1#![allow(clippy::legacy_numeric_constants)]
2
3use super::*;
4
5/// A trait indicating that:
6///
7/// 1. A type has an equivalent representation to some known integral type.
8/// 2. All instances of this type fall in a fixed range of values.
9/// 3. Within that range, there are no gaps.
10///
11/// This is generally useful for fieldless enums (aka "c-style" enums), however
12/// it's important that it only be used for those with an explicit `#[repr]`, as
13/// `#[repr(Rust)]` fieldess enums have an unspecified layout.
14///
15/// Additionally, you shouldn't assume that all implementations are enums. Any
16/// type which meets the requirements above while following the rules under
17/// "Safety" below is valid.
18///
19/// # Example
20///
21/// ```
22/// # use bytemuck::Contiguous;
23/// #[repr(u8)]
24/// #[derive(Debug, Copy, Clone, PartialEq)]
25/// enum Foo {
26/// A = 0,
27/// B = 1,
28/// C = 2,
29/// D = 3,
30/// E = 4,
31/// }
32/// unsafe impl Contiguous for Foo {
33/// type Int = u8;
34/// const MIN_VALUE: u8 = Foo::A as u8;
35/// const MAX_VALUE: u8 = Foo::E as u8;
36/// }
37/// assert_eq!(Foo::from_integer(3).unwrap(), Foo::D);
38/// assert_eq!(Foo::from_integer(8), None);
39/// assert_eq!(Foo::C.into_integer(), 2);
40/// ```
41/// # Safety
42///
43/// This is an unsafe trait, and incorrectly implementing it is undefined
44/// behavior.
45///
46/// Informally, by implementing it, you're asserting that `C` is identical to
47/// the integral type `C::Int`, and that every `C` falls between `C::MIN_VALUE`
48/// and `C::MAX_VALUE` exactly once, without any gaps.
49///
50/// Precisely, the guarantees you must uphold when implementing `Contiguous` for
51/// some type `C` are:
52///
53/// 1. The size of `C` and `C::Int` must be the same, and neither may be a ZST.
54/// (Note: alignment is explicitly allowed to differ)
55///
56/// 2. `C::Int` must be a primitive integer, and not a wrapper type. In the
57/// future, this may be lifted to include cases where the behavior is
58/// identical for a relevant set of traits (Ord, arithmetic, ...).
59///
60/// 3. All `C::Int`s which are in the *inclusive* range between `C::MIN_VALUE`
61/// and `C::MAX_VALUE` are bitwise identical to unique valid instances of
62/// `C`.
63///
64/// 4. There exist no instances of `C` such that their bitpatterns, when
65/// interpreted as instances of `C::Int`, fall outside of the `MAX_VALUE` /
66/// `MIN_VALUE` range -- It is legal for unsafe code to assume that if it
67/// gets a `C` that implements `Contiguous`, it is in the appropriate range.
68///
69/// 5. Finally, you promise not to provide overridden implementations of
70/// `Contiguous::from_integer` and `Contiguous::into_integer`.
71///
72/// For clarity, the following rules could be derived from the above, but are
73/// listed explicitly:
74///
75/// - `C::MAX_VALUE` must be greater or equal to `C::MIN_VALUE` (therefore, `C`
76/// must be an inhabited type).
77///
78/// - There exist no two values between `MIN_VALUE` and `MAX_VALUE` such that
79/// when interpreted as a `C` they are considered identical (by, say, match).
80pub unsafe trait Contiguous: Copy + 'static {
81 /// The primitive integer type with an identical representation to this
82 /// type.
83 ///
84 /// Contiguous is broadly intended for use with fieldless enums, and for
85 /// these the correct integer type is easy: The enum should have a
86 /// `#[repr(Int)]` or `#[repr(C)]` attribute, (if it does not, it is
87 /// *unsound* to implement `Contiguous`!).
88 ///
89 /// - For `#[repr(Int)]`, use the listed `Int`. e.g. `#[repr(u8)]` should use
90 /// `type Int = u8`.
91 ///
92 /// - For `#[repr(C)]`, use whichever type the C compiler will use to
93 /// represent the given enum. This is usually `c_int` (from `std::os::raw`
94 /// or `libc`), but it's up to you to make the determination as the
95 /// implementer of the unsafe trait.
96 ///
97 /// For precise rules, see the list under "Safety" above.
98 type Int: Copy + Ord;
99
100 /// The upper *inclusive* bound for valid instances of this type.
101 const MAX_VALUE: Self::Int;
102
103 /// The lower *inclusive* bound for valid instances of this type.
104 const MIN_VALUE: Self::Int;
105
106 /// If `value` is within the range for valid instances of this type,
107 /// returns `Some(converted_value)`, otherwise, returns `None`.
108 ///
109 /// This is a trait method so that you can write `value.into_integer()` in
110 /// your code. It is a contract of this trait that if you implement
111 /// `Contiguous` on your type you **must not** override this method.
112 ///
113 /// # Panics
114 ///
115 /// We will not panic for any correct implementation of `Contiguous`, but
116 /// *may* panic if we detect an incorrect one.
117 ///
118 /// This is undefined behavior regardless, so it could have been the nasal
119 /// demons at that point anyway ;).
120 #[inline]
121 #[cfg_attr(feature = "track_caller", track_caller)]
122 fn from_integer(value: Self::Int) -> Option<Self> {
123 // Guard against an illegal implementation of Contiguous. Annoyingly we
124 // can't rely on `transmute` to do this for us (see below), but
125 // whatever, this gets compiled into nothing in release.
126 assert!(size_of::<Self>() == size_of::<Self::Int>());
127 if Self::MIN_VALUE <= value && value <= Self::MAX_VALUE {
128 // SAFETY: We've checked their bounds (and their size, even though
129 // they've sworn under the Oath Of Unsafe Rust that that already
130 // matched) so this is allowed by `Contiguous`'s unsafe contract.
131 //
132 // So, the `transmute!`. ideally we'd use transmute here, which
133 // is more obviously safe. Sadly, we can't, as these types still
134 // have unspecified sizes.
135 Some(unsafe { transmute!(value) })
136 } else {
137 None
138 }
139 }
140
141 /// Perform the conversion from `C` into the underlying integral type. This
142 /// mostly exists otherwise generic code would need unsafe for the `value as
143 /// integer`
144 ///
145 /// This is a trait method so that you can write `value.into_integer()` in
146 /// your code. It is a contract of this trait that if you implement
147 /// `Contiguous` on your type you **must not** override this method.
148 ///
149 /// # Panics
150 ///
151 /// We will not panic for any correct implementation of `Contiguous`, but
152 /// *may* panic if we detect an incorrect one.
153 ///
154 /// This is undefined behavior regardless, so it could have been the nasal
155 /// demons at that point anyway ;).
156 #[inline]
157 #[cfg_attr(feature = "track_caller", track_caller)]
158 fn into_integer(self) -> Self::Int {
159 // Guard against an illegal implementation of Contiguous. Annoyingly we
160 // can't rely on `transmute` to do the size check for us (see
161 // `from_integer's comment`), but whatever, this gets compiled into
162 // nothing in release. Note that we don't check the result of cast
163 assert!(size_of::<Self>() == size_of::<Self::Int>());
164
165 // SAFETY: The unsafe contract requires that these have identical
166 // representations, and that the range be entirely valid. Using
167 // transmute! instead of transmute here is annoying, but is required
168 // as `Self` and `Self::Int` have unspecified sizes still.
169 unsafe { transmute!(self) }
170 }
171}
172
173macro_rules! impl_contiguous {
174 ($($src:ty as $repr:ident in [$min:expr, $max:expr];)*) => {$(
175 unsafe impl Contiguous for $src {
176 type Int = $repr;
177 const MAX_VALUE: $repr = $max;
178 const MIN_VALUE: $repr = $min;
179 }
180 )*};
181}
182
183impl_contiguous! {
184 bool as u8 in [0, 1];
185
186 u8 as u8 in [0, u8::max_value()];
187 u16 as u16 in [0, u16::max_value()];
188 u32 as u32 in [0, u32::max_value()];
189 u64 as u64 in [0, u64::max_value()];
190 u128 as u128 in [0, u128::max_value()];
191 usize as usize in [0, usize::max_value()];
192
193 i8 as i8 in [i8::min_value(), i8::max_value()];
194 i16 as i16 in [i16::min_value(), i16::max_value()];
195 i32 as i32 in [i32::min_value(), i32::max_value()];
196 i64 as i64 in [i64::min_value(), i64::max_value()];
197 i128 as i128 in [i128::min_value(), i128::max_value()];
198 isize as isize in [isize::min_value(), isize::max_value()];
199
200 NonZeroU8 as u8 in [1, u8::max_value()];
201 NonZeroU16 as u16 in [1, u16::max_value()];
202 NonZeroU32 as u32 in [1, u32::max_value()];
203 NonZeroU64 as u64 in [1, u64::max_value()];
204 NonZeroU128 as u128 in [1, u128::max_value()];
205 NonZeroUsize as usize in [1, usize::max_value()];
206}
207