reprs have no effect on a struct.

repr(packed) forces Rust to strip any padding, and only align the type to a byte. This may improve the memory footprint, but will likely have other negative side-effects.

In particular, most architectures strongly prefer values to be aligned. This may mean the unaligned loads are penalized (x86), or even fault (some ARM chips). For simple cases like directly loading or storing a packed field, the compiler might be able to paper over alignment issues with shifts and masks. However if you take a reference to a packed field, it's unlikely that the compiler will be able to emit code to avoid an unaligned load.

As of Rust 1.0 this can cause undefined behavior.

repr(packed) is not to be used lightly. Unless you have extreme requirements, this should not be used.

This repr is a modifier on repr(C) and repr(rust).