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% The Rust Design FAQ

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This document describes decisions that were arrived at after lengthy discussion and experimenting with alternatives. Please do not propose reversing them unless you have a new, extremely compelling argument. Note that this document specifically talks about the *language* and not any library or implementation.

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A few general guidelines define the philosophy:

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- [Memory safety][mem] must never be compromised - [Abstraction][abs] should be zero-cost, while still maintaining safety - Practicality is key

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[mem]: http://en.wikipedia.org/wiki/Memory_safety [abs]: http://en.wikipedia.org/wiki/Abstraction_%28computer_science%29

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# Semantics

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## Data layout is unspecified

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In the general case, `enum` and `struct` layout is undefined. This allows the compiler to potentially do optimizations like re-using padding for the discriminant, compacting variants of nested enums, reordering fields to remove padding, etc. `enum`s which carry no data ("C-like") are eligible to have a defined representation. Such `enum`s are easily distinguished in that they are simply a list of names that carry no data:

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```
enum CLike {
A,
B = 32,
C = 34,
D
}
```

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The [repr attribute][repr] can be applied to such `enum`s to give them the same representation as a primitive. This allows using Rust `enum`s in FFI where C `enum`s are also used, for most use cases. The attribute can also be applied to `struct`s to get the same layout as a C struct would.