src/doc/complement-design-faq.md

   1 % The Rust Design FAQ
   2
   3 This document describes decisions that were arrived at after lengthy discussion and
   4 experimenting with alternatives. Please do not propose reversing them unless
   5 you have a new, extremely compelling argument. Note that this document
   6 specifically talks about the *language* and not any library or implementation.
   7
   8 A few general guidelines define the philosophy:
   9
  10 - [Memory safety][mem] must never be compromised
  11 - [Abstraction][abs] should be zero-cost, while still maintaining safety
  12 - Practicality is key
  13
  14 [mem]: http://en.wikipedia.org/wiki/Memory_safety
  15 [abs]: http://en.wikipedia.org/wiki/Abstraction_%28computer_science%29
  16
  17 # Semantics
  18
  19 ## Data layout is unspecified
  20
  21 In the general case, `enum` and `struct` layout is undefined. This allows the
  22 compiler to potentially do optimizations like re-using padding for the
  23 discriminant, compacting variants of nested enums, reordering fields to remove
  24 padding, etc. `enum`s which carry no data ("C-like") are eligible to have a
  25 defined representation. Such `enum`s are easily distinguished in that they are
  26 simply a list of names that carry no data:
  27
  28 ```
  29 enum CLike {
  30     A,
  31     B = 32,
  32     C = 34,
  33     D
  34 }
  35 ```
  36
  37 The [repr attribute][repr] can be applied to such `enum`s to give them the same
  38 representation as a primitive. This allows using Rust `enum`s in FFI where C
  39 `enum`s are also used, for most use cases. The attribute can also be applied
  40 to `struct`s to get the same layout as a C struct would.
  41
  42 [repr]: reference.html#ffi-attributes
  43
  44 ## There is no GC
  45
  46 A language that requires a GC is a language that opts into a larger, more
  47 complex runtime than Rust cares for. Rust is usable on bare metal with no
  48 extra runtime. Additionally, garbage collection is frequently a source of
  49 non-deterministic behavior. Rust provides the tools to make using a GC
  50 possible and even pleasant, but it should not be a requirement for
  51 implementing the language.
  52
  53 ## Non-`Sync` `static mut` is unsafe
  54
  55 Types which are [`Sync`][sync] are thread-safe when multiple shared
  56 references to them are used concurrently. Types which are not `Sync` are not
  57 thread-safe, and thus when used in a global require unsafe code to use.
  58
  59 [sync]: core/marker/trait.Sync.html
  60
  61 ### If mutable static items that implement `Sync` are safe, why is taking &mut SHARABLE unsafe?
  62
  63 Having multiple aliasing `&mut T`s is never allowed. Due to the nature of
  64 globals, the borrow checker cannot possibly ensure that a static obeys the
  65 borrowing rules, so taking a mutable reference to a static is always unsafe.
  66
  67 ## There is no life before or after main (no static ctors/dtors)
  68
  69 Globals can not have a non-constant-expression constructor and cannot have a
  70 destructor at all. This is an opinion of the language. Static constructors are
  71 undesirable because they can slow down program startup. Life before main is
  72 often considered a misfeature, never to be used. Rust helps this along by just
  73 not having the feature.
  74
  75 See [the C++ FQA][fqa]  about the "static initialization order fiasco", and
  76 [Eric Lippert's blog][elp] for the challenges in C#, which also has this
  77 feature.
  78
  79 A nice replacement is the [lazy constructor macro][lcm] by [Marvin
  80 Löbel][kim].
  81
  82 [fqa]: https://mail.mozilla.org/pipermail/rust-dev/2013-April/003815.html
  83 [elp]: http://ericlippert.com/2013/02/06/static-constructors-part-one/
  84 [lcm]: https://gist.github.com/Kimundi/8782487
  85 [kim]: https://github.com/Kimundi
  86
  87 ## The language does not require a runtime
  88
  89 See the above entry on GC. Requiring a runtime limits the utility of the
  90 language, and makes it undeserving of the title "systems language". All Rust
  91 code should need to run is a stack.
  92
  93 ## `match` must be exhaustive
  94
  95 `match` being exhaustive has some useful properties. First, if every
  96 possibility is covered by the `match`, adding further variants to the `enum`
  97 in the future will prompt a compilation failure, rather than runtime panic.
  98 Second, it makes cost explicit. In general, the only safe way to have a
  99 non-exhaustive match would be to panic the thread if nothing is matched, though
 100 it could fall through if the type of the `match` expression is `()`. This sort
 101 of hidden cost and special casing is against the language's philosophy. It's
 102 easy to ignore certain cases by using the `_` wildcard:
 103
 104 ```rust,ignore
 105 match val.do_something() {
 106     Cat(a) => { /* ... */ }
 107     _      => { /* ... */ }
 108 }
 109 ```
 110
 111 [#3101][iss] is the issue that proposed making this the only behavior, with
 112 rationale and discussion.
 113
 114 [iss]: https://github.com/rust-lang/rust/issues/3101
 115
 116 ## No guaranteed tail-call optimization
 117
 118 In general, tail-call optimization is not guaranteed: see [here][tml] for a
 119 detailed explanation with references. There is a [proposed extension][tce] that
 120 would allow tail-call elimination in certain contexts. The compiler is still
 121 free to optimize tail-calls [when it pleases][sco], however.
 122
 123 [tml]: https://mail.mozilla.org/pipermail/rust-dev/2013-April/003557.html
 124 [sco]: http://llvm.org/docs/CodeGenerator.html#sibling-call-optimization
 125 [tce]: https://github.com/rust-lang/rfcs/pull/81
 126
 127 ## No constructors
 128
 129 Functions can serve the same purpose as constructors without adding any
 130 language complexity.
 131
 132 ## No copy constructors
 133
 134 Types which implement [`Copy`][copy], will do a standard C-like "shallow copy"
 135 with no extra work (similar to "plain old data" in C++). It is impossible to
 136 implement `Copy` types that require custom copy behavior. Instead, in Rust
 137 "copy constructors" are created by implementing the [`Clone`][clone] trait,
 138 and explicitly calling the `clone` method. Making user-defined copy operators
 139 explicit surfaces the underlying complexity, forcing the developer to opt-in
 140 to potentially expensive operations.
 141
 142 [copy]: core/marker/trait.Copy.html
 143 [clone]: core/clone/trait.Clone.html
 144
 145 ## No move constructors
 146
 147 Values of all types are moved via `memcpy`. This makes writing generic unsafe
 148 code much simpler since assignment, passing and returning are known to never
 149 have a side effect like unwinding.
 150
 151 # Syntax
 152
 153 ## Macros require balanced delimiters
 154
 155 This is to make the language easier to parse for machines. Since the body of a
 156 macro can contain arbitrary tokens, some restriction is needed to allow simple
 157 non-macro-expanding lexers and parsers. This comes in the form of requiring
 158 that all delimiters be balanced.
 159
 160 ## `->` for function return type
 161
 162 This is to make the language easier to parse for humans, especially in the face
 163 of higher-order functions. `fn foo<T>(f: fn(int): int, fn(T): U): U` is not
 164 particularly easy to read.
 165
 166 ## Why is `let` used to introduce variables?
 167
 168 Instead of the term "variable", we use "variable bindings". The
 169 simplest way for creating a binding is by using the `let` syntax.
 170 Other ways include `if let`, `while let`, and `match`. Bindings also
 171 exist in function argument positions.
 172
 173 Bindings always happen in pattern matching positions, and it's also Rust's way
 174 to declare mutability. One can also re-declare mutability of a binding in
 175 pattern matching. This is useful to avoid unnecessary `mut` annotations. An
 176 interesting historical note is that Rust comes, syntactically, most closely
 177 from ML, which also uses `let` to introduce bindings.
 178
 179 See also [a long thread][alt] on renaming `let mut` to `var`.
 180
 181 [alt]: https://mail.mozilla.org/pipermail/rust-dev/2014-January/008319.html
 182
 183 ## Why no `--x` or `x++`?
 184
 185 Preincrement and postincrement, while convenient, are also fairly complex. They
 186 require knowledge of evaluation order, and often lead to subtle bugs and
 187 undefined behavior in C and C++. `x = x + 1` or `x += 1` is only slightly
 188 longer, but unambiguous.