3535/// original `Src` will be forgotten, and the value of type `Dst` will be
3636/// returned.
3737///
38+ /// # `#![allow(shrink)]`
39+ ///
40+ /// If `#![allow(shrink)]` is provided, `transmute!` additionally supports
41+ /// transmutations that shrink the size of the value; e.g.:
42+ ///
43+ /// ```
44+ /// # use zerocopy::transmute;
45+ /// let u: u32 = transmute!(#![allow(shrink)] 0u64);
46+ /// assert_eq!(u, 0u32);
47+ /// ```
48+ ///
3849/// # Examples
3950///
4051/// ```
5162/// This macro can be invoked in `const` contexts.
5263#[ macro_export]
5364macro_rules! transmute {
54- ( $e: expr) => { {
55- // NOTE: This must be a macro (rather than a function with trait bounds)
56- // because there's no way, in a generic context, to enforce that two
57- // types have the same size. `core::mem::transmute` uses compiler magic
58- // to enforce this so long as the types are concrete.
65+ // NOTE: This must be a macro (rather than a function with trait bounds)
66+ // because there's no way, in a generic context, to enforce that two types
67+ // have the same size. `core::mem::transmute` uses compiler magic to enforce
68+ // this so long as the types are concrete.
69+ ( #![ allow( shrink) ] $e: expr) => { {
70+ let mut e = $e;
71+ if false {
72+ // This branch, though never taken, ensures that the type of `e` is
73+ // `IntoBytes` and that the type of the outer macro invocation
74+ // expression is `FromBytes`.
75+
76+ fn transmute<Src , Dst >( src: Src ) -> Dst
77+ where
78+ Src : $crate:: IntoBytes ,
79+ Dst : $crate:: FromBytes ,
80+ {
81+ let _ = src;
82+ loop { }
83+ }
84+ loop { }
85+ #[ allow( unreachable_code) ]
86+ transmute( e)
87+ } else {
88+ use $crate:: util:: macro_util:: core_reexport:: mem:: ManuallyDrop ;
89+
90+ // NOTE: `repr(packed)` is important! It ensures that the size of
91+ // `Transmute` won't be rounded up to accomodate `Src`'s or `Dst`'s
92+ // alignment, which would break the size comparison logic below.
93+ //
94+ // As an example of why this is problematic, consider `Src = [u8;
95+ // 5]`, `Dst = u32`. The total size of `Transmute<Src, Dst>` would
96+ // be 8, and so we would reject a `[u8; 5]` to `u32` transmute as
97+ // being size-increasing, which it isn't.
98+ #[ repr( C , packed) ]
99+ union Transmute <Src , Dst > {
100+ src: ManuallyDrop <Src >,
101+ dst: ManuallyDrop <Dst >,
102+ }
59103
104+ // SAFETY: `Transmute` is a `reper(C)` union whose `src` field has
105+ // type `ManuallyDrop<Src>`. Thus, the `src` field starts at byte
106+ // offset 0 within `Transmute` [1]. `ManuallyDrop<T>` has the same
107+ // layout and bit validity as `T`, so it is sound to transmute `Src`
108+ // to `Transmute`.
109+ //
110+ // [1] https://doc.rust-lang.org/1.85.0/reference/type-layout.html#reprc-unions
111+ //
112+ // [2] Per https://doc.rust-lang.org/1.85.0/std/mem/struct.ManuallyDrop.html:
113+ //
114+ // `ManuallyDrop<T>` is guaranteed to have the same layout and bit
115+ // validity as `T`
116+ let u: Transmute <_, _> = unsafe {
117+ // Clippy: We can't annotate the types; this macro is designed
118+ // to infer the types from the calling context.
119+ #[ allow( clippy:: missing_transmute_annotations) ]
120+ $crate:: util:: macro_util:: core_reexport:: mem:: transmute( e)
121+ } ;
122+
123+ if false {
124+ // SAFETY: This code is never executed.
125+ e = ManuallyDrop :: into_inner( unsafe { u. src } ) ;
126+ // Suppress the `unused_assignments` lint on the previous line.
127+ let _ = e;
128+ loop { }
129+ } else {
130+ // SAFETY: Per the safety comment on `let u` above, the `dst`
131+ // field in `Transmute` starts at byte offset 0, and has the
132+ // same layout and bit validity as `Dst`.
133+ //
134+ // Transmuting `Src` to `Transmute<Src, Dst>` above using
135+ // `core::mem::transmute` ensures that `size_of::<Src>() ==
136+ // size_of::<Transmute<Src, Dst>>()`. A `#[repr(C, packed)]`
137+ // union has the maximum size of all of its fields [1], so this
138+ // is equivalent to `size_of::<Src>() >= size_of::<Dst>()`.
139+ //
140+ // The outer `if`'s `false` branch ensures that `Src: IntoBytes`
141+ // and `Dst: FromBytes`. This, combined with the size bound,
142+ // ensures that this transmute is sound.
143+ //
144+ // [1] Per https://doc.rust-lang.org/1.85.0/reference/type-layout.html#reprc-unions:
145+ //
146+ // The union will have a size of the maximum size of all of
147+ // its fields rounded to its alignment
148+ let dst = unsafe { u. dst } ;
149+ $crate:: util:: macro_util:: must_use( ManuallyDrop :: into_inner( dst) )
150+ }
151+ }
152+ } } ;
153+ ( $e: expr) => { {
60154 let e = $e;
61155 if false {
62156 // This branch, though never taken, ensures that the type of `e` is
63- // `IntoBytes` and that the type of this macro invocation expression
64- // is `FromBytes`.
65-
66- struct AssertIsIntoBytes <T : $crate:: IntoBytes >( T ) ;
67- let _ = AssertIsIntoBytes ( e) ;
157+ // `IntoBytes` and that the type of the outer macro invocation
158+ // expression is `FromBytes`.
68159
69- struct AssertIsFromBytes <U : $crate:: FromBytes >( U ) ;
70- #[ allow( unused, unreachable_code) ]
71- let u = AssertIsFromBytes ( loop { } ) ;
72- u. 0
160+ fn transmute<Src , Dst >( src: Src ) -> Dst
161+ where
162+ Src : $crate:: IntoBytes ,
163+ Dst : $crate:: FromBytes ,
164+ {
165+ let _ = src;
166+ loop { }
167+ }
168+ loop { }
169+ #[ allow( unreachable_code) ]
170+ transmute( e)
73171 } else {
74172 // SAFETY: `core::mem::transmute` ensures that the type of `e` and
75173 // the type of this macro invocation expression have the same size.
76174 // We know this transmute is safe thanks to the `IntoBytes` and
77175 // `FromBytes` bounds enforced by the `false` branch.
78- //
79- // We use this reexport of `core::mem::transmute` because we know it
80- // will always be available for crates which are using the 2015
81- // edition of Rust. By contrast, if we were to use
82- // `std::mem::transmute`, this macro would not work for such crates
83- // in `no_std` contexts, and if we were to use
84- // `core::mem::transmute`, this macro would not work in `std`
85- // contexts in which `core` was not manually imported. This is not a
86- // problem for 2018 edition crates.
87176 let u = unsafe {
88177 // Clippy: We can't annotate the types; this macro is designed
89178 // to infer the types from the calling context.
@@ -92,7 +181,7 @@ macro_rules! transmute {
92181 } ;
93182 $crate:: util:: macro_util:: must_use( u)
94183 }
95- } }
184+ } } ;
96185}
97186
98187/// Safely transmutes a mutable or immutable reference of one type to an
@@ -1046,6 +1135,10 @@ mod tests {
10461135 let x: [ u8 ; 8 ] = transmute ! ( array_of_arrays) ;
10471136 assert_eq ! ( x, array_of_u8s) ;
10481137
1138+ // Test that memory is transmuted as expected when shrinking.
1139+ let x: [ [ u8 ; 2 ] ; 3 ] = transmute ! ( #![ allow( shrink) ] array_of_u8s) ;
1140+ assert_eq ! ( x, [ [ 0u8 , 1 ] , [ 2 , 3 ] , [ 4 , 5 ] ] ) ;
1141+
10491142 // Test that the source expression's value is forgotten rather than
10501143 // dropped.
10511144 #[ derive( IntoBytes ) ]
@@ -1058,12 +1151,16 @@ mod tests {
10581151 }
10591152 #[ allow( clippy:: let_unit_value) ]
10601153 let _: ( ) = transmute ! ( PanicOnDrop ( ( ) ) ) ;
1154+ #[ allow( clippy:: let_unit_value) ]
1155+ let _: ( ) = transmute ! ( #![ allow( shrink) ] PanicOnDrop ( ( ) ) ) ;
10611156
10621157 // Test that `transmute!` is legal in a const context.
10631158 const ARRAY_OF_U8S : [ u8 ; 8 ] = [ 0u8 , 1 , 2 , 3 , 4 , 5 , 6 , 7 ] ;
10641159 const ARRAY_OF_ARRAYS : [ [ u8 ; 2 ] ; 4 ] = [ [ 0 , 1 ] , [ 2 , 3 ] , [ 4 , 5 ] , [ 6 , 7 ] ] ;
10651160 const X : [ [ u8 ; 2 ] ; 4 ] = transmute ! ( ARRAY_OF_U8S ) ;
10661161 assert_eq ! ( X , ARRAY_OF_ARRAYS ) ;
1162+ const X_SHRINK : [ [ u8 ; 2 ] ; 3 ] = transmute ! ( #![ allow( shrink) ] ARRAY_OF_U8S ) ;
1163+ assert_eq ! ( X_SHRINK , [ [ 0u8 , 1 ] , [ 2 , 3 ] , [ 4 , 5 ] ] ) ;
10671164
10681165 // Test that `transmute!` works with `!Immutable` types.
10691166 let x: usize = transmute ! ( UnsafeCell :: new( 1usize ) ) ;
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