diff --git a/src/libcore/marker.rs b/src/libcore/marker.rs index c22c9f0d1c717..5a1a034a36358 100644 --- a/src/libcore/marker.rs +++ b/src/libcore/marker.rs @@ -8,11 +8,11 @@ // 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. +//! Primitive traits and types representing basic properties 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. +//! their intrinsic properties. These classifications are represented +//! as traits. #![stable(feature = "rust1", since = "1.0.0")] @@ -22,7 +22,21 @@ use hash::Hasher; /// Types that can be transferred across thread boundaries. /// -/// This trait is automatically derived when the compiler determines it's appropriate. +/// This trait is automatically implemented when the compiler determines it's +/// appropriate. +/// +/// An example of a non-`Send` type is the reference-counting pointer +/// [`rc::Rc`][rc]. If two threads attempt to clone `Rc`s that point to the same +/// reference-counted value, they might try to update the reference count at the +/// same time, which is [undefined behavior][ub] because `Rc` doesn't use atomic +/// operations. Its cousin [`sync::Arc`][arc] does use atomic operations (incurring +/// some overhead) and thus is `Send`. +/// +/// See [the Nomicon](../../nomicon/send-and-sync.html) for more details. +/// +/// [rc]: ../../std/rc/struct.Rc.html +/// [arc]: ../../std/sync/struct.Arc.html +/// [ub]: ../../reference.html#behavior-considered-undefined #[stable(feature = "rust1", since = "1.0.0")] #[lang = "send"] #[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"] @@ -38,10 +52,10 @@ impl !Send for *const T { } #[stable(feature = "rust1", since = "1.0.0")] impl !Send for *mut T { } -/// Types with a constant size known at compile-time. +/// Types with a constant size known at compile time. /// -/// All type parameters which can be bounded have an implicit bound of `Sized`. The special syntax -/// `?Sized` can be used to remove this bound if it is not appropriate. +/// All type parameters have an implicit bound of `Sized`. The special syntax +/// `?Sized` can be used to remove this bound if it's not appropriate. /// /// ``` /// # #![allow(dead_code)] @@ -51,6 +65,26 @@ impl !Send for *mut T { } /// // struct FooUse(Foo<[i32]>); // error: Sized is not implemented for [i32] /// struct BarUse(Bar<[i32]>); // OK /// ``` +/// +/// The one exception is the implicit `Self` type of a trait, which does not +/// get an implicit `Sized` bound. This is because a `Sized` bound prevents +/// the trait from being used to form a [trait object]: +/// +/// ``` +/// # #![allow(unused_variables)] +/// trait Foo { } +/// trait Bar: Sized { } +/// +/// struct Impl; +/// impl Foo for Impl { } +/// impl Bar for Impl { } +/// +/// let x: &Foo = &Impl; // OK +/// // let y: &Bar = &Impl; // error: the trait `Bar` cannot +/// // be made into an object +/// ``` +/// +/// [trait object]: ../../book/trait-objects.html #[stable(feature = "rust1", since = "1.0.0")] #[lang = "sized"] #[rustc_on_unimplemented = "`{Self}` does not have a constant size known at compile-time"] @@ -59,14 +93,27 @@ pub trait Sized { // Empty. } -/// Types that can be "unsized" to a dynamically sized type. +/// Types that can be "unsized" to a dynamically-sized type. +/// +/// For example, the sized array type `[i8; 2]` implements `Unsize<[i8]>` and +/// `Unsize`. +/// +/// All implementations of `Unsize` are provided automatically by the compiler. +/// +/// `Unsize` is used along with [`ops::CoerceUnsized`][coerceunsized] to allow +/// "user-defined" containers such as [`rc::Rc`][rc] to contain dynamically-sized +/// types. See the [DST coercion RFC][RFC982] for more details. +/// +/// [coerceunsized]: ../ops/trait.CoerceUnsized.html +/// [rc]: ../../std/rc/struct.Rc.html +/// [RFC982]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md #[unstable(feature = "unsize", issue = "27732")] #[lang="unsize"] pub trait Unsize { // Empty. } -/// Types that can be copied by simply copying bits (i.e. `memcpy`). +/// Types whose values can be duplicated simply by copying bits. /// /// By default, variable bindings have 'move semantics.' In other /// words: @@ -87,7 +134,8 @@ pub trait Unsize { /// However, if a type implements `Copy`, it instead has 'copy semantics': /// /// ``` -/// // we can just derive a `Copy` implementation +/// // We can derive a `Copy` implementation. `Clone` is also required, as it's +/// // a supertrait of `Copy`. /// #[derive(Debug, Copy, Clone)] /// struct Foo; /// @@ -100,13 +148,59 @@ pub trait Unsize { /// 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. +/// It's important to note that in these two examples, the only difference is whether you +/// are allowed to access `x` after the assignment. Under the hood, both a copy and a move +/// can result in bits being copied in memory, although this is sometimes optimized away. +/// +/// ## How can I implement `Copy`? +/// +/// There are two ways to implement `Copy` on your type. The simplest is to use `derive`: +/// +/// ``` +/// #[derive(Copy, Clone)] +/// struct MyStruct; +/// ``` +/// +/// You can also implement `Copy` and `Clone` manually: +/// +/// ``` +/// struct MyStruct; +/// +/// impl Copy for MyStruct { } +/// +/// impl Clone for MyStruct { +/// fn clone(&self) -> MyStruct { +/// *self +/// } +/// } +/// ``` +/// +/// 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. +/// +/// ## What's the difference between `Copy` and `Clone`? +/// +/// Copies happen implicitly, for example as part of an assignment `y = x`. The behavior of +/// `Copy` is not overloadable; it is always a simple bit-wise copy. +/// +/// Cloning is an explicit action, `x.clone()`. The implementation of [`Clone`][clone] can +/// provide any type-specific behavior necessary to duplicate values safely. For example, +/// the implementation of `Clone` for [`String`][string] needs to copy the pointed-to string +/// buffer in the heap. A simple bitwise copy of `String` values would merely copy the +/// pointer, leading to a double free down the line. For this reason, `String` is `Clone` +/// but not `Copy`. +/// +/// `Clone` is a supertrait of `Copy`, so everything which is `Copy` must also implement +/// `Clone`. If a type is `Copy` then its `Clone` implementation need only return `*self` +/// (see the example above). +/// +/// [clone]: ../clone/trait.Clone.html +/// [string]: ../../std/string/struct.String.html /// /// ## 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 can be `Copy`: /// /// ``` /// # #[allow(dead_code)] @@ -116,7 +210,8 @@ pub trait Unsize { /// } /// ``` /// -/// A `struct` can be `Copy`, and `i32` is `Copy`, so therefore, `Point` is eligible to be `Copy`. +/// A struct can be `Copy`, and `i32` is `Copy`, therefore `Point` is eligible to be `Copy`. +/// By contrast, consider /// /// ``` /// # #![allow(dead_code)] @@ -126,57 +221,35 @@ pub trait Unsize { /// } /// ``` /// -/// The `PointList` `struct` cannot implement `Copy`, because [`Vec`] is not `Copy`. If we +/// The struct `PointList` cannot implement `Copy`, because [`Vec`] is not `Copy`. If we /// attempt to derive a `Copy` implementation, we'll get an error: /// /// ```text /// the trait `Copy` may not be implemented for this type; field `points` does not implement `Copy` /// ``` /// -/// ## When can my type _not_ be `Copy`? +/// ## When *can't* my type 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. +/// mutable reference. Copying [`String`] would duplicate responsibility for managing the `String`'s +/// buffer, leading to a double free. /// /// Generalizing the latter case, any type implementing [`Drop`] can't be `Copy`, because it's /// managing some resource besides its own [`size_of::()`] bytes. /// -/// ## What if I derive `Copy` on a type that can't? -/// -/// If you try to derive `Copy` on a struct or enum, you will get a compile-time error. -/// Specifically, with structs you'll get [E0204](https://doc.rust-lang.org/error-index.html#E0204) -/// and with enums you'll get [E0205](https://doc.rust-lang.org/error-index.html#E0205). -/// -/// ## 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`. +/// If you try to implement `Copy` on a struct or enum containing non-`Copy` data, you will get a +/// compile-time error. Specifically, with structs you'll get [E0204] and with enums you'll get +/// [E0205]. /// -/// ## Derivable +/// [E0204]: https://doc.rust-lang.org/error-index.html#E0204 +/// [E0205]: https://doc.rust-lang.org/error-index.html#E0205 /// -/// This trait can be used with `#[derive]` if all of its components implement `Copy` and the type. +/// ## When *should* my type be `Copy`? /// -/// ## How can I implement `Copy`? -/// -/// There are two ways to implement `Copy` on your type: -/// -/// ``` -/// #[derive(Copy, Clone)] -/// struct MyStruct; -/// ``` -/// -/// and -/// -/// ``` -/// struct MyStruct; -/// impl Copy for MyStruct {} -/// impl Clone for MyStruct { fn clone(&self) -> MyStruct { *self } } -/// ``` -/// -/// 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. +/// Generally speaking, if your type _can_ implement `Copy`, it should. Keep in mind, though, +/// that implementing `Copy` is part of the public API of your type. If the type might become +/// non-`Copy` in the future, it could be prudent to omit the `Copy` implementation now, to +/// avoid a breaking API change. /// /// [`Vec`]: ../../std/vec/struct.Vec.html /// [`String`]: ../../std/string/struct.String.html @@ -188,64 +261,74 @@ pub trait Copy : Clone { // Empty. } -/// Types that can be safely shared between threads when aliased. +/// Types for which it is safe to share references between threads. +/// +/// This trait is automatically implemented when the compiler determines +/// it's appropriate. /// /// 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 unsynchronized 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. +/// [`Send`][send]. In other words, if there is no possibility of +/// [undefined behavior][ub] (including data races) when passing +/// `&T` references between threads. +/// +/// As one would expect, primitive types like [`u8`][u8] and [`f64`][f64] +/// are all `Sync`, and so are simple aggregate types containing them, +/// like tuples, structs and enums. More examples of basic `Sync` +/// types include "immutable" types like `&T`, and those with simple +/// inherited mutability, such as [`Box`][box], [`Vec`][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 that `&mut T` +/// is `Sync` (if `T` is `Sync`) even though it seems like that might +/// provide unsynchronized mutation. The trick is that a mutable +/// reference behind a shared 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`][`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`][`Rc`], because any reference [`&Rc`][`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`][`sync`] and [`Mutex`] / [`RwLock`] in -/// the [`sync`] crate do ensure that any mutation cannot cause data -/// races. Hence these types are `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 behavior (for example, -/// [`transmute`]-ing from `&T` to `&mut T` is invalid). +/// mutability" in a non-thread-safe form, such as [`cell::Cell`][cell] +/// and [`cell::RefCell`][refcell]. These types allow for mutation of +/// their contents even through an immutable, shared reference. For +/// example the `set` method on `Cell` takes `&self`, so it requires +/// only a shared reference `&Cell`. The method performs no +/// synchronization, thus `Cell` cannot be `Sync`. /// -/// This trait is automatically derived when the compiler determines it's appropriate. +/// Another example of a non-`Sync` type is the reference-counting +/// pointer [`rc::Rc`][rc]. Given any reference `&Rc`, you can clone +/// a new `Rc`, modifying the reference counts in a non-atomic way. /// -/// [`u8`]: ../../std/primitive.u8.html -/// [`f64`]: ../../std/primitive.f64.html -/// [`Vec`]: ../../std/vec/struct.Vec.html -/// [`Box`]: ../../std/boxed/struct.Box.html -/// [`Cell`]: ../../std/cell/struct.Cell.html -/// [`RefCell`]: ../../std/cell/struct.RefCell.html -/// [`std::cell`]: ../../std/cell/index.html -/// [`.set`]: ../../std/cell/struct.Cell.html#method.set -/// [`Rc`]: ../../std/rc/struct.Rc.html -/// [`sync`]: ../../std/sync/index.html -/// [`Mutex`]: ../../std/sync/struct.Mutex.html -/// [`RwLock`]: ../../std/sync/struct.RwLock.html -/// [`std::cell::UnsafeCell`]: ../../std/cell/struct.UnsafeCell.html -/// [`transmute`]: ../../std/mem/fn.transmute.html +/// For cases when one does need thread-safe interior mutability, +/// Rust provides [atomic data types], as well as explicit locking via +/// [`sync::Mutex`][mutex] and [`sync::RWLock`][rwlock]. These types +/// ensure that any mutation cannot cause data races, hence the types +/// are `Sync`. Likewise, [`sync::Arc`][arc] provides a thread-safe +/// analogue of `Rc`. +/// +/// Any types with interior mutability must also use the +/// [`cell::UnsafeCell`][unsafecell] wrapper around the value(s) which +/// can be mutated through a shared reference. Failing to doing this is +/// [undefined behavior][ub]. For example, [`transmute`][transmute]-ing +/// from `&T` to `&mut T` is invalid. +/// +/// See [the Nomicon](../../nomicon/send-and-sync.html) for more +/// details about `Sync`. +/// +/// [send]: trait.Send.html +/// [u8]: ../../std/primitive.u8.html +/// [f64]: ../../std/primitive.f64.html +/// [box]: ../../std/boxed/struct.Box.html +/// [vec]: ../../std/vec/struct.Vec.html +/// [cell]: ../cell/struct.Cell.html +/// [refcell]: ../cell/struct.RefCell.html +/// [rc]: ../../std/rc/struct.Rc.html +/// [arc]: ../../std/sync/struct.Arc.html +/// [atomic data types]: ../sync/atomic/index.html +/// [mutex]: ../../std/sync/struct.Mutex.html +/// [rwlock]: ../../std/sync/struct.RwLock.html +/// [unsafecell]: ../cell/struct.UnsafeCell.html +/// [ub]: ../../reference.html#behavior-considered-undefined +/// [transmute]: ../../std/mem/fn.transmute.html #[stable(feature = "rust1", since = "1.0.0")] #[lang = "sync"] #[rustc_on_unimplemented = "`{Self}` cannot be shared between threads safely"] @@ -314,29 +397,30 @@ macro_rules! impls{ ) } -/// `PhantomData` allows you to describe that a type acts as if it stores a value of type `T`, -/// even though it does not. This allows you to inform the compiler about certain safety properties -/// of your code. +/// Zero-sized type used to mark things that "act like" they own a `T`. /// -/// For a more in-depth explanation of how to use `PhantomData`, please see [the Nomicon]. +/// Adding a `PhantomData` field to your type tells the compiler that your +/// type acts as though it stores a value of type `T`, even though it doesn't +/// really. This information is used when computing certain safety properties. /// -/// [the Nomicon]: ../../nomicon/phantom-data.html +/// For a more in-depth explanation of how to use `PhantomData`, please see +/// [the Nomicon](../../nomicon/phantom-data.html). /// /// # A ghastly note 👻👻👻 /// -/// Though they both have scary names, `PhantomData` and 'phantom types' are related, but not -/// identical. Phantom types are a more general concept that don't require `PhantomData` to -/// implement, but `PhantomData` is the most common way to implement them in a correct manner. +/// Though they both have scary names, `PhantomData` and 'phantom types' are +/// related, but not identical. A phantom type parameter is simply a type +/// parameter which is never used. In Rust, this often causes the compiler to +/// complain, and the solution is to add a "dummy" use by way of `PhantomData`. /// /// # Examples /// -/// ## Unused lifetime parameter +/// ## Unused lifetime parameters /// -/// Perhaps the most common time that `PhantomData` is required is -/// with a struct that has an unused lifetime parameter, typically as -/// part of some unsafe code. For example, here is a struct `Slice` -/// that has two pointers of type `*const T`, presumably pointing into -/// an array somewhere: +/// Perhaps the most common use case for `PhantomData` is a struct that has an +/// unused lifetime parameter, typically as part of some unsafe code. For +/// example, here is a struct `Slice` that has two pointers of type `*const T`, +/// presumably pointing into an array somewhere: /// /// ```ignore /// struct Slice<'a, T> { @@ -350,7 +434,7 @@ macro_rules! impls{ /// intent is not expressed in the code, since there are no uses of /// the lifetime `'a` and hence it is not clear what data it applies /// to. We can correct this by telling the compiler to act *as if* the -/// `Slice` struct contained a borrowed reference `&'a T`: +/// `Slice` struct contained a reference `&'a T`: /// /// ``` /// use std::marker::PhantomData; @@ -359,29 +443,53 @@ macro_rules! impls{ /// struct Slice<'a, T: 'a> { /// start: *const T, /// end: *const T, -/// phantom: PhantomData<&'a T> +/// phantom: PhantomData<&'a T>, /// } /// ``` /// -/// This also in turn requires that we annotate `T:'a`, indicating -/// that `T` is a type that can be borrowed for the lifetime `'a`. +/// This also in turn requires the annotation `T: 'a`, indicating +/// that any references in `T` are valid over the lifetime `'a`. +/// +/// When initializing a `Slice` you simply provide the value +/// `PhantomData` for the field `phantom`: +/// +/// ``` +/// # #![allow(dead_code)] +/// # use std::marker::PhantomData; +/// # struct Slice<'a, T: 'a> { +/// # start: *const T, +/// # end: *const T, +/// # phantom: PhantomData<&'a T>, +/// # } +/// fn borrow_vec<'a, T>(vec: &'a Vec) -> Slice<'a, T> { +/// let ptr = vec.as_ptr(); +/// Slice { +/// start: ptr, +/// end: unsafe { ptr.offset(vec.len() as isize) }, +/// phantom: PhantomData, +/// } +/// } +/// ``` /// /// ## Unused type parameters /// -/// It sometimes happens that there are unused type parameters that +/// It sometimes happens that you have unused type parameters which /// indicate what type of data a struct is "tied" to, even though that /// data is not actually found in the struct itself. Here is an -/// example where this arises when handling external resources over a -/// foreign function interface. `PhantomData` can prevent -/// mismatches by enforcing types in the method implementations: +/// example where this arises with [FFI]. The foreign interface uses +/// handles of type `*mut ()` to refer to Rust values of different +/// types. We track the Rust type using a phantom type parameter on +/// the struct `ExternalResource` which wraps a handle. +/// +/// [FFI]: ../../book/ffi.html /// /// ``` /// # #![allow(dead_code)] -/// # trait ResType { fn foo(&self); } +/// # trait ResType { } /// # struct ParamType; /// # mod foreign_lib { -/// # pub fn new(_: usize) -> *mut () { 42 as *mut () } -/// # pub fn do_stuff(_: *mut (), _: usize) {} +/// # pub fn new(_: usize) -> *mut () { 42 as *mut () } +/// # pub fn do_stuff(_: *mut (), _: usize) {} /// # } /// # fn convert_params(_: ParamType) -> usize { 42 } /// use std::marker::PhantomData; @@ -408,21 +516,20 @@ macro_rules! impls{ /// } /// ``` /// -/// ## Indicating ownership +/// ## Ownership and the drop check /// -/// Adding a field of type `PhantomData` also indicates that your -/// struct owns data of type `T`. This in turn implies that when your -/// struct is dropped, it may in turn drop one or more instances of -/// the type `T`, though that may not be apparent from the other -/// structure of the type itself. This is commonly necessary if the -/// structure is using a raw pointer like `*mut T` whose referent -/// may be dropped when the type is dropped, as a `*mut T` is -/// otherwise not treated as owned. +/// Adding a field of type `PhantomData` indicates that your +/// type owns data of type `T`. This in turn implies that when your +/// type is dropped, it may drop one or more instances of the type +/// `T`. This has bearing on the Rust compiler's [drop check] +/// analysis. /// /// If your struct does not in fact *own* the data of type `T`, it is /// better to use a reference type, like `PhantomData<&'a T>` /// (ideally) or `PhantomData<*const T>` (if no lifetime applies), so /// as not to indicate ownership. +/// +/// [drop check]: ../../nomicon/dropck.html #[lang = "phantom_data"] #[stable(feature = "rust1", since = "1.0.0")] pub struct PhantomData; @@ -438,10 +545,13 @@ mod impls { /// Types that can be reflected over. /// -/// This trait is implemented for all types. Its purpose is to ensure -/// that when you write a generic function that will employ -/// reflection, that must be reflected (no pun intended) in the -/// generic bounds of that function. Here is an example: +/// By "reflection" we mean use of the [`Any`][any] trait, or related +/// machinery such as [`TypeId`][typeid]. +/// +/// `Reflect` is implemented for all types. Its purpose is to ensure +/// that when you write a generic function that will employ reflection, +/// that must be reflected (no pun intended) in the generic bounds of +/// that function. /// /// ``` /// #![feature(reflect_marker)] @@ -455,21 +565,24 @@ mod impls { /// } /// ``` /// -/// Without the declaration `T: Reflect`, `foo` would not type check -/// (note: as a matter of style, it would be preferable to write -/// `T: Any`, because `T: Any` implies `T: Reflect` and `T: 'static`, but -/// we use `Reflect` here to show how it works). The `Reflect` bound -/// thus serves to alert `foo`'s caller to the fact that `foo` may -/// behave differently depending on whether `T = u32` or not. In -/// particular, thanks to the `Reflect` bound, callers know that a -/// function declared like `fn bar(...)` will always act in -/// precisely the same way no matter what type `T` is supplied, -/// because there are no bounds declared on `T`. (The ability for a -/// caller to reason about what a function may do based solely on what -/// generic bounds are declared is often called the ["parametricity -/// property"][1].) -/// -/// [1]: http://en.wikipedia.org/wiki/Parametricity +/// Without the bound `T: Reflect`, `foo` would not typecheck. (As +/// a matter of style, it would be preferable to write `T: Any`, +/// because `T: Any` implies `T: Reflect` and `T: 'static`, but we +/// use `Reflect` here for illustrative purposes.) +/// +/// The `Reflect` bound serves to alert `foo`'s caller to the +/// fact that `foo` may behave differently depending on whether +/// `T` is `u32` or not. The ability for a caller to reason about what +/// a function may do based solely on what generic bounds are declared +/// is often called the "[parametricity property][param]". Despite the +/// use of `Reflect`, Rust lacks true parametricity because a generic +/// function can, at the very least, call [`mem::size_of`][size_of] +/// without employing any trait bounds whatsoever. +/// +/// [any]: ../any/trait.Any.html +/// [typeid]: ../any/struct.TypeId.html +/// [param]: http://en.wikipedia.org/wiki/Parametricity +/// [size_of]: ../mem/fn.size_of.html #[rustc_reflect_like] #[unstable(feature = "reflect_marker", reason = "requires RFC and more experience",