2594d56e32
into variance inference; fix various bugs in variance inference so that it considers the correct set of constraints; modify infer to consider the results of variance inference for type arguments.
320 lines
11 KiB
Rust
320 lines
11 KiB
Rust
// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! Primitive traits and marker types representing basic 'kinds' of types.
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//!
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//! Rust types can be classified in various useful ways according to
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//! intrinsic properties of the type. These classifications, often called
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//! 'kinds', are represented as traits.
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//!
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//! They cannot be implemented by user code, but are instead implemented
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//! by the compiler automatically for the types to which they apply.
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//!
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//! Marker types are special types that are used with unsafe code to
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//! inform the compiler of special constraints. Marker types should
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//! only be needed when you are creating an abstraction that is
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//! implemented using unsafe code. In that case, you may want to embed
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//! some of the marker types below into your type.
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#![stable(feature = "rust1", since = "1.0.0")]
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use clone::Clone;
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use cmp;
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use option::Option;
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use hash::Hash;
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use hash::Hasher;
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/// Types able to be transferred across thread boundaries.
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#[unstable(feature = "core",
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reason = "will be overhauled with new lifetime rules; see RFC 458")]
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#[lang="send"]
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#[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"]
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#[cfg(stage0)]
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pub unsafe trait Send: 'static {
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// empty.
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}
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/// Types able to be transferred across thread boundaries.
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#[unstable(feature = "core",
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reason = "will be overhauled with new lifetime rules; see RFC 458")]
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#[lang="send"]
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#[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"]
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#[cfg(not(stage0))]
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pub unsafe trait Send : MarkerTrait {
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// empty.
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}
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/// Types with a constant size known at compile-time.
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#[stable(feature = "rust1", since = "1.0.0")]
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#[lang="sized"]
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#[rustc_on_unimplemented = "`{Self}` does not have a constant size known at compile-time"]
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pub trait Sized : MarkerTrait {
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// Empty.
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}
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/// Types that can be copied by simply copying bits (i.e. `memcpy`).
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///
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/// By default, variable bindings have 'move semantics.' In other
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/// words:
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///
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/// ```
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/// #[derive(Debug)]
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/// struct Foo;
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///
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/// let x = Foo;
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///
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/// let y = x;
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///
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/// // `x` has moved into `y`, and so cannot be used
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///
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/// // println!("{:?}", x); // error: use of moved value
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/// ```
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///
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/// However, if a type implements `Copy`, it instead has 'copy semantics':
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///
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/// ```
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/// // we can just derive a `Copy` implementation
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/// #[derive(Debug, Copy)]
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/// struct Foo;
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///
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/// let x = Foo;
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///
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/// let y = x;
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///
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/// // `y` is a copy of `x`
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///
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/// println!("{:?}", x); // A-OK!
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/// ```
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///
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/// It's important to note that in these two examples, the only difference is if you are allowed to
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/// access `x` after the assignment: a move is also a bitwise copy under the hood.
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///
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/// ## When can my type be `Copy`?
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///
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/// A type can implement `Copy` if all of its components implement `Copy`. For example, this
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/// `struct` can be `Copy`:
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///
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/// ```
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/// struct Point {
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/// x: i32,
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/// y: i32,
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/// }
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/// ```
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///
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/// A `struct` can be `Copy`, and `i32` is `Copy`, so therefore, `Point` is eligible to be `Copy`.
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///
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/// ```
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/// # struct Point;
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/// struct PointList {
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/// points: Vec<Point>,
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/// }
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/// ```
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///
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/// The `PointList` `struct` cannot implement `Copy`, because `Vec<T>` is not `Copy`. If we
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/// attempt to derive a `Copy` implementation, we'll get an error.
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///
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/// ```text
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/// error: the trait `Copy` may not be implemented for this type; field `points` does not implement
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/// `Copy`
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/// ```
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///
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/// ## How can I implement `Copy`?
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///
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/// There are two ways to implement `Copy` on your type:
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///
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/// ```
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/// #[derive(Copy)]
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/// struct MyStruct;
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/// ```
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///
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/// and
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///
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/// ```
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/// struct MyStruct;
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/// impl Copy for MyStruct {}
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/// ```
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///
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/// There is a small difference between the two: the `derive` strategy will also place a `Copy`
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/// bound on type parameters, which isn't always desired.
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///
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/// ## When can my type _not_ be `Copy`?
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///
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/// Some types can't be copied safely. For example, copying `&mut T` would create an aliased
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/// mutable reference, and copying `String` would result in two attempts to free the same buffer.
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///
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/// Generalizing the latter case, any type implementing `Drop` can't be `Copy`, because it's
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/// managing some resource besides its own `size_of::<T>()` bytes.
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///
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/// ## When should my type be `Copy`?
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///
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/// Generally speaking, if your type _can_ implement `Copy`, it should. There's one important thing
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/// to consider though: if you think your type may _not_ be able to implement `Copy` in the future,
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/// then it might be prudent to not implement `Copy`. This is because removing `Copy` is a breaking
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/// change: that second example would fail to compile if we made `Foo` non-`Copy`.
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#[stable(feature = "rust1", since = "1.0.0")]
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#[lang="copy"]
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pub trait Copy : MarkerTrait {
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// Empty.
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}
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/// Types that can be safely shared between threads when aliased.
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///
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/// The precise definition is: a type `T` is `Sync` if `&T` is
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/// thread-safe. In other words, there is no possibility of data races
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/// when passing `&T` references between threads.
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///
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/// As one would expect, primitive types like `u8` and `f64` are all
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/// `Sync`, and so are simple aggregate types containing them (like
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/// tuples, structs and enums). More instances of basic `Sync` types
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/// include "immutable" types like `&T` and those with simple
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/// inherited mutability, such as `Box<T>`, `Vec<T>` and most other
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/// collection types. (Generic parameters need to be `Sync` for their
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/// container to be `Sync`.)
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///
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/// A somewhat surprising consequence of the definition is `&mut T` is
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/// `Sync` (if `T` is `Sync`) even though it seems that it might
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/// provide unsynchronised mutation. The trick is a mutable reference
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/// stored in an aliasable reference (that is, `& &mut T`) becomes
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/// read-only, as if it were a `& &T`, hence there is no risk of a data
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/// race.
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///
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/// Types that are not `Sync` are those that have "interior
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/// mutability" in a non-thread-safe way, such as `Cell` and `RefCell`
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/// in `std::cell`. These types allow for mutation of their contents
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/// even when in an immutable, aliasable slot, e.g. the contents of
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/// `&Cell<T>` can be `.set`, and do not ensure data races are
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/// impossible, hence they cannot be `Sync`. A higher level example
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/// of a non-`Sync` type is the reference counted pointer
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/// `std::rc::Rc`, because any reference `&Rc<T>` can clone a new
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/// reference, which modifies the reference counts in a non-atomic
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/// way.
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///
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/// For cases when one does need thread-safe interior mutability,
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/// types like the atomics in `std::sync` and `Mutex` & `RWLock` in
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/// the `sync` crate do ensure that any mutation cannot cause data
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/// races. Hence these types are `Sync`.
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///
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/// Users writing their own types with interior mutability (or anything
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/// else that is not thread-safe) should use the `NoSync` marker type
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/// (from `std::marker`) to ensure that the compiler doesn't
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/// consider the user-defined type to be `Sync`. Any types with
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/// interior mutability must also use the `std::cell::UnsafeCell` wrapper
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/// around the value(s) which can be mutated when behind a `&`
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/// reference; not doing this is undefined behaviour (for example,
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/// `transmute`-ing from `&T` to `&mut T` is illegal).
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#[unstable(feature = "core",
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reason = "will be overhauled with new lifetime rules; see RFC 458")]
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#[lang="sync"]
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#[rustc_on_unimplemented = "`{Self}` cannot be shared between threads safely"]
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pub unsafe trait Sync : MarkerTrait {
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// Empty
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}
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/// A type which is considered "not POD", meaning that it is not
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/// implicitly copyable. This is typically embedded in other types to
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/// ensure that they are never copied, even if they lack a destructor.
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#[unstable(feature = "core",
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reason = "likely to change with new variance strategy")]
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#[lang="no_copy_bound"]
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#[derive(Clone, PartialEq, Eq, PartialOrd, Ord)]
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pub struct NoCopy;
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/// A type which is considered managed by the GC. This is typically
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/// embedded in other types.
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#[unstable(feature = "core",
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reason = "likely to change with new variance strategy")]
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#[lang="managed_bound"]
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#[derive(Clone, PartialEq, Eq, PartialOrd, Ord)]
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pub struct Managed;
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macro_rules! impls{
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($t: ident) => (
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impl<T:?Sized, S: Hasher> Hash<S> for $t<T> {
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#[inline]
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fn hash(&self, _: &mut S) {
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}
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}
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impl<T:?Sized> cmp::PartialEq for $t<T> {
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fn eq(&self, _other: &$t<T>) -> bool {
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true
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}
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}
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impl<T:?Sized> cmp::Eq for $t<T> {
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}
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impl<T:?Sized> cmp::PartialOrd for $t<T> {
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fn partial_cmp(&self, _other: &$t<T>) -> Option<cmp::Ordering> {
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Option::Some(cmp::Ordering::Equal)
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}
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}
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impl<T:?Sized> cmp::Ord for $t<T> {
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fn cmp(&self, _other: &$t<T>) -> cmp::Ordering {
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cmp::Ordering::Equal
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}
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}
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impl<T:?Sized> Copy for $t<T> { }
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impl<T:?Sized> Clone for $t<T> {
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fn clone(&self) -> $t<T> {
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$t
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}
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}
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)
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}
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/// `MarkerTrait` is intended to be used as the supertrait for traits
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/// that don't have any methods but instead serve just to designate
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/// categories of types. An example would be the `Send` trait, which
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/// indicates types that are sendable: `Send` does not itself offer
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/// any methods, but instead is used to gate access to data.
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///
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/// FIXME. Better documentation needed here!
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pub trait MarkerTrait : PhantomFn<Self> { }
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impl<T:?Sized> MarkerTrait for T { }
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/// `PhantomFn` is a marker trait for use with traits that do not
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/// include any methods.
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///
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/// FIXME. Better documentation needed here!
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#[lang="phantom_fn"]
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pub trait PhantomFn<A:?Sized,R:?Sized=()> { }
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#[cfg(stage0)] // built into the trait matching system after stage0
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impl<A:?Sized, R:?Sized, U:?Sized> PhantomFn<A,R> for U { }
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/// Specific to stage0. You should not be seeing these docs!
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#[cfg(stage0)]
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#[lang="covariant_type"] // only relevant to stage0
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pub struct PhantomData<T:?Sized>;
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/// `PhantomData` is a way to tell the compiler about fake fields.
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/// The idea is that if the compiler encounters a `PhantomData<T>`
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/// instance, it will behave *as if* an instance of the type `T` were
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/// present for the purpose of various automatic analyses.
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///
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/// FIXME. Better documentation needed here!
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#[cfg(not(stage0))]
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#[lang="phantom_data"]
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pub struct PhantomData<T:?Sized>;
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impls! { PhantomData }
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#[cfg(not(stage0))]
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mod impls {
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use super::{Send, Sync, Sized};
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unsafe impl<'a, T: Sync + ?Sized> Send for &'a T {}
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unsafe impl<'a, T: Send + ?Sized> Send for &'a mut T {}
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}
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