1 // Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Primitive traits and marker types representing basic 'kinds' of types.
13 //! Rust types can be classified in various useful ways according to
14 //! intrinsic properties of the type. These classifications, often called
15 //! 'kinds', are represented as traits.
17 #![stable(feature = "rust1", since = "1.0.0")]
25 /// Types able to be transferred across thread boundaries.
26 #[stable(feature = "rust1", since = "1.0.0")]
28 #[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"]
29 pub unsafe trait Send {
33 unsafe impl Send for .. { }
35 impl<T> !Send for *const T { }
36 impl<T> !Send for *mut T { }
38 /// Types with a constant size known at compile-time.
40 /// All type parameters which can be bounded have an implicit bound of `Sized`. The special syntax
41 /// `?Sized` can be used to remove this bound if it is not appropriate.
45 /// struct Bar<T: ?Sized>(T);
47 /// // struct FooUse(Foo<[i32]>); // error: Sized is not implemented for [i32]
48 /// struct BarUse(Bar<[i32]>); // OK
50 #[stable(feature = "rust1", since = "1.0.0")]
52 #[rustc_on_unimplemented = "`{Self}` does not have a constant size known at compile-time"]
53 #[fundamental] // for Default, for example, which requires that `[T]: !Default` be evaluatable
58 /// Types that can be "unsized" to a dynamically sized type.
59 #[unstable(feature = "unsize", issue = "27732")]
61 pub trait Unsize<T: ?Sized> {
65 /// Types that can be copied by simply copying bits (i.e. `memcpy`).
67 /// By default, variable bindings have 'move semantics.' In other
78 /// // `x` has moved into `y`, and so cannot be used
80 /// // println!("{:?}", x); // error: use of moved value
83 /// However, if a type implements `Copy`, it instead has 'copy semantics':
86 /// // we can just derive a `Copy` implementation
87 /// #[derive(Debug, Copy, Clone)]
94 /// // `y` is a copy of `x`
96 /// println!("{:?}", x); // A-OK!
99 /// It's important to note that in these two examples, the only difference is if you are allowed to
100 /// access `x` after the assignment: a move is also a bitwise copy under the hood.
102 /// ## When can my type be `Copy`?
104 /// A type can implement `Copy` if all of its components implement `Copy`. For example, this
105 /// `struct` can be `Copy`:
114 /// A `struct` can be `Copy`, and `i32` is `Copy`, so therefore, `Point` is eligible to be `Copy`.
118 /// struct PointList {
119 /// points: Vec<Point>,
123 /// The `PointList` `struct` cannot implement `Copy`, because `Vec<T>` is not `Copy`. If we
124 /// attempt to derive a `Copy` implementation, we'll get an error:
127 /// the trait `Copy` may not be implemented for this type; field `points` does not implement `Copy`
130 /// ## How can I implement `Copy`?
132 /// There are two ways to implement `Copy` on your type:
135 /// #[derive(Copy, Clone)]
143 /// impl Copy for MyStruct {}
144 /// impl Clone for MyStruct { fn clone(&self) -> MyStruct { *self } }
147 /// There is a small difference between the two: the `derive` strategy will also place a `Copy`
148 /// bound on type parameters, which isn't always desired.
150 /// ## When can my type _not_ be `Copy`?
152 /// Some types can't be copied safely. For example, copying `&mut T` would create an aliased
153 /// mutable reference, and copying `String` would result in two attempts to free the same buffer.
155 /// Generalizing the latter case, any type implementing `Drop` can't be `Copy`, because it's
156 /// managing some resource besides its own `size_of::<T>()` bytes.
158 /// ## When should my type be `Copy`?
160 /// Generally speaking, if your type _can_ implement `Copy`, it should. There's one important thing
161 /// to consider though: if you think your type may _not_ be able to implement `Copy` in the future,
162 /// then it might be prudent to not implement `Copy`. This is because removing `Copy` is a breaking
163 /// change: that second example would fail to compile if we made `Foo` non-`Copy`.
164 #[stable(feature = "rust1", since = "1.0.0")]
166 pub trait Copy : Clone {
170 /// Types that can be safely shared between threads when aliased.
172 /// The precise definition is: a type `T` is `Sync` if `&T` is
173 /// thread-safe. In other words, there is no possibility of data races
174 /// when passing `&T` references between threads.
176 /// As one would expect, primitive types like `u8` and `f64` are all
177 /// `Sync`, and so are simple aggregate types containing them (like
178 /// tuples, structs and enums). More instances of basic `Sync` types
179 /// include "immutable" types like `&T` and those with simple
180 /// inherited mutability, such as `Box<T>`, `Vec<T>` and most other
181 /// collection types. (Generic parameters need to be `Sync` for their
182 /// container to be `Sync`.)
184 /// A somewhat surprising consequence of the definition is `&mut T` is
185 /// `Sync` (if `T` is `Sync`) even though it seems that it might
186 /// provide unsynchronized mutation. The trick is a mutable reference
187 /// stored in an aliasable reference (that is, `& &mut T`) becomes
188 /// read-only, as if it were a `& &T`, hence there is no risk of a data
191 /// Types that are not `Sync` are those that have "interior
192 /// mutability" in a non-thread-safe way, such as `Cell` and `RefCell`
193 /// in `std::cell`. These types allow for mutation of their contents
194 /// even when in an immutable, aliasable slot, e.g. the contents of
195 /// `&Cell<T>` can be `.set`, and do not ensure data races are
196 /// impossible, hence they cannot be `Sync`. A higher level example
197 /// of a non-`Sync` type is the reference counted pointer
198 /// `std::rc::Rc`, because any reference `&Rc<T>` can clone a new
199 /// reference, which modifies the reference counts in a non-atomic
202 /// For cases when one does need thread-safe interior mutability,
203 /// types like the atomics in `std::sync` and `Mutex` & `RWLock` in
204 /// the `sync` crate do ensure that any mutation cannot cause data
205 /// races. Hence these types are `Sync`.
207 /// Any types with interior mutability must also use the `std::cell::UnsafeCell`
208 /// wrapper around the value(s) which can be mutated when behind a `&`
209 /// reference; not doing this is undefined behavior (for example,
210 /// `transmute`-ing from `&T` to `&mut T` is invalid).
211 #[stable(feature = "rust1", since = "1.0.0")]
213 #[rustc_on_unimplemented = "`{Self}` cannot be shared between threads safely"]
214 pub unsafe trait Sync {
218 unsafe impl Sync for .. { }
220 impl<T> !Sync for *const T { }
221 impl<T> !Sync for *mut T { }
225 impl<T:?Sized> Hash for $t<T> {
227 fn hash<H: Hasher>(&self, _: &mut H) {
231 impl<T:?Sized> cmp::PartialEq for $t<T> {
232 fn eq(&self, _other: &$t<T>) -> bool {
237 impl<T:?Sized> cmp::Eq for $t<T> {
240 impl<T:?Sized> cmp::PartialOrd for $t<T> {
241 fn partial_cmp(&self, _other: &$t<T>) -> Option<cmp::Ordering> {
242 Option::Some(cmp::Ordering::Equal)
246 impl<T:?Sized> cmp::Ord for $t<T> {
247 fn cmp(&self, _other: &$t<T>) -> cmp::Ordering {
252 impl<T:?Sized> Copy for $t<T> { }
254 impl<T:?Sized> Clone for $t<T> {
255 fn clone(&self) -> $t<T> {
262 /// `PhantomData<T>` allows you to describe that a type acts as if it stores a value of type `T`,
263 /// even though it does not. This allows you to inform the compiler about certain safety properties
266 /// # A ghastly note 👻👻👻
268 /// Though they both have scary names, `PhantomData<T>` and 'phantom types' are related, but not
269 /// identical. Phantom types are a more general concept that don't require `PhantomData<T>` to
270 /// implement, but `PhantomData<T>` is the most common way to implement them in a correct manner.
274 /// ## Unused lifetime parameter
276 /// Perhaps the most common time that `PhantomData` is required is
277 /// with a struct that has an unused lifetime parameter, typically as
278 /// part of some unsafe code. For example, here is a struct `Slice`
279 /// that has two pointers of type `*const T`, presumably pointing into
280 /// an array somewhere:
283 /// struct Slice<'a, T> {
289 /// The intention is that the underlying data is only valid for the
290 /// lifetime `'a`, so `Slice` should not outlive `'a`. However, this
291 /// intent is not expressed in the code, since there are no uses of
292 /// the lifetime `'a` and hence it is not clear what data it applies
293 /// to. We can correct this by telling the compiler to act *as if* the
294 /// `Slice` struct contained a borrowed reference `&'a T`:
297 /// use std::marker::PhantomData;
299 /// struct Slice<'a, T:'a> {
302 /// phantom: PhantomData<&'a T>
306 /// This also in turn requires that we annotate `T:'a`, indicating
307 /// that `T` is a type that can be borrowed for the lifetime `'a`.
309 /// ## Unused type parameters
311 /// It sometimes happens that there are unused type parameters that
312 /// indicate what type of data a struct is "tied" to, even though that
313 /// data is not actually found in the struct itself. Here is an
314 /// example where this arises when handling external resources over a
315 /// foreign function interface. `PhantomData<T>` can prevent
316 /// mismatches by enforcing types in the method implementations:
319 /// # trait ResType { fn foo(&self); }
320 /// # struct ParamType;
321 /// # mod foreign_lib {
322 /// # pub fn new(_: usize) -> *mut () { 42 as *mut () }
323 /// # pub fn do_stuff(_: *mut (), _: usize) {}
325 /// # fn convert_params(_: ParamType) -> usize { 42 }
326 /// use std::marker::PhantomData;
329 /// struct ExternalResource<R> {
330 /// resource_handle: *mut (),
331 /// resource_type: PhantomData<R>,
334 /// impl<R: ResType> ExternalResource<R> {
335 /// fn new() -> ExternalResource<R> {
336 /// let size_of_res = mem::size_of::<R>();
337 /// ExternalResource {
338 /// resource_handle: foreign_lib::new(size_of_res),
339 /// resource_type: PhantomData,
343 /// fn do_stuff(&self, param: ParamType) {
344 /// let foreign_params = convert_params(param);
345 /// foreign_lib::do_stuff(self.resource_handle, foreign_params);
350 /// ## Indicating ownership
352 /// Adding a field of type `PhantomData<T>` also indicates that your
353 /// struct owns data of type `T`. This in turn implies that when your
354 /// struct is dropped, it may in turn drop one or more instances of
355 /// the type `T`, though that may not be apparent from the other
356 /// structure of the type itself. This is commonly necessary if the
357 /// structure is using a raw pointer like `*mut T` whose referent
358 /// may be dropped when the type is dropped, as a `*mut T` is
359 /// otherwise not treated as owned.
361 /// If your struct does not in fact *own* the data of type `T`, it is
362 /// better to use a reference type, like `PhantomData<&'a T>`
363 /// (ideally) or `PhantomData<*const T>` (if no lifetime applies), so
364 /// as not to indicate ownership.
365 #[lang = "phantom_data"]
366 #[stable(feature = "rust1", since = "1.0.0")]
367 pub struct PhantomData<T:?Sized>;
369 impls! { PhantomData }
372 use super::{Send, Sync, Sized};
374 unsafe impl<'a, T: Sync + ?Sized> Send for &'a T {}
375 unsafe impl<'a, T: Send + ?Sized> Send for &'a mut T {}
378 /// A marker trait indicates a type that can be reflected over. This
379 /// trait is implemented for all types. Its purpose is to ensure that
380 /// when you write a generic function that will employ reflection,
381 /// that must be reflected (no pun intended) in the generic bounds of
382 /// that function. Here is an example:
385 /// #![feature(reflect_marker)]
386 /// use std::marker::Reflect;
387 /// use std::any::Any;
388 /// fn foo<T:Reflect+'static>(x: &T) {
389 /// let any: &Any = x;
390 /// if any.is::<u32>() { println!("u32"); }
394 /// Without the declaration `T:Reflect`, `foo` would not type check
395 /// (note: as a matter of style, it would be preferable to write
396 /// `T:Any`, because `T:Any` implies `T:Reflect` and `T:'static`, but
397 /// we use `Reflect` here to show how it works). The `Reflect` bound
398 /// thus serves to alert `foo`'s caller to the fact that `foo` may
399 /// behave differently depending on whether `T=u32` or not. In
400 /// particular, thanks to the `Reflect` bound, callers know that a
401 /// function declared like `fn bar<T>(...)` will always act in
402 /// precisely the same way no matter what type `T` is supplied,
403 /// because there are no bounds declared on `T`. (The ability for a
404 /// caller to reason about what a function may do based solely on what
405 /// generic bounds are declared is often called the ["parametricity
408 /// [1]: http://en.wikipedia.org/wiki/Parametricity
409 #[rustc_reflect_like]
410 #[unstable(feature = "reflect_marker",
411 reason = "requires RFC and more experience",
413 #[rustc_on_unimplemented = "`{Self}` does not implement `Any`; \
414 ensure all type parameters are bounded by `Any`"]
417 impl Reflect for .. { }