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% Alternative representations
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Rust allows you to specify alternative data layout strategies from the default.
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# repr(C)
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This is the most important `repr`. It has fairly simple intent: do what C does.
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The order, size, and alignment of fields is exactly what you would expect from C
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or C++. Any type you expect to pass through an FFI boundary should have
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`repr(C)`, as C is the lingua-franca of the programming world. This is also
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necessary to soundly do more elaborate tricks with data layout such as
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reinterpreting values as a different type.
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However, the interaction with Rust's more exotic data layout features must be
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kept in mind. Due to its dual purpose as "for FFI" and "for layout control",
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`repr(C)` can be applied to types that will be nonsensical or problematic if
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passed through the FFI boundary.
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* ZSTs are still zero-sized, even though this is not a standard behaviour in
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C, and is explicitly contrary to the behaviour of an empty type in C++, which
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still consumes a byte of space.
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* DSTs, tuples, and tagged unions are not a concept in C and as such are never
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FFI safe.
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* **If the type would have any [drop flags][], they will still be added**
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* This is equivalent to one of `repr(u*)` (see the next section) for enums. The
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chosen size is the default enum size for the target platform's C ABI. Note that
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enum representation in C is implementation defined, so this is really a "best
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guess". In particular, this may be incorrect when the C code of interest is
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compiled with certain flags.
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# repr(u8), repr(u16), repr(u32), repr(u64)
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These specify the size to make a C-like enum. If the discriminant overflows the
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integer it has to fit in, it will produce a compile-time error. You can manually
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ask Rust to allow this by setting the overflowing element to explicitly be 0.
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However Rust will not allow you to create an enum where two variants have the
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same discriminant.
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On non-C-like enums, this will inhibit certain optimizations like the null-
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pointer optimization.
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These reprs have no effect on a struct.
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# repr(packed)
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`repr(packed)` forces rust to strip any padding, and only align the type to a
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byte. This may improve the memory footprint, but will likely have other negative
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side-effects.
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In particular, most architectures *strongly* prefer values to be aligned. This
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may mean the unaligned loads are penalized (x86), or even fault (some ARM
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chips). For simple cases like directly loading or storing a packed field, the
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compiler might be able to paper over alignment issues with shifts and masks.
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However if you take a reference to a packed field, it's unlikely that the
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compiler will be able to emit code to avoid an unaligned load.
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**[As of Rust 1.0 this can cause undefined behaviour.][ub loads]**
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`repr(packed)` is not to be used lightly. Unless you have extreme requirements,
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this should not be used.
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This repr is a modifier on `repr(C)` and `repr(rust)`.
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[drop flags]: drop-flags.html
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[ub loads]: https://github.com/rust-lang/rust/issues/27060
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