// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 or the MIT license // , at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! Primitive traits and marker types representing basic 'kinds' of types. //! //! Rust types can be classified in various useful ways according to //! intrinsic properties of the type. These classifications, often called //! 'kinds', are represented as traits. //! //! They cannot be implemented by user code, but are instead implemented //! by the compiler automatically for the types to which they apply. //! //! Marker types are special types that are used with unsafe code to //! inform the compiler of special constraints. Marker types should //! only be needed when you are creating an abstraction that is //! implemented using unsafe code. In that case, you may want to embed //! some of the marker types below into your type. #![stable(feature = "rust1", since = "1.0.0")] use clone::Clone; /// Types able to be transferred across thread boundaries. #[unstable(feature = "core", reason = "will be overhauled with new lifetime rules; see RFC 458")] #[lang="send"] #[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"] pub unsafe trait Send: 'static { // empty. } /// Types with a constant size known at compile-time. #[stable(feature = "rust1", since = "1.0.0")] #[lang="sized"] #[rustc_on_unimplemented = "`{Self}` does not have a constant size known at compile-time"] pub trait Sized { // Empty. } /// Types that can be copied by simply copying bits (i.e. `memcpy`). /// /// By default, variable bindings have 'move semantics.' In other /// words: /// /// ``` /// #[derive(Debug)] /// struct Foo; /// /// let x = Foo; /// /// let y = x; /// /// // `x` has moved into `y`, and so cannot be used /// /// // println!("{:?}", x); // error: use of moved value /// ``` /// /// However, if a type implements `Copy`, it instead has 'copy semantics': /// /// ``` /// // we can just derive a `Copy` implementation /// #[derive(Debug, Copy)] /// struct Foo; /// /// let x = Foo; /// /// let y = x; /// /// // `y` is a copy of `x` /// /// println!("{:?}", x); // A-OK! /// ``` /// /// It's important to note that in these two examples, the only difference is if you are allowed to /// access `x` after the assignment: a move is also a bitwise copy under the hood. /// /// ## When can my type be `Copy`? /// /// A type can implement `Copy` if all of its components implement `Copy`. For example, this /// `struct` can be `Copy`: /// /// ``` /// struct Point { /// x: i32, /// y: i32, /// } /// ``` /// /// A `struct` can be `Copy`, and `i32` is `Copy`, so therefore, `Point` is eligible to be `Copy`. /// /// ``` /// # struct Point; /// struct PointList { /// points: Vec, /// } /// ``` /// /// The `PointList` `struct` cannot implement `Copy`, because `Vec` is not `Copy`. If we /// attempt to derive a `Copy` implementation, we'll get an error. /// /// ```text /// error: the trait `Copy` may not be implemented for this type; field `points` does not implement /// `Copy` /// ``` /// /// ## How can I implement `Copy`? /// /// There are two ways to implement `Copy` on your type: /// /// ``` /// #[derive(Copy)] /// struct MyStruct; /// ``` /// /// and /// /// ``` /// struct MyStruct; /// impl Copy for MyStruct {} /// ``` /// /// There is a small difference between the two: the `derive` strategy will also place a `Copy` /// bound on type parameters, which isn't always desired. /// /// ## When can my type _not_ be `Copy`? /// /// Some types can't be copied safely. For example, copying `&mut T` would create an aliased /// mutable reference, and copying `String` would result in two attempts to free the same buffer. /// /// Generalizing the latter case, any type implementing `Drop` can't be `Copy`, because it's /// managing some resource besides its own `size_of::()` bytes. /// /// ## When should my type be `Copy`? /// /// Generally speaking, if your type _can_ implement `Copy`, it should. There's one important thing /// to consider though: if you think your type may _not_ be able to implement `Copy` in the future, /// then it might be prudent to not implement `Copy`. This is because removing `Copy` is a breaking /// change: that second example would fail to compile if we made `Foo` non-`Copy`. #[stable(feature = "rust1", since = "1.0.0")] #[lang="copy"] pub trait Copy { // Empty. } /// Types that can be safely shared between threads when aliased. /// /// The precise definition is: a type `T` is `Sync` if `&T` is /// thread-safe. In other words, there is no possibility of data races /// when passing `&T` references between threads. /// /// As one would expect, primitive types like `u8` and `f64` are all /// `Sync`, and so are simple aggregate types containing them (like /// tuples, structs and enums). More instances of basic `Sync` types /// include "immutable" types like `&T` and those with simple /// inherited mutability, such as `Box`, `Vec` and most other /// collection types. (Generic parameters need to be `Sync` for their /// container to be `Sync`.) /// /// A somewhat surprising consequence of the definition is `&mut T` is /// `Sync` (if `T` is `Sync`) even though it seems that it might /// provide unsynchronised mutation. The trick is a mutable reference /// stored in an aliasable reference (that is, `& &mut T`) becomes /// read-only, as if it were a `& &T`, hence there is no risk of a data /// race. /// /// Types that are not `Sync` are those that have "interior /// mutability" in a non-thread-safe way, such as `Cell` and `RefCell` /// in `std::cell`. These types allow for mutation of their contents /// even when in an immutable, aliasable slot, e.g. the contents of /// `&Cell` can be `.set`, and do not ensure data races are /// impossible, hence they cannot be `Sync`. A higher level example /// of a non-`Sync` type is the reference counted pointer /// `std::rc::Rc`, because any reference `&Rc` can clone a new /// reference, which modifies the reference counts in a non-atomic /// way. /// /// For cases when one does need thread-safe interior mutability, /// types like the atomics in `std::sync` and `Mutex` & `RWLock` in /// the `sync` crate do ensure that any mutation cannot cause data /// races. Hence these types are `Sync`. /// /// Users writing their own types with interior mutability (or anything /// else that is not thread-safe) should use the `NoSync` marker type /// (from `std::marker`) to ensure that the compiler doesn't /// consider the user-defined type to be `Sync`. Any types with /// interior mutability must also use the `std::cell::UnsafeCell` wrapper /// around the value(s) which can be mutated when behind a `&` /// reference; not doing this is undefined behaviour (for example, /// `transmute`-ing from `&T` to `&mut T` is illegal). #[unstable(feature = "core", reason = "will be overhauled with new lifetime rules; see RFC 458")] #[lang="sync"] #[rustc_on_unimplemented = "`{Self}` cannot be shared between threads safely"] pub unsafe trait Sync { // Empty } /// A marker type that indicates to the compiler that the instances /// of the type itself owns instances of the type parameter `T`. /// /// This is used to indicate that one or more instances of the type /// `T` could be dropped when instances of the type itself is dropped, /// though that may not be apparent from the other structure of the /// type itself. For example, the type may hold a `*mut T`, which the /// compiler does not automatically treat as owned. #[unstable(feature = "core", reason = "Newly added to deal with scoping and destructor changes")] #[lang="phantom_data"] #[derive(PartialEq, Eq, PartialOrd, Ord)] pub struct PhantomData; impl Copy for PhantomData {} impl Clone for PhantomData { fn clone(&self) -> PhantomData { *self } } /// A marker type whose type parameter `T` is considered to be /// covariant with respect to the type itself. This is (typically) /// used to indicate that an instance of the type `T` is being stored /// into memory and read from, even though that may not be apparent. /// /// For more information about variance, refer to this Wikipedia /// article . /// /// *Note:* It is very unusual to have to add a covariant constraint. /// If you are not sure, you probably want to use `InvariantType`. /// /// # Example /// /// Given a struct `S` that includes a type parameter `T` /// but does not actually *reference* that type parameter: /// /// ```ignore /// use std::mem; /// /// struct S { x: *() } /// fn get(s: &S) -> T { /// unsafe { /// let x: *T = mem::transmute(s.x); /// *x /// } /// } /// ``` /// /// The type system would currently infer that the value of /// the type parameter `T` is irrelevant, and hence a `S` is /// a subtype of `S>` (or, for that matter, `S` for /// any `U`). But this is incorrect because `get()` converts the /// `*()` into a `*T` and reads from it. Therefore, we should include the /// a marker field `CovariantType` to inform the type checker that /// `S` is a subtype of `S` if `T` is a subtype of `U` /// (for example, `S<&'static int>` is a subtype of `S<&'a int>` /// for some lifetime `'a`, but not the other way around). #[unstable(feature = "core", reason = "likely to change with new variance strategy")] #[lang="covariant_type"] #[derive(PartialEq, Eq, PartialOrd, Ord)] pub struct CovariantType; impl Copy for CovariantType {} impl Clone for CovariantType { fn clone(&self) -> CovariantType { *self } } /// A marker type whose type parameter `T` is considered to be /// contravariant with respect to the type itself. This is (typically) /// used to indicate that an instance of the type `T` will be consumed /// (but not read from), even though that may not be apparent. /// /// For more information about variance, refer to this Wikipedia /// article . /// /// *Note:* It is very unusual to have to add a contravariant constraint. /// If you are not sure, you probably want to use `InvariantType`. /// /// # Example /// /// Given a struct `S` that includes a type parameter `T` /// but does not actually *reference* that type parameter: /// /// ``` /// use std::mem; /// /// struct S { x: *const () } /// fn get(s: &S, v: T) { /// unsafe { /// let x: fn(T) = mem::transmute(s.x); /// x(v) /// } /// } /// ``` /// /// The type system would currently infer that the value of /// the type parameter `T` is irrelevant, and hence a `S` is /// a subtype of `S>` (or, for that matter, `S` for /// any `U`). But this is incorrect because `get()` converts the /// `*()` into a `fn(T)` and then passes a value of type `T` to it. /// /// Supplying a `ContravariantType` marker would correct the /// problem, because it would mark `S` so that `S` is only a /// subtype of `S` if `U` is a subtype of `T`; given that the /// function requires arguments of type `T`, it must also accept /// arguments of type `U`, hence such a conversion is safe. #[unstable(feature = "core", reason = "likely to change with new variance strategy")] #[lang="contravariant_type"] #[derive(PartialEq, Eq, PartialOrd, Ord)] pub struct ContravariantType; impl Copy for ContravariantType {} impl Clone for ContravariantType { fn clone(&self) -> ContravariantType { *self } } /// A marker type whose type parameter `T` is considered to be /// invariant with respect to the type itself. This is (typically) /// used to indicate that instances of the type `T` may be read or /// written, even though that may not be apparent. /// /// For more information about variance, refer to this Wikipedia /// article . /// /// # Example /// /// The Cell type is an example of an `InvariantType` which uses unsafe /// code to achieve "interior" mutability: /// /// ``` /// struct Cell { value: T } /// ``` /// /// The type system would infer that `value` is only read here /// and never written, but in fact `Cell` uses unsafe code to achieve /// interior mutability. In order to get correct behavior, the /// `InvariantType` marker must be applied. #[unstable(feature = "core", reason = "likely to change with new variance strategy")] #[lang="invariant_type"] #[derive(PartialEq, Eq, PartialOrd, Ord)] pub struct InvariantType; #[unstable(feature = "core", reason = "likely to change with new variance strategy")] impl Copy for InvariantType {} #[unstable(feature = "core", reason = "likely to change with new variance strategy")] impl Clone for InvariantType { fn clone(&self) -> InvariantType { *self } } /// As `CovariantType`, but for lifetime parameters. Using /// `CovariantLifetime<'a>` indicates that it is ok to substitute /// a *longer* lifetime for `'a` than the one you originally /// started with (e.g., you could convert any lifetime `'foo` to /// `'static`). You almost certainly want `ContravariantLifetime` /// instead, or possibly `InvariantLifetime`. The only case where /// it would be appropriate is that you have a (type-casted, and /// hence hidden from the type system) function pointer with a /// signature like `fn(&'a T)` (and no other uses of `'a`). In /// this case, it is ok to substitute a larger lifetime for `'a` /// (e.g., `fn(&'static T)`), because the function is only /// becoming more selective in terms of what it accepts as /// argument. /// /// For more information about variance, refer to this Wikipedia /// article . #[unstable(feature = "core", reason = "likely to change with new variance strategy")] #[lang="covariant_lifetime"] #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)] pub struct CovariantLifetime<'a>; /// As `ContravariantType`, but for lifetime parameters. Using /// `ContravariantLifetime<'a>` indicates that it is ok to /// substitute a *shorter* lifetime for `'a` than the one you /// originally started with (e.g., you could convert `'static` to /// any lifetime `'foo`). This is appropriate for cases where you /// have an unsafe pointer that is actually a pointer into some /// memory with lifetime `'a`, and thus you want to limit the /// lifetime of your data structure to `'a`. An example of where /// this is used is the iterator for vectors. /// /// For more information about variance, refer to this Wikipedia /// article . #[unstable(feature = "core", reason = "likely to change with new variance strategy")] #[lang="contravariant_lifetime"] #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)] pub struct ContravariantLifetime<'a>; /// As `InvariantType`, but for lifetime parameters. Using /// `InvariantLifetime<'a>` indicates that it is not ok to /// substitute any other lifetime for `'a` besides its original /// value. This is appropriate for cases where you have an unsafe /// pointer that is actually a pointer into memory with lifetime `'a`, /// and this pointer is itself stored in an inherently mutable /// location (such as a `Cell`). #[unstable(feature = "core", reason = "likely to change with new variance strategy")] #[lang="invariant_lifetime"] #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)] pub struct InvariantLifetime<'a>; /// A type which is considered "not POD", meaning that it is not /// implicitly copyable. This is typically embedded in other types to /// ensure that they are never copied, even if they lack a destructor. #[unstable(feature = "core", reason = "likely to change with new variance strategy")] #[lang="no_copy_bound"] #[derive(Clone, PartialEq, Eq, PartialOrd, Ord)] pub struct NoCopy; /// A type which is considered managed by the GC. This is typically /// embedded in other types. #[unstable(feature = "core", reason = "likely to change with new variance strategy")] #[lang="managed_bound"] #[derive(Clone, PartialEq, Eq, PartialOrd, Ord)] pub struct Managed;