core/mem/
transmutability.rs

1use crate::marker::{ConstParamTy_, UnsizedConstParamTy};
2
3/// Marks that `Src` is transmutable into `Self`.
4///
5/// # Implementation
6///
7/// This trait cannot be implemented explicitly. It is implemented on-the-fly by
8/// the compiler for all types `Src` and `Self` such that, given a set of safety
9/// obligations on the programmer (see [`Assume`]), the compiler has proved that
10/// the bits of a value of type `Src` can be soundly reinterpreted as a `Self`.
11///
12/// # Safety
13///
14/// If `Dst: TransmuteFrom<Src, ASSUMPTIONS>`, the compiler guarantees that
15/// `Src` is soundly *union-transmutable* into a value of type `Dst`, provided
16/// that the programmer has guaranteed that the given [`ASSUMPTIONS`](Assume)
17/// are satisfied.
18///
19/// A union-transmute is any bit-reinterpretation conversion in the form of:
20///
21/// ```rust
22/// pub unsafe fn transmute_via_union<Src, Dst>(src: Src) -> Dst {
23///     use core::mem::ManuallyDrop;
24///
25///     #[repr(C)]
26///     union Transmute<Src, Dst> {
27///         src: ManuallyDrop<Src>,
28///         dst: ManuallyDrop<Dst>,
29///     }
30///
31///     let transmute = Transmute {
32///         src: ManuallyDrop::new(src),
33///     };
34///
35///     let dst = transmute.dst;
36///
37///     ManuallyDrop::into_inner(dst)
38/// }
39/// ```
40///
41/// Note that this construction is more permissive than
42/// [`mem::transmute_copy`](super::transmute_copy); union-transmutes permit
43/// conversions that extend the bits of `Src` with trailing padding to fill
44/// trailing uninitialized bytes of `Self`; e.g.:
45///
46/// ```rust
47/// #![feature(transmutability)]
48///
49/// use core::mem::{Assume, TransmuteFrom};
50///
51/// let src = 42u8; // size = 1
52///
53/// #[repr(C, align(2))]
54/// struct Dst(u8); // size = 2
55//
56/// let _ = unsafe {
57///     <Dst as TransmuteFrom<u8, { Assume::SAFETY }>>::transmute(src)
58/// };
59/// ```
60///
61/// # Caveats
62///
63/// ## Portability
64///
65/// Implementations of this trait do not provide any guarantee of portability
66/// across toolchains, targets or compilations. This trait may be implemented
67/// for certain combinations of `Src`, `Self` and `ASSUME` on some toolchains,
68/// targets or compilations, but not others. For example, if the layouts of
69/// `Src` or `Self` are non-deterministic, the presence or absence of an
70/// implementation of this trait may also be non-deterministic. Even if `Src`
71/// and `Self` have deterministic layouts (e.g., they are `repr(C)` structs),
72/// Rust does not specify the alignments of its primitive integer types, and
73/// layouts that involve these types may vary across toolchains, targets or
74/// compilations.
75///
76/// ## Stability
77///
78/// Implementations of this trait do not provide any guarantee of SemVer
79/// stability across the crate versions that define the `Src` and `Self` types.
80/// If SemVer stability is crucial to your application, you must consult the
81/// documentation of `Src` and `Self`s' defining crates. Note that the presence
82/// of `repr(C)`, alone, does not carry a safety invariant of SemVer stability.
83/// Furthermore, stability does not imply portability. For example, the size of
84/// `usize` is stable, but not portable.
85#[unstable(feature = "transmutability", issue = "99571")]
86#[lang = "transmute_trait"]
87#[rustc_deny_explicit_impl]
88#[rustc_do_not_implement_via_object]
89#[rustc_coinductive]
90pub unsafe trait TransmuteFrom<Src, const ASSUME: Assume = { Assume::NOTHING }>
91where
92    Src: ?Sized,
93{
94    /// Transmutes a `Src` value into a `Self`.
95    ///
96    /// # Safety
97    ///
98    /// The safety obligations of the caller depend on the value of `ASSUME`:
99    /// - If [`ASSUME.alignment`](Assume::alignment), the caller must guarantee
100    ///   that the addresses of references in the returned `Self` satisfy the
101    ///   alignment requirements of their referent types.
102    /// - If [`ASSUME.lifetimes`](Assume::lifetimes), the caller must guarantee
103    ///   that references in the returned `Self` will not outlive their
104    ///   referents.
105    /// - If [`ASSUME.safety`](Assume::safety), the returned value might not
106    ///   satisfy the library safety invariants of `Self`, and the caller must
107    ///   guarantee that undefined behavior does not arise from uses of the
108    ///   returned value.
109    /// - If [`ASSUME.validity`](Assume::validity), the caller must guarantee
110    ///   that `src` is a bit-valid instance of `Self`.
111    ///
112    /// When satisfying the above obligations (if any), the caller must *not*
113    /// assume that this trait provides any inherent guarantee of layout
114    /// [portability](#portability) or [stability](#stability).
115    unsafe fn transmute(src: Src) -> Self
116    where
117        Src: Sized,
118        Self: Sized,
119    {
120        use super::ManuallyDrop;
121
122        #[repr(C)]
123        union Transmute<Src, Dst> {
124            src: ManuallyDrop<Src>,
125            dst: ManuallyDrop<Dst>,
126        }
127
128        let transmute = Transmute { src: ManuallyDrop::new(src) };
129
130        // SAFETY: It is safe to reinterpret the bits of `src` as a value of
131        // type `Self`, because, by combination of invariant on this trait and
132        // contract on the caller, `src` has been proven to satisfy both the
133        // language and library invariants of `Self`. For all invariants not
134        // `ASSUME`'d by the caller, the safety obligation is supplied by the
135        // compiler. Conversely, for all invariants `ASSUME`'d by the caller,
136        // the safety obligation is supplied by contract on the caller.
137        let dst = unsafe { transmute.dst };
138
139        ManuallyDrop::into_inner(dst)
140    }
141}
142
143/// Configurable proof assumptions of [`TransmuteFrom`].
144///
145/// When `false`, the respective proof obligation belongs to the compiler. When
146/// `true`, the onus of the safety proof belongs to the programmer.
147#[unstable(feature = "transmutability", issue = "99571")]
148#[lang = "transmute_opts"]
149#[derive(PartialEq, Eq, Clone, Copy, Debug)]
150pub struct Assume {
151    /// When `false`, [`TransmuteFrom`] is not implemented for transmutations
152    /// that might violate the alignment requirements of references; e.g.:
153    ///
154    /// ```compile_fail,E0277
155    /// #![feature(transmutability)]
156    /// use core::mem::{align_of, TransmuteFrom};
157    ///
158    /// assert_eq!(align_of::<[u8; 2]>(), 1);
159    /// assert_eq!(align_of::<u16>(), 2);
160    ///
161    /// let src: &[u8; 2] = &[0xFF, 0xFF];
162    ///
163    /// // SAFETY: No safety obligations.
164    /// let dst: &u16 = unsafe {
165    ///     <_ as TransmuteFrom<_>>::transmute(src)
166    /// };
167    /// ```
168    ///
169    /// When `true`, [`TransmuteFrom`] assumes that *you* have ensured
170    /// that references in the transmuted value satisfy the alignment
171    /// requirements of their referent types; e.g.:
172    ///
173    /// ```rust
174    /// #![feature(pointer_is_aligned_to, transmutability)]
175    /// use core::mem::{align_of, Assume, TransmuteFrom};
176    ///
177    /// let src: &[u8; 2] = &[0xFF, 0xFF];
178    ///
179    /// let maybe_dst: Option<&u16> = if <*const _>::is_aligned_to(src, align_of::<u16>()) {
180    ///     // SAFETY: We have checked above that the address of `src` satisfies the
181    ///     // alignment requirements of `u16`.
182    ///     Some(unsafe {
183    ///         <_ as TransmuteFrom<_, { Assume::ALIGNMENT }>>::transmute(src)
184    ///     })
185    /// } else {
186    ///     None
187    /// };
188    ///
189    /// assert!(matches!(maybe_dst, Some(&u16::MAX) | None));
190    /// ```
191    pub alignment: bool,
192
193    /// When `false`, [`TransmuteFrom`] is not implemented for transmutations
194    /// that extend the lifetimes of references.
195    ///
196    /// When `true`, [`TransmuteFrom`] assumes that *you* have ensured that
197    /// references in the transmuted value do not outlive their referents.
198    pub lifetimes: bool,
199
200    /// When `false`, [`TransmuteFrom`] is not implemented for transmutations
201    /// that might violate the library safety invariants of the destination
202    /// type; e.g.:
203    ///
204    /// ```compile_fail,E0277
205    /// #![feature(transmutability)]
206    /// use core::mem::TransmuteFrom;
207    ///
208    /// let src: u8 = 3;
209    ///
210    /// struct EvenU8 {
211    ///     // SAFETY: `val` must be an even number.
212    ///     val: u8,
213    /// }
214    ///
215    /// // SAFETY: No safety obligations.
216    /// let dst: EvenU8 = unsafe {
217    ///     <_ as TransmuteFrom<_>>::transmute(src)
218    /// };
219    /// ```
220    ///
221    /// When `true`, [`TransmuteFrom`] assumes that *you* have ensured
222    /// that undefined behavior does not arise from using the transmuted value;
223    /// e.g.:
224    ///
225    /// ```rust
226    /// #![feature(transmutability)]
227    /// use core::mem::{Assume, TransmuteFrom};
228    ///
229    /// let src: u8 = 42;
230    ///
231    /// struct EvenU8 {
232    ///     // SAFETY: `val` must be an even number.
233    ///     val: u8,
234    /// }
235    ///
236    /// let maybe_dst: Option<EvenU8> = if src % 2 == 0 {
237    ///     // SAFETY: We have checked above that the value of `src` is even.
238    ///     Some(unsafe {
239    ///         <_ as TransmuteFrom<_, { Assume::SAFETY }>>::transmute(src)
240    ///     })
241    /// } else {
242    ///     None
243    /// };
244    ///
245    /// assert!(matches!(maybe_dst, Some(EvenU8 { val: 42 })));
246    /// ```
247    pub safety: bool,
248
249    /// When `false`, [`TransmuteFrom`] is not implemented for transmutations
250    /// that might violate the language-level bit-validity invariant of the
251    /// destination type; e.g.:
252    ///
253    /// ```compile_fail,E0277
254    /// #![feature(transmutability)]
255    /// use core::mem::TransmuteFrom;
256    ///
257    /// let src: u8 = 3;
258    ///
259    /// // SAFETY: No safety obligations.
260    /// let dst: bool = unsafe {
261    ///     <_ as TransmuteFrom<_>>::transmute(src)
262    /// };
263    /// ```
264    ///
265    /// When `true`, [`TransmuteFrom`] assumes that *you* have ensured
266    /// that the value being transmuted is a bit-valid instance of the
267    /// transmuted value; e.g.:
268    ///
269    /// ```rust
270    /// #![feature(transmutability)]
271    /// use core::mem::{Assume, TransmuteFrom};
272    ///
273    /// let src: u8 = 1;
274    ///
275    /// let maybe_dst: Option<bool> = if src == 0 || src == 1 {
276    ///     // SAFETY: We have checked above that the value of `src` is a bit-valid
277    ///     // instance of `bool`.
278    ///     Some(unsafe {
279    ///         <_ as TransmuteFrom<_, { Assume::VALIDITY }>>::transmute(src)
280    ///     })
281    /// } else {
282    ///     None
283    /// };
284    ///
285    /// assert_eq!(maybe_dst, Some(true));
286    /// ```
287    pub validity: bool,
288}
289
290#[unstable(feature = "transmutability", issue = "99571")]
291impl ConstParamTy_ for Assume {}
292#[unstable(feature = "transmutability", issue = "99571")]
293impl UnsizedConstParamTy for Assume {}
294
295impl Assume {
296    /// With this, [`TransmuteFrom`] does not assume you have ensured any safety
297    /// obligations are met, and relies only upon its own analysis to (dis)prove
298    /// transmutability.
299    #[unstable(feature = "transmutability", issue = "99571")]
300    pub const NOTHING: Self =
301        Self { alignment: false, lifetimes: false, safety: false, validity: false };
302
303    /// With this, [`TransmuteFrom`] assumes only that you have ensured that
304    /// references in the transmuted value satisfy the alignment requirements of
305    /// their referent types. See [`Assume::alignment`] for examples.
306    #[unstable(feature = "transmutability", issue = "99571")]
307    pub const ALIGNMENT: Self = Self { alignment: true, ..Self::NOTHING };
308
309    /// With this, [`TransmuteFrom`] assumes only that you have ensured that
310    /// references in the transmuted value do not outlive their referents. See
311    /// [`Assume::lifetimes`] for examples.
312    #[unstable(feature = "transmutability", issue = "99571")]
313    pub const LIFETIMES: Self = Self { lifetimes: true, ..Self::NOTHING };
314
315    /// With this, [`TransmuteFrom`] assumes only that you have ensured that
316    /// undefined behavior does not arise from using the transmuted value. See
317    /// [`Assume::safety`] for examples.
318    #[unstable(feature = "transmutability", issue = "99571")]
319    pub const SAFETY: Self = Self { safety: true, ..Self::NOTHING };
320
321    /// With this, [`TransmuteFrom`] assumes only that you have ensured that the
322    /// value being transmuted is a bit-valid instance of the transmuted value.
323    /// See [`Assume::validity`] for examples.
324    #[unstable(feature = "transmutability", issue = "99571")]
325    pub const VALIDITY: Self = Self { validity: true, ..Self::NOTHING };
326
327    /// Combine the assumptions of `self` and `other_assumptions`.
328    ///
329    /// This is especially useful for extending [`Assume`] in generic contexts;
330    /// e.g.:
331    ///
332    /// ```rust
333    /// #![feature(
334    ///     adt_const_params,
335    ///     generic_const_exprs,
336    ///     pointer_is_aligned_to,
337    ///     transmutability,
338    /// )]
339    /// #![allow(incomplete_features)]
340    /// use core::mem::{align_of, Assume, TransmuteFrom};
341    ///
342    /// /// Attempts to transmute `src` to `&Dst`.
343    /// ///
344    /// /// Returns `None` if `src` violates the alignment requirements of `&Dst`.
345    /// ///
346    /// /// # Safety
347    /// ///
348    /// /// The caller guarantees that the obligations required by `ASSUME`, except
349    /// /// alignment, are satisfied.
350    /// unsafe fn try_transmute_ref<'a, Src, Dst, const ASSUME: Assume>(src: &'a Src) -> Option<&'a Dst>
351    /// where
352    ///     &'a Dst: TransmuteFrom<&'a Src, { ASSUME.and(Assume::ALIGNMENT) }>,
353    /// {
354    ///     if <*const _>::is_aligned_to(src, align_of::<Dst>()) {
355    ///         // SAFETY: By the above dynamic check, we have ensured that the address
356    ///         // of `src` satisfies the alignment requirements of `&Dst`. By contract
357    ///         // on the caller, the safety obligations required by `ASSUME` have also
358    ///         // been satisfied.
359    ///         Some(unsafe {
360    ///             <_ as TransmuteFrom<_, { ASSUME.and(Assume::ALIGNMENT) }>>::transmute(src)
361    ///         })
362    ///     } else {
363    ///         None
364    ///     }
365    /// }
366    ///
367    /// let src: &[u8; 2] = &[0xFF, 0xFF];
368    ///
369    /// // SAFETY: No safety obligations.
370    /// let maybe_dst: Option<&u16> = unsafe {
371    ///     try_transmute_ref::<_, _, { Assume::NOTHING }>(src)
372    /// };
373    ///```
374    #[unstable(feature = "transmutability", issue = "99571")]
375    pub const fn and(self, other_assumptions: Self) -> Self {
376        Self {
377            alignment: self.alignment || other_assumptions.alignment,
378            lifetimes: self.lifetimes || other_assumptions.lifetimes,
379            safety: self.safety || other_assumptions.safety,
380            validity: self.validity || other_assumptions.validity,
381        }
382    }
383
384    /// Remove `other_assumptions` the obligations of `self`; e.g.:
385    ///
386    /// ```rust
387    /// #![feature(transmutability)]
388    /// use core::mem::Assume;
389    ///
390    /// let assumptions = Assume::ALIGNMENT.and(Assume::SAFETY);
391    /// let to_be_removed = Assume::SAFETY.and(Assume::VALIDITY);
392    ///
393    /// assert_eq!(
394    ///     assumptions.but_not(to_be_removed),
395    ///     Assume::ALIGNMENT,
396    /// );
397    /// ```
398    #[unstable(feature = "transmutability", issue = "99571")]
399    pub const fn but_not(self, other_assumptions: Self) -> Self {
400        Self {
401            alignment: self.alignment && !other_assumptions.alignment,
402            lifetimes: self.lifetimes && !other_assumptions.lifetimes,
403            safety: self.safety && !other_assumptions.safety,
404            validity: self.validity && !other_assumptions.validity,
405        }
406    }
407}
408
409// FIXME(jswrenn): This const op is not actually usable. Why?
410// https://github.com/rust-lang/rust/pull/100726#issuecomment-1219928926
411#[unstable(feature = "transmutability", issue = "99571")]
412impl core::ops::Add for Assume {
413    type Output = Assume;
414
415    fn add(self, other_assumptions: Assume) -> Assume {
416        self.and(other_assumptions)
417    }
418}
419
420// FIXME(jswrenn): This const op is not actually usable. Why?
421// https://github.com/rust-lang/rust/pull/100726#issuecomment-1219928926
422#[unstable(feature = "transmutability", issue = "99571")]
423impl core::ops::Sub for Assume {
424    type Output = Assume;
425
426    fn sub(self, other_assumptions: Assume) -> Assume {
427        self.but_not(other_assumptions)
428    }
429}