core/marker/
variance.rs

1#![unstable(feature = "phantom_variance_markers", issue = "135806")]
2
3use super::PhantomData;
4use crate::any::type_name;
5use crate::cmp::Ordering;
6use crate::fmt;
7use crate::hash::{Hash, Hasher};
8
9macro_rules! first_token {
10    ($first:tt $($rest:tt)*) => {
11        $first
12    };
13}
14
15macro_rules! phantom_type {
16    ($(
17        $(#[$attr:meta])*
18        pub struct $name:ident <$t:ident> ($($inner:tt)*);
19    )*) => {$(
20        $(#[$attr])*
21        pub struct $name<$t>($($inner)*) where T: ?Sized;
22
23        impl<T> $name<T>
24            where T: ?Sized
25        {
26            /// Constructs a new instance of the variance marker.
27            pub const fn new() -> Self {
28                Self(PhantomData)
29            }
30        }
31
32        impl<T> self::sealed::Sealed for $name<T> where T: ?Sized {
33            const VALUE: Self = Self::new();
34        }
35        impl<T> Variance for $name<T> where T: ?Sized {}
36
37        impl<T> Default for $name<T>
38            where T: ?Sized
39        {
40            fn default() -> Self {
41                Self(PhantomData)
42            }
43        }
44
45        impl<T> fmt::Debug for $name<T>
46            where T: ?Sized
47        {
48            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
49                write!(f, "{}<{}>", stringify!($name), type_name::<T>())
50            }
51        }
52
53        impl<T> Clone for $name<T>
54            where T: ?Sized
55        {
56            fn clone(&self) -> Self {
57                *self
58            }
59        }
60
61        impl<T> Copy for $name<T> where T: ?Sized {}
62
63        impl<T> PartialEq for $name<T>
64            where T: ?Sized
65        {
66            fn eq(&self, _: &Self) -> bool {
67                true
68            }
69        }
70
71        impl<T> Eq for $name<T> where T: ?Sized {}
72
73        impl<T> PartialOrd for $name<T>
74            where T: ?Sized
75        {
76            fn partial_cmp(&self, _: &Self) -> Option<Ordering> {
77                Some(Ordering::Equal)
78            }
79        }
80
81        impl<T> Ord for $name<T>
82            where T: ?Sized
83        {
84            fn cmp(&self, _: &Self) -> Ordering {
85                Ordering::Equal
86            }
87        }
88
89        impl<T> Hash for $name<T>
90            where T: ?Sized
91        {
92            fn hash<H: Hasher>(&self, _: &mut H) {}
93        }
94    )*};
95}
96
97macro_rules! phantom_lifetime {
98    ($(
99        $(#[$attr:meta])*
100        pub struct $name:ident <$lt:lifetime> ($($inner:tt)*);
101    )*) => {$(
102        $(#[$attr])*
103        #[derive(Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
104        pub struct $name<$lt>($($inner)*);
105
106        impl $name<'_> {
107            /// Constructs a new instance of the variance marker.
108            pub const fn new() -> Self {
109                Self(first_token!($($inner)*)(PhantomData))
110            }
111        }
112
113        impl self::sealed::Sealed for $name<'_> {
114            const VALUE: Self = Self::new();
115        }
116        impl Variance for $name<'_> {}
117
118        impl fmt::Debug for $name<'_> {
119            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
120                write!(f, "{}", stringify!($name))
121            }
122        }
123    )*};
124}
125
126phantom_lifetime! {
127    /// Zero-sized type used to mark a lifetime as covariant.
128    ///
129    /// Covariant lifetimes must live at least as long as declared. See [the reference][1] for more
130    /// information.
131    ///
132    /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
133    ///
134    /// ## Layout
135    ///
136    /// For all `'a`, the following are guaranteed:
137    /// * `size_of::<PhantomCovariantLifetime<'a>>() == 0`
138    /// * `align_of::<PhantomCovariantLifetime<'a>>() == 1`
139    pub struct PhantomCovariantLifetime<'a>(PhantomCovariant<&'a ()>);
140    /// Zero-sized type used to mark a lifetime as contravariant.
141    ///
142    /// Contravariant lifetimes must live at most as long as declared. See [the reference][1] for
143    /// more information.
144    ///
145    /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
146    ///
147    /// ## Layout
148    ///
149    /// For all `'a`, the following are guaranteed:
150    /// * `size_of::<PhantomContravariantLifetime<'a>>() == 0`
151    /// * `align_of::<PhantomContravariantLifetime<'a>>() == 1`
152    pub struct PhantomContravariantLifetime<'a>(PhantomContravariant<&'a ()>);
153    /// Zero-sized type used to mark a lifetime as invariant.
154    ///
155    /// Invariant lifetimes must be live for the exact length declared, neither shorter nor longer.
156    /// See [the reference][1] for more information.
157    ///
158    /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
159    ///
160    /// ## Layout
161    ///
162    /// For all `'a`, the following are guaranteed:
163    /// * `size_of::<PhantomInvariantLifetime<'a>>() == 0`
164    /// * `align_of::<PhantomInvariantLifetime<'a>>() == 1`
165    pub struct PhantomInvariantLifetime<'a>(PhantomInvariant<&'a ()>);
166}
167
168phantom_type! {
169    /// Zero-sized type used to mark a type parameter as covariant.
170    ///
171    /// Types used as part of the return value from a function are covariant. If the type is _also_
172    /// passed as a parameter then it is [invariant][PhantomInvariant]. See [the reference][1] for
173    /// more information.
174    ///
175    /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
176    ///
177    /// ## Layout
178    ///
179    /// For all `T`, the following are guaranteed:
180    /// * `size_of::<PhantomCovariant<T>>() == 0`
181    /// * `align_of::<PhantomCovariant<T>>() == 1`
182    pub struct PhantomCovariant<T>(PhantomData<fn() -> T>);
183    /// Zero-sized type used to mark a type parameter as contravariant.
184    ///
185    /// Types passed as arguments to a function are contravariant. If the type is _also_ part of the
186    /// return value from a function then it is [invariant][PhantomInvariant]. See [the
187    /// reference][1] for more information.
188    ///
189    /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
190    ///
191    /// ## Layout
192    ///
193    /// For all `T`, the following are guaranteed:
194    /// * `size_of::<PhantomContravariant<T>>() == 0`
195    /// * `align_of::<PhantomContravariant<T>>() == 1`
196    pub struct PhantomContravariant<T>(PhantomData<fn(T)>);
197    /// Zero-sized type used to mark a type parameter as invariant.
198    ///
199    /// Types that are both passed as an argument _and_ used as part of the return value from a
200    /// function are invariant. See [the reference][1] for more information.
201    ///
202    /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
203    ///
204    /// ## Layout
205    ///
206    /// For all `T`, the following are guaranteed:
207    /// * `size_of::<PhantomInvariant<T>>() == 0`
208    /// * `align_of::<PhantomInvariant<T>>() == 1`
209    pub struct PhantomInvariant<T>(PhantomData<fn(T) -> T>);
210}
211
212mod sealed {
213    pub trait Sealed {
214        const VALUE: Self;
215    }
216}
217
218/// A marker trait for phantom variance types.
219pub trait Variance: sealed::Sealed + Default {}
220
221/// Construct a variance marker; equivalent to [`Default::default`].
222///
223/// This type can be any of the following. You generally should not need to explicitly name the
224/// type, however.
225///
226/// - [`PhantomCovariant`]
227/// - [`PhantomContravariant`]
228/// - [`PhantomInvariant`]
229/// - [`PhantomCovariantLifetime`]
230/// - [`PhantomContravariantLifetime`]
231/// - [`PhantomInvariantLifetime`]
232///
233/// # Example
234///
235/// ```rust
236/// #![feature(phantom_variance_markers)]
237///
238/// use core::marker::{PhantomCovariant, variance};
239///
240/// struct BoundFn<F, P, R>
241/// where
242///     F: Fn(P) -> R,
243/// {
244///     function: F,
245///     parameter: P,
246///     return_value: PhantomCovariant<R>,
247/// }
248///
249/// let bound_fn = BoundFn {
250///     function: core::convert::identity,
251///     parameter: 5u8,
252///     return_value: variance(),
253/// };
254/// ```
255pub const fn variance<T>() -> T
256where
257    T: Variance,
258{
259    T::VALUE
260}