11use crate :: marker:: { ConstParamTy_ , UnsizedConstParamTy } ;
22
3- /// Are values of a type transmutable into values of another type?
3+ /// Marks that `Src` is transmutable into `Self`.
44///
5- /// This trait is implemented on-the-fly by the compiler for types `Src` and `Self` when the bits of
6- /// any value of type `Self` are safely transmutable into a value of type `Dst`, in a given `Context`,
7- /// notwithstanding whatever safety checks you have asked the compiler to [`Assume`] are satisfied.
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+ /// Specifically, this trait models (and
13+ /// [implements](BikeshedIntrinsicFrom::transmute)) the semantics of
14+ /// transmute-via-union; i.e.:
15+ ///
16+ /// ```rust
17+ /// pub unsafe fn transmute_via_union<Src, Dst>(src: Src) -> Dst {
18+ /// use core::mem::ManuallyDrop;
19+ ///
20+ /// #[repr(C)]
21+ /// union Transmute<Src, Dst> {
22+ /// src: ManuallyDrop<Src>,
23+ /// dst: ManuallyDrop<Dst>,
24+ /// }
25+ ///
26+ /// let transmute = Transmute {
27+ /// src: ManuallyDrop::new(src),
28+ /// };
29+ ///
30+ /// let dst = transmute.dst;
31+ ///
32+ /// ManuallyDrop::into_inner(dst)
33+ /// }
34+ /// ```
35+ ///
36+ /// Note that this construction supports some transmutations forbidden by
37+ /// [`mem::transmute_copy`](super::transmute_copy); namely, transmutations that
38+ /// extend the bits of `Src` with trailing padding to fill trailing
39+ /// uninitialized bytes of `Self`; e.g.:
40+ ///
41+ /// ```
42+ /// #![feature(transmutability)]
43+ ///
44+ /// use core::mem::{Assume, BikeshedIntrinsicFrom};
45+ ///
46+ /// let src = 42u8; // size = 1
47+ ///
48+ /// #[repr(C, align(2))]
49+ /// struct Dst(u8); // size = 2
50+ //
51+ /// let _ = unsafe { <Dst as BikeshedIntrinsicFrom<u8, { Assume::SAFETY
52+ /// }>>::transmute(src) };
53+ /// ```
54+ ///
55+ /// ## Portability
56+ ///
57+ /// Implementations of this trait do not provide any guarantee of portability
58+ /// across toolchains, targets or compilations. This trait may be implemented
59+ /// for certain combinations of `Src`, `Self` and `ASSUME` on some toolchains,
60+ /// targets or compilations, but not others. For example, if the layouts of
61+ /// `Src` or `Self` are non-deterministic, the presence or absence of an
62+ /// implementation of this trait may also be non-deterministic. Even if `Src`
63+ /// and `Self` have deterministic layouts (e.g., they are `repr(C)` structs),
64+ /// Rust does not specify the alignments of its primitive integer types, and
65+ /// layouts that involve these types may vary across toolchains, targets or
66+ /// compilations.
67+ ///
68+ /// ## Stability
69+ ///
70+ /// Implementations of this trait do not provide any guarantee of SemVer
71+ /// stability across the crate versions that define the `Src` and `Self` types.
72+ /// If SemVer stability is crucial to your application, you must consult the
73+ /// documentation of `Src` and `Self`s' defining crates. Note that the presence
74+ /// of `repr(C)`, alone, does not carry a safety invariant of SemVer stability.
75+ /// Furthermore, stability does not imply portability. For example, the size of
76+ /// `usize` is stable, but not portable.
877#[ unstable( feature = "transmutability" , issue = "99571" ) ]
978#[ lang = "transmute_trait" ]
1079#[ rustc_deny_explicit_impl( implement_via_object = false ) ]
@@ -13,28 +82,96 @@ pub unsafe trait BikeshedIntrinsicFrom<Src, const ASSUME: Assume = { Assume::NOT
1382where
1483 Src : ?Sized ,
1584{
85+ /// Transmutes a `Src` value into a `Self`.
86+ ///
87+ /// # Safety
88+ ///
89+ /// The safety obligations of the caller depend on the value of `ASSUME`:
90+ /// - If [`ASSUME.alignment`](Assume::alignment), the caller must guarantee
91+ /// that the addresses of references in the returned `Self` satisfy the
92+ /// alignment requirements of their referent types.
93+ /// - If [`ASSUME.lifetimes`](Assume::lifetimes), the caller must guarantee
94+ /// that references in the returned `Self` will not outlive their
95+ /// referents.
96+ /// - If [`ASSUME.safety`](Assume::safety), the returned value might not
97+ /// satisfy the library safety invariants of `Self`, and the caller must
98+ /// guarantee that undefined behavior does not arise from uses of the
99+ /// returned value.
100+ /// - If [`ASSUME.validity`](Assume::validity), the caller must guarantee
101+ /// that `src` is a bit-valid instance of `Self`.
102+ ///
103+ /// When satisfying the above obligations (if any), the caller must *not*
104+ /// assume that this trait provides any inherent guarantee of layout
105+ /// [portability](#portability) or [stability](#stability).
106+ unsafe fn transmute ( src : Src ) -> Self
107+ where
108+ Src : Sized ,
109+ Self : Sized ,
110+ {
111+ use super :: ManuallyDrop ;
112+
113+ #[ repr( C ) ]
114+ union Transmute < Src , Dst > {
115+ src : ManuallyDrop < Src > ,
116+ dst : ManuallyDrop < Dst > ,
117+ }
118+
119+ let transmute = Transmute { src : ManuallyDrop :: new ( src) } ;
120+
121+ // SAFETY: It is safe to reinterpret the bits of `src` as a value of
122+ // type `Self`, because, by combination of invariant on this trait and
123+ // contract on the caller, `src` has been proven to satisfy both the
124+ // language and library invariants of `Self`. For all invariants not
125+ // `ASSUME`'d by the caller, the safety obligation is supplied by the
126+ // compiler. Conversely, for all invariants `ASSUME`'d by the caller,
127+ // the safety obligation is supplied by contract on the caller.
128+ let dst = unsafe { transmute. dst } ;
129+
130+ ManuallyDrop :: into_inner ( dst)
131+ }
16132}
17133
18- /// What transmutation safety conditions shall the compiler assume that *you* are checking?
134+ /// Configurable proof assumptions of [`BikeshedIntrinsicFrom`].
135+ ///
136+ /// When `false`, the respective proof obligation belongs to the compiler. When
137+ /// `true`, the onus of the safety proof belongs to the programmer.
138+ /// [`BikeshedIntrinsicFrom`].
19139#[ unstable( feature = "transmutability" , issue = "99571" ) ]
20140#[ lang = "transmute_opts" ]
21141#[ derive( PartialEq , Eq , Clone , Copy , Debug ) ]
22142pub struct Assume {
23- /// When `true`, the compiler assumes that *you* are ensuring (either dynamically or statically) that
24- /// destination referents do not have stricter alignment requirements than source referents.
143+ /// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
144+ /// transmutations that might violate the the alignment requirements of
145+ /// references.
146+ ///
147+ /// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
148+ /// that that references in the transmuted value satisfy the alignment
149+ /// requirements of their referent types.
25150pub alignment : bool ,
26151
27- /// When `true`, the compiler assume that *you* are ensuring that lifetimes are not extended in a manner
28- /// that violates Rust's memory model.
152+ /// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
153+ /// transmutations that extend the lifetimes of references.
154+ ///
155+ /// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
156+ /// that that references in the transmuted value do not outlive their
157+ /// referents.
29158pub lifetimes : bool ,
30159
31- /// When `true`, the compiler assumes that *you* have ensured that no
32- /// unsoundness will arise from violating the safety invariants of the
33- /// destination type (and sometimes of the source type, too).
160+ /// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
161+ /// transmutations that might violate the library safety invariants of the
162+ /// return type.
163+ ///
164+ /// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
165+ /// that undefined behavior does not arise from using the returned value.
34166pub safety : bool ,
35167
36- /// When `true`, the compiler assumes that *you* are ensuring that the source type is actually a valid
37- /// instance of the destination type.
168+ /// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
169+ /// transmutations that might violate the language-level bit-validity
170+ /// invariant of the return type.
171+ ///
172+ /// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
173+ /// that the value being transmuted is a bit-valid instance of the return
174+ /// type.
38175pub validity : bool ,
39176}
40177
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