core: Inherit the kinds module

2 files changed, 285 insertions, 0 deletions

diff --git a/src/libcore/kinds.rs b/src/libcore/kinds.rs
new file mode 100644
index 00000000000..6ef71d3360a
--- /dev/null
+++ b/src/libcore/kinds.rs
@@ -0,0 +1,281 @@
+// 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::cast;
+    ///
+    /// struct S<T> { x: *() }
+    /// fn get<T>(s: &S<T>) -> T {
+    ///    unsafe {
+    ///        let x: *T = cast::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::cast;
+    ///
+    /// struct S<T> { x: *() }
+    /// fn get<T>(s: &S<T>, v: T) {
+    ///    unsafe {
+    ///        let x: fn(T) = cast::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 sharable", 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;
+}
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index 4f215793a38..630f84f6579 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -27,3 +27,7 @@ pub mod cast;
 pub mod intrinsics;
 pub mod mem;
 pub mod ptr;
+
+/* Core language traits */
+
+pub mod kinds;