rust/src/libcore/kinds.rs

// Copyright 2012 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 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

/*!
Primitive traits 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.

*/

/// Types able to be transferred across task boundaries.
#[lang="send"]
pub trait Send {
    // empty.
}

/// Types with a constant size known at compile-time.
#[lang="sized"]
pub trait Sized {
    // Empty.
}

/// Types that can be copied by simply copying bits (i.e. `memcpy`).
#[lang="copy"]
pub trait Copy {
    // Empty.
}

/// Types that can be safely shared between tasks when aliased.
///
/// The precise definition is: a type `T` is `Share` if `&T` is
/// thread-safe. In other words, there is no possibility of data races
/// when passing `&T` references between tasks.
///
/// As one would expect, primitive types like `u8` and `f64` are all
/// `Share`, and so are simple aggregate types containing them (like
/// tuples, structs and enums). More instances of basic `Share` types
/// include "immutable" types like `&T` and those with simple
/// inherited mutability, such as `Box<T>`, `Vec<T>` and most other
/// collection types. (Generic parameters need to be `Share` for their
/// container to be `Share`.)
///
/// A somewhat surprising consequence of the definition is `&mut T` is
/// `Share` (if `T` is `Share`) 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 `Share` 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<T>` can be `.set`, and do not ensure data races are
/// impossible, hence they cannot be `Share`. A higher level example
/// of a non-`Share` type is the reference counted pointer
/// `std::rc::Rc`, because any reference `&Rc<T>` 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 `Share`.
///
/// Users writing their own types with interior mutability (or anything
/// else that is not thread-safe) should use the `NoShare` marker type
/// (from `std::kinds::marker`) to ensure that the compiler doesn't
/// consider the user-defined type to be `Share`.  Any types with
/// interior mutability must also use the `std::ty::Unsafe` 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).
#[lang="share"]
pub trait Share {
    // Empty
}

/// 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.
pub mod marker {

    /// 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 <http://en.wikipedia.org/wiki/Variance_%28computer_science%29>.
    ///
    /// *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<T> { x: *() }
    /// fn get<T>(s: &S<T>) -> 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<int>` is
    /// a subtype of `S<~[int]>` (or, for that matter, `S<U>` 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<T>` to inform the type checker that
    /// `S<T>` is a subtype of `S<U>` 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).
    #[lang="covariant_type"]
    #[deriving(Eq,Clone)]
    pub struct CovariantType<T>;

    /// 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 <http://en.wikipedia.org/wiki/Variance_%28computer_science%29>.
    ///
    /// *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<T> { x: *() }
    /// fn get<T>(s: &S<T>, 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<int>` is
    /// a subtype of `S<~[int]>` (or, for that matter, `S<U>` 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<T>` is only a
    /// subtype of `S<U>` 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.
    #[lang="contravariant_type"]
    #[deriving(Eq,Clone)]
    pub struct ContravariantType<T>;

    /// 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 <http://en.wikipedia.org/wiki/Variance_%28computer_science%29>.
    ///
    /// # Example
    ///
    /// The Cell type is an example which uses unsafe code to achieve
    /// "interior" mutability:
    ///
    /// ```
    /// pub struct Cell<T> { value: T }
    /// # fn main() {}
    /// ```
    ///
    /// 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.
    #[lang="invariant_type"]
    #[deriving(Eq,Clone)]
    pub struct InvariantType<T>;

    /// 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 <http://en.wikipedia.org/wiki/Variance_%28computer_science%29>.
    #[lang="covariant_lifetime"]
    #[deriving(Eq,Clone)]
    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 <http://en.wikipedia.org/wiki/Variance_%28computer_science%29>.
    #[lang="contravariant_lifetime"]
    #[deriving(Eq,Clone)]
    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`).
    #[lang="invariant_lifetime"]
    #[deriving(Eq,Clone)]
    pub struct InvariantLifetime<'a>;

    /// A type which is considered "not sendable", meaning that it cannot
    /// be safely sent between tasks, even if it is owned. This is
    /// typically embedded in other types, such as `Gc`, to ensure that
    /// their instances remain thread-local.
    #[lang="no_send_bound"]
    #[deriving(Eq,Clone)]
    pub struct NoSend;

    /// 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.
    #[lang="no_copy_bound"]
    #[deriving(Eq,Clone)]
    pub struct NoCopy;

    /// A type which is considered "not shareable", meaning that
    /// its contents are not threadsafe, hence they cannot be
    /// shared between tasks.
    #[lang="no_share_bound"]
    #[deriving(Eq,Clone)]
    pub struct NoShare;

    /// A type which is considered managed by the GC. This is typically
    /// embedded in other types.
    #[lang="managed_bound"]
    #[deriving(Eq,Clone)]
    pub struct Managed;
}