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| 1 | +use repr::Maybe; |
| 2 | +use std::mem::MaybeUninit; |
| 3 | + |
| 4 | +/// # Safety |
| 5 | +/// This trait should only be implemented in `workerd-cxx` on types |
| 6 | +/// which contain a specialization of `kj::Maybe` that needs to be represented in |
| 7 | +/// Rust. |
| 8 | +unsafe trait HasNiche: Sized { |
| 9 | + fn is_niche(value: &MaybeUninit<Self>) -> bool; |
| 10 | +} |
| 11 | + |
| 12 | +// In `kj`, null pointers are considered niche, and `kj::none` if null |
| 13 | +unsafe impl<T> HasNiche for *const T { |
| 14 | + fn is_niche(value: &MaybeUninit<*const T>) -> bool { |
| 15 | + unsafe { value.assume_init().is_null() } |
| 16 | + } |
| 17 | +} |
| 18 | + |
| 19 | +unsafe impl<T> HasNiche for *mut T { |
| 20 | + fn is_niche(value: &MaybeUninit<*mut T>) -> bool { |
| 21 | + unsafe { value.assume_init().is_null() } |
| 22 | + } |
| 23 | +} |
| 24 | + |
| 25 | +// In Rust, references are not allowed to be null, so a null `MaybeUninit<&T>` is a niche |
| 26 | +unsafe impl<T> HasNiche for &T { |
| 27 | + fn is_niche(value: &MaybeUninit<&T>) -> bool { |
| 28 | + unsafe { |
| 29 | + std::mem::transmute_copy::<MaybeUninit<&T>, MaybeUninit<*const T>>(value) |
| 30 | + // This is to avoid potentially zero-initalizing a reference, which is always undefined behavior |
| 31 | + .assume_init() |
| 32 | + .is_null() |
| 33 | + } |
| 34 | + } |
| 35 | +} |
| 36 | + |
| 37 | +unsafe impl<T> HasNiche for &mut T { |
| 38 | + fn is_niche(value: &MaybeUninit<&mut T>) -> bool { |
| 39 | + unsafe { |
| 40 | + std::mem::transmute_copy::<MaybeUninit<&mut T>, MaybeUninit<*mut T>>(value) |
| 41 | + // This is to avoid potentially zero-initalizing a reference, which is always undefined behavior |
| 42 | + .assume_init() |
| 43 | + .is_null() |
| 44 | + } |
| 45 | + } |
| 46 | +} |
| 47 | + |
| 48 | +// In `kj`, `kj::Own<T>` are considered `none` in a `Maybe` if the pointer is null |
| 49 | +unsafe impl<T> HasNiche for crate::repr::Own<T> { |
| 50 | + fn is_niche(value: &MaybeUninit<crate::repr::Own<T>>) -> bool { |
| 51 | + unsafe { value.assume_init_ref().as_ptr().is_null() } |
| 52 | + } |
| 53 | +} |
| 54 | + |
| 55 | +/// Trait that is used as the bounds for what can be in a Maybe |
| 56 | +/// |
| 57 | +/// # Safety |
| 58 | +/// This trait should only be implemented from macro expansion and should |
| 59 | +/// never be manually implemented. |
| 60 | +pub unsafe trait MaybeItem: Sized { |
| 61 | + type Discriminant: Copy; |
| 62 | + fn is_some(value: &Maybe<Self>) -> bool; |
| 63 | + fn is_none(value: &Maybe<Self>) -> bool; |
| 64 | + fn from_option(value: Option<Self>) -> Maybe<Self>; |
| 65 | + fn drop_in_place(value: &mut Maybe<Self>) { |
| 66 | + if <Self as MaybeItem>::is_some(value) { |
| 67 | + unsafe { |
| 68 | + value.some.assume_init_drop(); |
| 69 | + } |
| 70 | + } |
| 71 | + } |
| 72 | +} |
| 73 | + |
| 74 | +/// Macro to implement [`MaybeItem`] for `T` which implment [`HasNiche`] |
| 75 | +macro_rules! impl_maybe_item_for_has_niche { |
| 76 | + ($ty:ty) => { |
| 77 | + unsafe impl<T> MaybeItem for $ty { |
| 78 | + type Discriminant = (); |
| 79 | + |
| 80 | + fn is_some(value: &Maybe<Self>) -> bool { |
| 81 | + !<$ty as HasNiche>::is_niche(&value.some) |
| 82 | + } |
| 83 | + |
| 84 | + fn is_none(value: &Maybe<Self>) -> bool { |
| 85 | + <$ty as HasNiche>::is_niche(&value.some) |
| 86 | + } |
| 87 | + |
| 88 | + fn from_option(value: Option<Self>) -> Maybe<Self> { |
| 89 | + match value { |
| 90 | + None => Maybe { |
| 91 | + is_set: (), |
| 92 | + some: MaybeUninit::zeroed(), |
| 93 | + }, |
| 94 | + Some(val) => Maybe { |
| 95 | + is_set: (), |
| 96 | + some: MaybeUninit::new(val), |
| 97 | + } |
| 98 | + } |
| 99 | + } |
| 100 | + } |
| 101 | + }; |
| 102 | + ($ty:ty, $($tail:ty),+) => { |
| 103 | + impl_maybe_item_for_has_niche!($ty); |
| 104 | + impl_maybe_item_for_has_niche!($($tail),*); |
| 105 | + }; |
| 106 | +} |
| 107 | + |
| 108 | +/// Macro to implement [`MaybeItem`] for primitives |
| 109 | +macro_rules! impl_maybe_item_for_primitive { |
| 110 | + ($ty:ty) => { |
| 111 | + unsafe impl MaybeItem for $ty { |
| 112 | + type Discriminant = bool; |
| 113 | + |
| 114 | + fn is_some(value: &Maybe<Self>) -> bool { |
| 115 | + value.is_set |
| 116 | + } |
| 117 | + |
| 118 | + fn is_none(value: &Maybe<Self>) -> bool { |
| 119 | + !value.is_set |
| 120 | + } |
| 121 | + |
| 122 | + fn from_option(value: Option<Self>) -> Maybe<Self> { |
| 123 | + match value { |
| 124 | + None => Maybe { |
| 125 | + is_set: false, |
| 126 | + some: MaybeUninit::zeroed(), |
| 127 | + }, |
| 128 | + Some(val) => Maybe { |
| 129 | + is_set: true, |
| 130 | + some: MaybeUninit::new(val), |
| 131 | + } |
| 132 | + } |
| 133 | + } |
| 134 | + } |
| 135 | + }; |
| 136 | + ($ty:ty, $($tail:ty),+) => { |
| 137 | + impl_maybe_item_for_primitive!($ty); |
| 138 | + impl_maybe_item_for_primitive!($($tail),*); |
| 139 | + }; |
| 140 | +} |
| 141 | + |
| 142 | +impl_maybe_item_for_has_niche!(crate::Own<T>, &T, &mut T, *const T, *mut T); |
| 143 | +impl_maybe_item_for_primitive!( |
| 144 | + u8, u16, u32, u64, u128, i8, i16, i32, i64, i128, usize, isize, bool |
| 145 | +); |
| 146 | + |
| 147 | +pub(crate) mod repr { |
| 148 | + use super::MaybeItem; |
| 149 | + use static_assertions::assert_eq_size; |
| 150 | + use std::fmt::{Debug, Display}; |
| 151 | + use std::mem::MaybeUninit; |
| 152 | + |
| 153 | + /// A [`Maybe`] represents bindings to the `kj::Maybe` class. |
| 154 | + /// It is an optional type, but represented using a struct, for alignment with kj. |
| 155 | + /// |
| 156 | + /// # Layout |
| 157 | + /// In kj, `Maybe` has 3 specializations, one without niche value optimization, and |
| 158 | + /// two with it. In order to maintain an identical layout in Rust, we include an associated type |
| 159 | + /// in the [`MaybeItem`] trait, which determines the discriminant of the `Maybe<T: MaybeItem>`. |
| 160 | + /// |
| 161 | + /// ## Niche Value Optimization |
| 162 | + /// This discriminant is used in tandem with the [`crate::maybe::HasNiche`] to implement |
| 163 | + /// [`MaybeItem`] properly for values which have a niche, which uses a discriminant of [`()`], |
| 164 | + /// the unit type. All other types use [`bool`]. |
| 165 | + #[repr(C)] |
| 166 | + pub struct Maybe<T: MaybeItem> { |
| 167 | + pub(super) is_set: T::Discriminant, |
| 168 | + pub(super) some: MaybeUninit<T>, |
| 169 | + } |
| 170 | + |
| 171 | + assert_eq_size!(Maybe<isize>, [usize; 2]); |
| 172 | + assert_eq_size!(Maybe<&isize>, usize); |
| 173 | + assert_eq_size!(Maybe<crate::Own<isize>>, [usize; 2]); |
| 174 | + assert_eq_size!(Maybe<*const isize>, usize); |
| 175 | + |
| 176 | + impl<T: MaybeItem> Maybe<T> { |
| 177 | + /// # Safety |
| 178 | + /// This function shouldn't be used except by macro generation. |
| 179 | + pub unsafe fn is_set(&self) -> T::Discriminant { |
| 180 | + self.is_set |
| 181 | + } |
| 182 | + |
| 183 | + /// # Safety |
| 184 | + /// This function shouldn't be used except by macro generation. |
| 185 | + pub unsafe fn from_parts_unchecked( |
| 186 | + is_set: T::Discriminant, |
| 187 | + some: MaybeUninit<T>, |
| 188 | + ) -> Maybe<T> { |
| 189 | + Maybe { is_set, some } |
| 190 | + } |
| 191 | + |
| 192 | + pub fn is_some(&self) -> bool { |
| 193 | + T::is_some(self) |
| 194 | + } |
| 195 | + |
| 196 | + pub fn is_none(&self) -> bool { |
| 197 | + T::is_none(self) |
| 198 | + } |
| 199 | + } |
| 200 | + |
| 201 | + impl<T: MaybeItem> From<Maybe<T>> for Option<T> { |
| 202 | + fn from(value: Maybe<T>) -> Self { |
| 203 | + if value.is_some() { |
| 204 | + // We can't move out of value so we copy it and forget it in |
| 205 | + // order to perform a "manual" move out of value |
| 206 | + let ret = unsafe { Some(value.some.assume_init_read()) }; |
| 207 | + std::mem::forget(value); |
| 208 | + ret |
| 209 | + } else { |
| 210 | + None |
| 211 | + } |
| 212 | + } |
| 213 | + } |
| 214 | + |
| 215 | + impl<T: MaybeItem> From<Option<T>> for Maybe<T> { |
| 216 | + fn from(value: Option<T>) -> Self { |
| 217 | + <T as MaybeItem>::from_option(value) |
| 218 | + } |
| 219 | + } |
| 220 | + |
| 221 | + impl<T: MaybeItem + Debug> Debug for Maybe<T> { |
| 222 | + fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { |
| 223 | + if self.is_none() { |
| 224 | + write!(f, "kj::None") |
| 225 | + } else { |
| 226 | + write!(f, "kj::Some({:?})", unsafe { self.some.assume_init_ref() }) |
| 227 | + } |
| 228 | + } |
| 229 | + } |
| 230 | + |
| 231 | + impl<T: MaybeItem + Display> Display for Maybe<T> { |
| 232 | + fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { |
| 233 | + if self.is_none() { |
| 234 | + write!(f, "kj::None") |
| 235 | + } else { |
| 236 | + write!(f, "kj::Some({})", unsafe { self.some.assume_init_ref() }) |
| 237 | + } |
| 238 | + } |
| 239 | + } |
| 240 | + |
| 241 | + impl<T: MaybeItem> Drop for Maybe<T> { |
| 242 | + fn drop(&mut self) { |
| 243 | + T::drop_in_place(self); |
| 244 | + } |
| 245 | + } |
| 246 | +} |
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