1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451
//! # Rust unsized enum implementation //! //! In stable Rust as of mid-2020, `?Sized` (aka unsized or trait //! object or "dynamically sized type" or DST) support is missing in //! various places: //! //! - Rust's built-in enums don't support `?Sized` variants //! //! - `Option` doesn't support `?Sized` (because it is an enum) //! //! - Rust's `union` type doesn't support `?Sized` //! //! - `MaybeUninit` also doesn't support `?Sized` (because it is //! currently built on top of `union`) //! //! So if there is a requirement for unsized types within an enum, //! some other approach must be taken. Currently this crate //! implements a single enum ([`UnsizedEnum`]), with one unsized //! variant and one sized variant, which is always returned boxed. //! (This approach can only support a *single* unsized variant, //! although it could be extended to provide additional *sized* //! variants.) //! //! The enum may be read and modified, including switching variants, //! even through a trait object reference. //! //! # Safety and soundness discussion //! //! This crate is intended to be sound, and if unsoundness can be //! demonstrated, it will be fixed (if possible) or else the API //! marked as unsafe until a safe way can be found. However right now //! the theoretical (rather than practical) soundness of the crate //! depends on aspects of Rust's safety contract which are as yet //! undecided. So if that makes you nervous, then don't use this //! crate for the time being. //! //! The boxed memory is accessed with two structures which represent //! two views of that memory: `UnsizedEnum` for the header plus the //! `V0` (unsized) variant, and `UnsizedEnum_V1` for the header plus //! the `V1` (sized) variant. `#[repr(C)]` is used to enforce the //! order of members and to ensure that the header part of the //! `UnsizedEnum` structure is compatible with `UnsizedEnum_V1`. //! //! It's necessary to include the `V0` instance directly in the //! `UnsizedEnum` structure, because its cleanup must be handled //! through the vtable. If no `V0` value is included in //! `UnsizedEnum`, it seems that the Drop handler doesn't receive a //! fat pointer, and so has no access to the vtable. However in the //! case of storing a `V1` variant, the `V0` value included in the //! `UnsizedEnum` must not be dropped because it will be invalid data //! for the `V0` type. So the `V0` value is made `ManuallyDrop` so //! that we can skip dropping that invalid `V0` data in the `V1` //! variant case. (`MaybeUninit` would be better but it doesn't //! support `?Sized` yet.) //! //! So strictly speaking in the case of storing the `V1` variant, //! because the `UnsizedEnum` struct contains `val: ManuallyDrop<V0>`, //! we're working with references to an invalid `UnsizedEnum` (invalid //! in the `val` part). However we never "produce" an invalid //! `UnsizedEnum` value. `V1` values are produced using //! `UnsizedEnum_V1`. The only code that is exposed to the entire //! invalid `UnsizedEnum` is the compiler-generated drop code. //! (Whether passing around a reference to invalid data is //! theoretically sound or not is undecided, but it seems like the //! [consensus is leaning towards it being //! sound](https://github.com/rust-lang/unsafe-code-guidelines/issues/77).) //! //! It's important that the niche-filling optimisation doesn't try to //! make use of any unused bit-patterns in the `V0` value to store //! data, because those may overwrite the value for the `V1` variant. //! However since this implementation is in total control of the //! structure and the structure is returned boxed, there is no way for //! a crate user to cause the `ManuallyDrop<V0>` value to be wrapped //! in an `enum`, so there should be no case where niche-filling would //! try to make use of the memory within the `ManuallyDrop`. So the //! compiler-generated drop code should have no reason to touch the //! `V0` variant memory in the `V1` case. So our Drop implementation //! is free to skip dropping the (invalid) `V0` value and drop the //! overlaid `V1` value instead. //! //! If Rust gains support for `?Sized` in more places, especially //! `MaybeUninit`, this implementation will be improved to use those //! features. That would also resolve the question about the //! theoretical soundness of holding a reference to invalid data. //! //! In addition it's necessary to compare vtable pointers in `set_v0`. //! This depends on the layout of fat pointers. This is much //! lower-risk, since it will fail immediately and very obviously in //! testing if the layout changes in the compiler. Also several other //! crates already depend on this layout. //! //! [`UnsizedEnum`]: struct.UnsizedEnum.html use std::alloc::Layout; use std::cell::Cell; use std::marker::PhantomData; use std::mem; use std::mem::ManuallyDrop; use std::ptr; /// An unsized enum with two variants /// /// As this is a DST, this enum can only be created on the heap. So /// for convenience, it allows a common base structure to be included /// in the header before the enum variants. Also, since a lot of /// space will generally be wasted by the discriminant due to /// alignment and packing (even if it is only a `u8`), a whole `usize` /// is used and the higher bits in the discriminant are made available /// to the caller for storage. /// /// The type parameters are: the common base type, the first variant /// (which may be unsized) and the second variant (which must be /// sized). #[repr(C)] pub struct UnsizedEnum<B, V0: ?Sized, V1> { header: Header<B>, phantomdata: PhantomData<V1>, val: ManuallyDrop<V0>, } #[repr(C)] struct UnsizedEnum_V1<B, V0: ?Sized, V1> { header: Header<B>, val: V1, phantomdata: PhantomData<V0>, } #[repr(C)] struct Header<B> { disc: usize, base: B, } // const max hack #[allow(dead_code)] const fn max(a: usize, b: usize) -> usize { [a, b][(a < b) as usize] } // If a non-fat pointer is passed, returns a null pointer #[inline(always)] fn vtable_of<T: ?Sized>(fat: &T) -> *const () { #[repr(C)] struct FatPointer { data: *const (), meta: *const (), } if mem::size_of_val(&fat) < mem::size_of::<FatPointer>() { std::ptr::null() } else { assert_eq!(mem::size_of_val(&fat), mem::size_of::<FatPointer>()); let repr = unsafe { mem::transmute_copy::<&T, FatPointer>(&fat) }; repr.meta } } impl<B, V0, V1> UnsizedEnum<B, V0, V1> { // Safe because we're using the largest alignment and the largest // size. These will be valid values for the Layout because they // come from existing types. So either we're using the size and // alignment from one type, or the size from one type and a larger // alignment from the other. Overaligning a type is safe. #[allow(dead_code)] const LAYOUT: Layout = unsafe { Layout::from_size_align_unchecked( max( mem::size_of::<UnsizedEnum<B, V0, V1>>(), mem::size_of::<UnsizedEnum_V1<B, V0, V1>>(), ), max( mem::align_of::<UnsizedEnum<B, V0, V1>>(), mem::align_of::<UnsizedEnum_V1<B, V0, V1>>(), ), ) }; // Returned memory is uninitialised unsafe fn alloc() -> *mut UnsizedEnum<B, V0, V1> { let p = std::alloc::alloc(Self::LAYOUT) as *mut UnsizedEnum<B, V0, V1>; if p.is_null() { std::alloc::handle_alloc_error(Self::LAYOUT); } p } /// Create a new instance of the V0 type pub fn new_v0(base: B, val: V0) -> Box<Self> { let inner = UnsizedEnum { header: Header { disc: 0, base }, phantomdata: PhantomData, val: ManuallyDrop::new(val), }; // Safe because all we're doing is writing a valid value into // a (possibly) bigger piece of memory. unsafe { let p = Self::alloc(); ptr::write(p, inner); Box::from_raw(p) } } /// Create a new instance of the V1 type pub fn new_v1(base: B, val: V1) -> Box<Self> { let inner = UnsizedEnum_V1 { header: Header { disc: 1, base }, val, phantomdata: PhantomData, }; // Safe because the rest of the code uses `disc` to decide how // to interpret the `val` part of the structure unsafe { let p = Self::alloc(); ptr::write(p as *mut UnsizedEnum_V1<B, V0, V1>, inner); Box::from_raw(p) } } } impl<B, V0: ?Sized, V1> UnsizedEnum<B, V0, V1> { /// Return a mutable reference to the common base structure `B` pub fn base(&mut self) -> &mut B { &mut self.header.base } /// Set a value in the spare bits above the discriminant. Bit 0 /// of this value will not be stored. pub fn set_spare(&mut self, spare: usize) { self.header.disc = (self.header.disc & 1) | (spare & !1); } /// Gets the discriminant value including any value stored in the /// spare bits above it. So bit 0 will be the disriminant (0 for /// `V0`, 1 for `V1`), and the remaining bits will be whatever /// value was saved by the most recent call to `set_spare`, or 0 /// initially. pub fn get_spare(&self) -> usize { self.header.disc } /// Change the value stored in the enum to the provided `V0` /// value, dropping the previous value (of whichever variant). #[inline] pub fn set_v0(&mut self, v0: &Move<V0>) { // Safe because the value is only temporarily in an // invalid/dropped state. The `Move` type lets us do a "move" // operation for an unsized value. Since the method API // doesn't prevent the caller providing a different underlying // type, it's necessary to check the vtable first. let p = v0.get_ref().expect("Already moved value passed to set_v0"); assert_eq!( vtable_of(p), vtable_of(self), "Passed a value to set_v0() from a different underlying type" ); unsafe { self.drop_value(); std::ptr::copy_nonoverlapping( p as *const V0 as *const u8, (&mut *self.val) as *mut V0 as *mut u8, std::mem::size_of_val(&*p), ); self.header.disc &= !1; } } /// Change the value stored in the enum to the provided V1 value, /// dropping the previous value (of whichever variant) #[inline] pub fn set_v1(&mut self, v1: V1) { // Safe because the value is only temporarily in an // invalid/dropped state unsafe { self.drop_value(); let p = self as *mut UnsizedEnum<B, V0, V1>; let p = p as *mut UnsizedEnum_V1<B, V0, V1>; ptr::write(&mut (*p).val, v1); self.header.disc |= 1; } } // Leaves the value part of structure in an uninitialised state. // Safe because we drop according to the `disc` value unsafe fn drop_value(&mut self) { if 0 == (self.header.disc & 1) { ManuallyDrop::drop(&mut self.val); } else { let p = self as *mut UnsizedEnum<B, V0, V1>; let p = p as *mut UnsizedEnum_V1<B, V0, V1>; ptr::drop_in_place(&mut (*p).val); } } /// Get a reference to the value. Since there are two possible /// variants, an enum is returned that provides either one /// reference or the other. pub fn get_mut(&mut self) -> EnumRef<V0, V1> { // Safe because we access the value according to `disc`. // Normal borrowing rules mean that no modifications can be // made whilst the borrow is active. unsafe { if 0 == (self.header.disc & 1) { EnumRef::V0(&mut *self.val) } else { let p = self as *mut UnsizedEnum<B, V0, V1>; let p = p as *mut UnsizedEnum_V1<B, V0, V1>; EnumRef::V1(&mut (*p).val) } } } } impl<B, V0: ?Sized, V1> Drop for UnsizedEnum<B, V0, V1> { fn drop(&mut self) { // Safe because we're in the drop handler, so leaving it // dropped is the idea. Nothing else will drop the value part // because it is `ManuallyDrop` unsafe { self.drop_value() }; } } /// A mutable reference to the active variant within the `UnsizedEnum` pub enum EnumRef<'a, V0: ?Sized, V1> { V0(&'a mut V0), V1(&'a mut V1), } /// A type that allows an unsized value to be moved pub struct Move<T: ?Sized> { moved: Cell<bool>, value: ManuallyDrop<T>, } impl<T> Move<T> { #[inline] pub fn new(val: T) -> Self { Self { moved: Cell::new(false), value: ManuallyDrop::new(val), } } } impl<T: ?Sized> Move<T> { /// Get a reference to the contained value and mark it as moved if /// it has not yet been moved, otherwise return `None`. If the /// caller does not move the value out (through copying in unsafe /// code), then the value will leak (like `mem::forget`). However /// leaking memory is safe, so this interface doesn't need to be /// marked unsafe. #[inline] pub fn get_ref(&self) -> Option<&T> { if self.moved.get() { None } else { self.moved.set(true); Some(&*self.value) } } } impl<T: ?Sized> Drop for Move<T> { fn drop(&mut self) { if !self.moved.get() { // Safe because we only drop it if it hasn't already been // moved unsafe { ManuallyDrop::drop(&mut self.value) }; } } } #[cfg(test)] mod tests { use super::{EnumRef, Move, UnsizedEnum}; struct Base(usize); struct A(u16); trait Sum { fn sum(&self) -> f64; } struct B { a: f64, b: f64, } impl Sum for B { fn sum(&self) -> f64 { self.a + self.b } } struct C { a: u32, b: u32, c: u32, } impl Sum for C { fn sum(&self) -> f64 { (self.a + self.b + self.c) as f64 } } fn calc_sum(r: &mut Box<UnsizedEnum<Base, dyn Sum, A>>) -> f64 { match r.get_mut() { EnumRef::V0(v) => v.sum(), EnumRef::V1(a) => a.0 as f64, } } #[test] fn test() { let mut e: Box<UnsizedEnum<Base, dyn Sum, A>>; // Assignments to `e` below are coercions e = UnsizedEnum::new_v0(Base(654321), B { a: 1.0, b: 2.0 }); assert_eq!(calc_sum(&mut e), 3.0); e.set_v1(A(54321)); assert_eq!(calc_sum(&mut e), 54321.0); e.set_v0(&Move::new(B { a: 3.0, b: 4.0 })); assert_eq!(calc_sum(&mut e), 7.0); e = UnsizedEnum::<Base, C, A>::new_v1(Base(654321), A(12345)); assert_eq!(calc_sum(&mut e), 12345.0); e.set_v0(&Move::new(C { a: 3, b: 4, c: 5 })); assert_eq!(calc_sum(&mut e), 12.0); e.set_v1(A(13542)); assert_eq!(calc_sum(&mut e), 13542.0); } #[test] #[should_panic] fn test_writing_wrong_type_1() { let mut e: Box<UnsizedEnum<Base, dyn Sum, A>>; e = UnsizedEnum::<Base, C, A>::new_v1(Base(654321), A(12345)); // Can't write a `B` to a `C` instance e.set_v0(&Move::new(B { a: 3.0, b: 4.0 })); } #[test] #[should_panic] fn test_writing_wrong_type_2() { let mut e: Box<UnsizedEnum<Base, dyn Sum, A>>; e = UnsizedEnum::new_v0(Base(654321), C { a: 3, b: 4, c: 5 }); // Can't write a `B` to a `C` instance e.set_v0(&Move::new(B { a: 3.0, b: 4.0 })); } #[test] fn test_sized() { // This tests that `set_v0` also works okay when there are no // fat pointers involved let mut e = UnsizedEnum::new_v0(Base(654321), B { a: 1.0, b: 2.0 }); e.set_v1(A(54321)); e.set_v0(&Move::new(B { a: 3.0, b: 4.0 })); } }