|
|
|
|
% Alternative representations
|
|
|
|
|
|
|
|
|
|
Rust allows you to specify alternative data layout strategies from the default.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
# repr(C)
|
|
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
|
|
* DSTs, tuples, and tagged unions are not a concept in C and as such are never
|
|
|
|
|
FFI safe.
|
|
|
|
|
|
|
|
|
|
* **If the type would have any [drop flags], they will still be added**
|
|
|
|
|
|
|
|
|
|
* 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 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.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
# repr(u8), repr(u16), repr(u32), repr(u64)
|
|
|
|
|
|
|
|
|
|
These specify the size to make a C-like 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.
|
|
|
|
|
|
|
|
|
|
On non-C-like enums, this will inhibit certain optimizations like the null-
|
|
|
|
|
pointer optimization.
|
|
|
|
|
|
|
|
|
|
These reprs have no effect on a struct.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
# repr(packed)
|
|
|
|
|
|
|
|
|
|
`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.][ub loads]**
|
|
|
|
|
|
|
|
|
|
`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)`.
|
|
|
|
|
|
|
|
|
|
[drop flags]: drop-flags.html
|
|
|
|
|
[ub loads]: https://github.com/rust-lang/rust/issues/27060
|
