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Replace transmute! macro with transmute fn #190

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Replace transmute! macro with transmute fn #190

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100 changes: 47 additions & 53 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1413,54 +1413,48 @@ impl<T: Unaligned + Display> Display for Unalign<T> {
}
}

// Used in `transmute!` below.
#[doc(hidden)]
pub use core::mem::transmute as __real_transmute;
pub trait TransmuteFrom<Src>: Sized {
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@kupiakos kupiakos Jul 21, 2023 •
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This trait does not need to be (and shouldn't be) pub, because it has a blanket implementation on T: Sized that transmute can use without declaring it in its where bounds.

const IS_TRANSMUTABLE: bool;
}

impl<Src, Dst> TransmuteFrom<Src> for Dst
where
Src: AsBytes,
Dst: FromBytes,
{
const IS_TRANSMUTABLE: bool = {
assert!(mem::size_of::<Src>() == mem::size_of::<Dst>());
true
};
}

/// Safely transmutes a value of one type to a value of another type of the same
/// size.
///
/// The expression `$e` must have a concrete type, `T`, which implements
/// `AsBytes`. The `transmute!` expression must also have a concrete type, `U`
/// (`U` is inferred from the calling context), and `U` must implement
/// `FromBytes`.
///
/// Note that the `T` produced by the expression `$e` will *not* be dropped.
/// Semantically, its bits will be copied into a new value of type `U`, the
/// original `T` will be forgotten, and the value of type `U` will be returned.
#[macro_export]
macro_rules! transmute {
($e:expr) => {{
// NOTE: This must be a macro (rather than a function with trait bounds)
// because there's no way, in a generic context, to enforce that two
// types have the same size. `core::mem::transmute` uses compiler magic
// to enforce this so long as the types are concrete.

let e = $e;
if false {
// This branch, though never taken, ensures that the type of `e` is
// `AsBytes` and that the type of this macro invocation expression
// is `FromBytes`.
const fn transmute<T: $crate::AsBytes, U: $crate::FromBytes>(_t: T) -> U {
unreachable!()
}
transmute(e)
} else {
// SAFETY: `core::mem::transmute` ensures that the type of `e` and
// the type of this macro invocation expression have the same size.
// We know this transmute is safe thanks to the `AsBytes` and
// `FromBytes` bounds enforced by the `false` branch.
//
// We use `$crate::__real_transmute` because we know it will always
// be available for crates which are using the 2015 edition of Rust.
// By contrast, if we were to use `std::mem::transmute`, this macro
// would not work for such crates in `no_std` contexts, and if we
// were to use `core::mem::transmute`, this macro would not work in
// `std` contexts in which `core` was not manually imported. This is
// not a problem for 2018 edition crates.
unsafe { $crate::__real_transmute(e) }
}
}}
/// ## `cargo check` and size errors
///
/// This function reports type size errors at a later compiler phase than
/// `mem::transmute`. Consequently, size errors may not be detected by `cargo
/// check`; only by `cargo build`. Run `cargo build` to be sure that your
/// transmute is valid.
pub const fn transmute<Src, Dst>(src: Src) -> Dst
where
Src: AsBytes,
Dst: FromBytes + TransmuteFrom<Src>,
{
#[repr(C)]
union Transmute<Src, Dst> {
src: ManuallyDrop<Src>,
dst: ManuallyDrop<Dst>,
}

assert!(<Dst as TransmuteFrom<Src>>::IS_TRANSMUTABLE);

// SAFETY: This is sound because `transmute`'s bounds ensure that `Src` is
// `AsBytes`, that `Dst` is `FromBytes`, and the above assert ensures that
// `Src` is at least as many bytes as `Dst`.
unsafe { ManuallyDrop::into_inner(Transmute { src: ManuallyDrop::new(src) }.dst) }
}

/// A length- and alignment-checked reference to a byte slice which can safely
Expand Down Expand Up @@ -3013,7 +3007,7 @@ mod tests {

// Converts an `AU64` to bytes using this platform's endianness.
fn au64_to_bytes(u: AU64) -> [u8; 8] {
transmute!(u)
transmute(u)
}

// An unsized type.
Expand Down Expand Up @@ -3142,12 +3136,12 @@ mod tests {
assert_eq!(u64::read_from(&VAL_BYTES[..]), Some(VAL));
// The first 8 bytes are from `VAL_BYTES` and the second 8 bytes are all
// zeroes.
let bytes_with_prefix: [u8; 16] = transmute!([VAL_BYTES, [0; 8]]);
let bytes_with_prefix: [u8; 16] = transmute([VAL_BYTES, [0; 8]]);
assert_eq!(u64::read_from_prefix(&bytes_with_prefix[..]), Some(VAL));
assert_eq!(u64::read_from_suffix(&bytes_with_prefix[..]), Some(0));
// The first 8 bytes are all zeroes and the second 8 bytes are from
// `VAL_BYTES`
let bytes_with_suffix: [u8; 16] = transmute!([[0; 8], VAL_BYTES]);
let bytes_with_suffix: [u8; 16] = transmute([[0; 8], VAL_BYTES]);
assert_eq!(u64::read_from_prefix(&bytes_with_suffix[..]), Some(0));
assert_eq!(u64::read_from_suffix(&bytes_with_suffix[..]), Some(VAL));

Expand All @@ -3158,11 +3152,11 @@ mod tests {
assert_eq!(bytes, VAL_BYTES);
let mut bytes = [0u8; 16];
assert_eq!(VAL.write_to_prefix(&mut bytes[..]), Some(()));
let want: [u8; 16] = transmute!([VAL_BYTES, [0; 8]]);
let want: [u8; 16] = transmute([VAL_BYTES, [0; 8]]);
assert_eq!(bytes, want);
let mut bytes = [0u8; 16];
assert_eq!(VAL.write_to_suffix(&mut bytes[..]), Some(()));
let want: [u8; 16] = transmute!([[0; 8], VAL_BYTES]);
let want: [u8; 16] = transmute([[0; 8], VAL_BYTES]);
assert_eq!(bytes, want);
}

Expand All @@ -3171,9 +3165,9 @@ mod tests {
// Test that memory is transmuted as expected.
let array_of_u8s = [0u8, 1, 2, 3, 4, 5, 6, 7];
let array_of_arrays = [[0, 1], [2, 3], [4, 5], [6, 7]];
let x: [[u8; 2]; 4] = transmute!(array_of_u8s);
let x: [[u8; 2]; 4] = transmute(array_of_u8s);
assert_eq!(x, array_of_arrays);
let x: [u8; 8] = transmute!(array_of_arrays);
let x: [u8; 8] = transmute(array_of_arrays);
assert_eq!(x, array_of_u8s);

// Test that the source expression's value is forgotten rather than
Expand All @@ -3186,12 +3180,12 @@ mod tests {
panic!("PanicOnDrop::drop");
}
}
let _: () = transmute!(PanicOnDrop(()));
let _: () = transmute(PanicOnDrop(()));

// Test that `transmute!` is legal in a const context.
// Test that `transmute` is legal in a const context.
const ARRAY_OF_U8S: [u8; 8] = [0u8, 1, 2, 3, 4, 5, 6, 7];
const ARRAY_OF_ARRAYS: [[u8; 2]; 4] = [[0, 1], [2, 3], [4, 5], [6, 7]];
const X: [[u8; 2]; 4] = transmute!(ARRAY_OF_U8S);
const X: [[u8; 2]; 4] = transmute(ARRAY_OF_U8S);
assert_eq!(X, ARRAY_OF_ARRAYS);
}

Expand Down
23 changes: 3 additions & 20 deletions tests/ui-msrv/transmute-bound-dst-not-frombytes.stderr
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Original file line number Diff line number Diff line change
@@ -1,22 +1,5 @@
error[E0277]: the trait bound `NotZerocopy<AU16>: FromBytes` is not satisfied
--> tests/ui-msrv/transmute-bound-dst-not-frombytes.rs:9:47
error[E0433]: failed to resolve: could not find `transmute` in `zerocopy`
--> tests/ui-msrv/transmute-bound-dst-not-frombytes.rs:9:57
|
9 | const DST_NOT_FROM_BYTES: NotZerocopy<AU16> = zerocopy::transmute!(AU16(0));
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `FromBytes` is not implemented for `NotZerocopy<AU16>`
|
= help: the following other types implement trait `FromBytes`:
()
AU16
F32<O>
F64<O>
I128<O>
I16<O>
I32<O>
I64<O>
and $N others
note: required by a bound in `DST_NOT_FROM_BYTES::transmute`
--> tests/ui-msrv/transmute-bound-dst-not-frombytes.rs:9:47
|
9 | const DST_NOT_FROM_BYTES: NotZerocopy<AU16> = zerocopy::transmute!(AU16(0));
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `DST_NOT_FROM_BYTES::transmute`
= note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
| ^^^^^^^^^ could not find `transmute` in `zerocopy`
37 changes: 29 additions & 8 deletions tests/ui-msrv/transmute-bound-src-not-asbytes.stderr
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Original file line number Diff line number Diff line change
@@ -1,10 +1,9 @@
error[E0277]: the trait bound `NotZerocopy<AU16>: AsBytes` is not satisfied
--> tests/ui-msrv/transmute-bound-src-not-asbytes.rs:9:32
--> tests/ui-msrv/transmute-bound-src-not-asbytes.rs:9:52
|
9 | const SRC_NOT_AS_BYTES: AU16 = zerocopy::transmute!(NotZerocopy(AU16(0)));
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
9 | const SRC_NOT_AS_BYTES: AU16 = zerocopy::transmute(NotZerocopy(AU16(0)));
| ------------------- ^^^^^^^^^^^^^^^^^^^^ the trait `AsBytes` is not implemented for `NotZerocopy<AU16>`
| |
| the trait `AsBytes` is not implemented for `NotZerocopy<AU16>`
| required by a bound introduced by this call
|
= help: the following other types implement trait `AsBytes`:
Expand All @@ -17,9 +16,31 @@ error[E0277]: the trait bound `NotZerocopy<AU16>: AsBytes` is not satisfied
I32<O>
I64<O>
and $N others
note: required by a bound in `SRC_NOT_AS_BYTES::transmute`
note: required by a bound in `zerocopy::transmute`
--> src/lib.rs
|
| Src: AsBytes,
| ^^^^^^^ required by this bound in `zerocopy::transmute`

error[E0277]: the trait bound `NotZerocopy<AU16>: AsBytes` is not satisfied
--> tests/ui-msrv/transmute-bound-src-not-asbytes.rs:9:32
|
9 | const SRC_NOT_AS_BYTES: AU16 = zerocopy::transmute!(NotZerocopy(AU16(0)));
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `SRC_NOT_AS_BYTES::transmute`
= note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
9 | const SRC_NOT_AS_BYTES: AU16 = zerocopy::transmute(NotZerocopy(AU16(0)));
| ^^^^^^^^^^^^^^^^^^^ the trait `AsBytes` is not implemented for `NotZerocopy<AU16>`
|
= help: the following other types implement trait `AsBytes`:
()
AU16
F32<O>
F64<O>
I128<O>
I16<O>
I32<O>
I64<O>
and $N others
= note: required for `AU16` to implement `TransmuteFrom<NotZerocopy<AU16>>`
note: required by a bound in `zerocopy::transmute`
--> src/lib.rs
|
| Dst: FromBytes + TransmuteFrom<Src>,
| ^^^^^^^^^^^^^^^^^^ required by this bound in `zerocopy::transmute`
37 changes: 29 additions & 8 deletions tests/ui-msrv/transmute-pointer.stderr
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Original file line number Diff line number Diff line change
@@ -1,10 +1,9 @@
error[E0277]: the trait bound `*const usize: AsBytes` is not satisfied
--> tests/ui-msrv/transmute-pointer.rs:7:30
--> tests/ui-msrv/transmute-pointer.rs:7:50
|
7 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
7 | const POINTER_VALUE: usize = zerocopy::transmute(&0usize as *const usize);
| ------------------- ^^^^^^^^^^^^^^^^^^^^^^^ the trait `AsBytes` is not implemented for `*const usize`
| |
| the trait `AsBytes` is not implemented for `*const usize`
| required by a bound introduced by this call
|
= help: the following other types implement trait `AsBytes`:
Expand All @@ -17,9 +16,31 @@ error[E0277]: the trait bound `*const usize: AsBytes` is not satisfied
i8
isize
and $N others
note: required by a bound in `POINTER_VALUE::transmute`
note: required by a bound in `zerocopy::transmute`
--> src/lib.rs
|
| Src: AsBytes,
| ^^^^^^^ required by this bound in `zerocopy::transmute`

error[E0277]: the trait bound `*const usize: AsBytes` is not satisfied
--> tests/ui-msrv/transmute-pointer.rs:7:30
|
7 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `POINTER_VALUE::transmute`
= note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
7 | const POINTER_VALUE: usize = zerocopy::transmute(&0usize as *const usize);
| ^^^^^^^^^^^^^^^^^^^ the trait `AsBytes` is not implemented for `*const usize`
|
= help: the following other types implement trait `AsBytes`:
f32
f64
i128
i16
i32
i64
i8
isize
and $N others
= note: required for `usize` to implement `TransmuteFrom<*const usize>`
note: required by a bound in `zerocopy::transmute`
--> src/lib.rs
|
| Dst: FromBytes + TransmuteFrom<Src>,
| ^^^^^^^^^^^^^^^^^^ required by this bound in `zerocopy::transmute`
31 changes: 24 additions & 7 deletions tests/ui-msrv/transmute-size-decrease.stderr
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Original file line number Diff line number Diff line change
@@ -1,9 +1,26 @@
error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
--> tests/ui-msrv/transmute-size-decrease.rs:7:32
error[E0080]: evaluation of `<[u8; 1] as zerocopy::TransmuteFrom<[u8; 2]>>::IS_TRANSMUTABLE` failed
--> src/lib.rs
|
7 | const SIZE_DECREASE: [u8; 1] = zerocopy::transmute!([0u8; 2]);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| assert!(mem::size_of::<Src>() == mem::size_of::<Dst>());
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: mem::size_of::<Src>() == mem::size_of::<Dst>()', $DIR/src/lib.rs:1427:9
|
= note: source type: `[u8; 2]` (16 bits)
= note: target type: `[u8; 1]` (8 bits)
= note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
= note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)

error: any use of this value will cause an error
--> src/lib.rs
|
| assert!(<Dst as TransmuteFrom<Src>>::IS_TRANSMUTABLE);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| referenced constant has errors
| inside `zerocopy::transmute::<[u8; 2], [u8; 1]>` at $DIR/src/lib.rs:1452:13
| inside `SIZE_DECREASE` at $DIR/tests/ui-msrv/transmute-size-decrease.rs:7:32
|
::: tests/ui-msrv/transmute-size-decrease.rs:7:1
|
7 | const SIZE_DECREASE: [u8; 1] = zerocopy::transmute([0u8; 2]);
| ----------------------------
|
= note: `#[deny(const_err)]` on by default
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
31 changes: 24 additions & 7 deletions tests/ui-msrv/transmute-size-increase.stderr
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Original file line number Diff line number Diff line change
@@ -1,9 +1,26 @@
error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
--> tests/ui-msrv/transmute-size-increase.rs:7:32
error[E0080]: evaluation of `<[u8; 2] as zerocopy::TransmuteFrom<[u8; 1]>>::IS_TRANSMUTABLE` failed
--> src/lib.rs
|
7 | const SIZE_INCREASE: [u8; 2] = zerocopy::transmute!([0u8; 1]);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| assert!(mem::size_of::<Src>() == mem::size_of::<Dst>());
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: mem::size_of::<Src>() == mem::size_of::<Dst>()', $DIR/src/lib.rs:1427:9
|
= note: source type: `[u8; 1]` (8 bits)
= note: target type: `[u8; 2]` (16 bits)
= note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
= note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)

error: any use of this value will cause an error
--> src/lib.rs
|
| assert!(<Dst as TransmuteFrom<Src>>::IS_TRANSMUTABLE);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| referenced constant has errors
| inside `zerocopy::transmute::<[u8; 1], [u8; 2]>` at $DIR/src/lib.rs:1452:13
| inside `SIZE_INCREASE` at $DIR/tests/ui-msrv/transmute-size-increase.rs:7:32
|
::: tests/ui-msrv/transmute-size-increase.rs:7:1
|
7 | const SIZE_INCREASE: [u8; 2] = zerocopy::transmute([0u8; 1]);
| ----------------------------
|
= note: `#[deny(const_err)]` on by default
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
25 changes: 7 additions & 18 deletions tests/ui-nightly/transmute-bound-dst-not-frombytes.stderr
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Original file line number Diff line number Diff line change
@@ -1,22 +1,11 @@
error[E0277]: the trait bound `NotZerocopy<AU16>: FromBytes` is not satisfied
--> tests/ui-nightly/transmute-bound-dst-not-frombytes.rs:9:47
error[E0433]: failed to resolve: could not find `transmute` in `zerocopy`
--> tests/ui-nightly/transmute-bound-dst-not-frombytes.rs:9:57
|
9 | const DST_NOT_FROM_BYTES: NotZerocopy<AU16> = zerocopy::transmute!(AU16(0));
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `FromBytes` is not implemented for `NotZerocopy<AU16>`
| ^^^^^^^^^ could not find `transmute` in `zerocopy`
|
= help: the following other types implement trait `FromBytes`:
()
AU16
F32<O>
F64<O>
I128<O>
I16<O>
I32<O>
I64<O>
and $N others
note: required by a bound in `DST_NOT_FROM_BYTES::transmute`
--> tests/ui-nightly/transmute-bound-dst-not-frombytes.rs:9:47
help: zerocopy::transmute is not a macro, but a function, try to remove `!`
|
9 - const DST_NOT_FROM_BYTES: NotZerocopy<AU16> = zerocopy::transmute!(AU16(0));
9 + const DST_NOT_FROM_BYTES: NotZerocopy<AU16> = zerocopy::transmute(AU16(0));
|
9 | const DST_NOT_FROM_BYTES: NotZerocopy<AU16> = zerocopy::transmute!(AU16(0));
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `transmute`
= note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
2 changes: 1 addition & 1 deletion tests/ui-nightly/transmute-bound-src-not-asbytes.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,7 @@ extern crate zerocopy;
include!("../../zerocopy-derive/tests/util.rs");

// The source type must be `AsBytes`.
const SRC_NOT_AS_BYTES: AU16 = zerocopy::transmute!(NotZerocopy(AU16(0)));
const SRC_NOT_AS_BYTES: AU16 = zerocopy::transmute(NotZerocopy(AU16(0)));

fn main() {
_ = SRC_NOT_AS_BYTES;
Expand Down
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