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 #[rustc_pub_transparent]
140 #[repr(transparent)]
141 pub struct PhantomCovariantLifetime<'a>(PhantomCovariant<&'a ()>);
142 /// Zero-sized type used to mark a lifetime as contravariant.
143 ///
144 /// Contravariant lifetimes must live at most as long as declared. See [the reference][1] for
145 /// more information.
146 ///
147 /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
148 ///
149 /// ## Layout
150 ///
151 /// For all `'a`, the following are guaranteed:
152 /// * `size_of::<PhantomContravariantLifetime<'a>>() == 0`
153 /// * `align_of::<PhantomContravariantLifetime<'a>>() == 1`
154 #[rustc_pub_transparent]
155 #[repr(transparent)]
156 pub struct PhantomContravariantLifetime<'a>(PhantomContravariant<&'a ()>);
157 /// Zero-sized type used to mark a lifetime as invariant.
158 ///
159 /// Invariant lifetimes must be live for the exact length declared, neither shorter nor longer.
160 /// See [the reference][1] for more information.
161 ///
162 /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
163 ///
164 /// ## Layout
165 ///
166 /// For all `'a`, the following are guaranteed:
167 /// * `size_of::<PhantomInvariantLifetime<'a>>() == 0`
168 /// * `align_of::<PhantomInvariantLifetime<'a>>() == 1`
169 #[rustc_pub_transparent]
170 #[repr(transparent)]
171 pub struct PhantomInvariantLifetime<'a>(PhantomInvariant<&'a ()>);
172}
173
174phantom_type! {
175 /// Zero-sized type used to mark a type parameter as covariant.
176 ///
177 /// Types used as part of the return value from a function are covariant. If the type is _also_
178 /// passed as a parameter then it is [invariant][PhantomInvariant]. See [the reference][1] for
179 /// more information.
180 ///
181 /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
182 ///
183 /// ## Layout
184 ///
185 /// For all `T`, the following are guaranteed:
186 /// * `size_of::<PhantomCovariant<T>>() == 0`
187 /// * `align_of::<PhantomCovariant<T>>() == 1`
188 #[rustc_pub_transparent]
189 #[repr(transparent)]
190 pub struct PhantomCovariant<T>(PhantomData<fn() -> T>);
191 /// Zero-sized type used to mark a type parameter as contravariant.
192 ///
193 /// Types passed as arguments to a function are contravariant. If the type is _also_ part of the
194 /// return value from a function then it is [invariant][PhantomInvariant]. See [the
195 /// reference][1] for more information.
196 ///
197 /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
198 ///
199 /// ## Layout
200 ///
201 /// For all `T`, the following are guaranteed:
202 /// * `size_of::<PhantomContravariant<T>>() == 0`
203 /// * `align_of::<PhantomContravariant<T>>() == 1`
204 #[rustc_pub_transparent]
205 #[repr(transparent)]
206 pub struct PhantomContravariant<T>(PhantomData<fn(T)>);
207 /// Zero-sized type used to mark a type parameter as invariant.
208 ///
209 /// Types that are both passed as an argument _and_ used as part of the return value from a
210 /// function are invariant. See [the reference][1] for more information.
211 ///
212 /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
213 ///
214 /// ## Layout
215 ///
216 /// For all `T`, the following are guaranteed:
217 /// * `size_of::<PhantomInvariant<T>>() == 0`
218 /// * `align_of::<PhantomInvariant<T>>() == 1`
219 #[rustc_pub_transparent]
220 #[repr(transparent)]
221 pub struct PhantomInvariant<T>(PhantomData<fn(T) -> T>);
222}
223
224mod sealed {
225 pub trait Sealed {
226 const VALUE: Self;
227 }
228}
229
230/// A marker trait for phantom variance types.
231pub trait Variance: sealed::Sealed + Default {}
232
233/// Construct a variance marker; equivalent to [`Default::default`].
234///
235/// This type can be any of the following. You generally should not need to explicitly name the
236/// type, however.
237///
238/// - [`PhantomCovariant`]
239/// - [`PhantomContravariant`]
240/// - [`PhantomInvariant`]
241/// - [`PhantomCovariantLifetime`]
242/// - [`PhantomContravariantLifetime`]
243/// - [`PhantomInvariantLifetime`]
244///
245/// # Example
246///
247/// ```rust
248/// #![feature(phantom_variance_markers)]
249///
250/// use core::marker::{PhantomCovariant, variance};
251///
252/// struct BoundFn<F, P, R>
253/// where
254/// F: Fn(P) -> R,
255/// {
256/// function: F,
257/// parameter: P,
258/// return_value: PhantomCovariant<R>,
259/// }
260///
261/// let bound_fn = BoundFn {
262/// function: core::convert::identity,
263/// parameter: 5u8,
264/// return_value: variance(),
265/// };
266/// ```
267pub const fn variance<T>() -> T
268where
269 T: Variance,
270{
271 T::VALUE
272}
273