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Aug 29, 2025
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Implement traits for unsafe function values #2697

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37 changes: 25 additions & 12 deletions src/impls.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -299,24 +299,25 @@ const _: () = unsafe {

// SAFETY: The following types can be transmuted from `[0u8; size_of::<T>()]`. [1]
//
// [1] Per https://doc.rust-lang.org/nightly/core/option/index.html#representation:
// [1] Per https://doc.rust-lang.org/1.89.0/core/option/index.html#representation:
//
// Rust guarantees to optimize the following types `T` such that [`Option<T>`]
// has the same size and alignment as `T`. In some of these cases, Rust
// further guarantees that `transmute::<_, Option<T>>([0u8; size_of::<T>()])`
// is sound and produces `Option::<T>::None`. These cases are identified by
// the second column:
//
// | `T` | `transmute::<_, Option<T>>([0u8; size_of::<T>()])` sound? |
// |-----------------------|-----------------------------------------------------------|
// | [`Box<U>`] | when `U: Sized` |
// | `&U` | when `U: Sized` |
// | `&mut U` | when `U: Sized` |
// | [`ptr::NonNull<U>`] | when `U: Sized` |
// | `fn`, `extern "C" fn` | always |
//
// FIXME(#429), FIXME(https://github.com/rust-lang/rust/pull/115333): Cite the
// Stable docs once they're available.
// | `T` | `transmute::<_, Option<T>>([0u8; size_of::<T>()])` sound? |
// |-----------------------------------|-----------------------------------------------------------|
// | [`Box<U>`] | when `U: Sized` |
// | `&U` | when `U: Sized` |
// | `&mut U` | when `U: Sized` |
// | [`ptr::NonNull<U>`] | when `U: Sized` |
// | `fn`, `extern "C" fn`[^extern_fn] | always |
//
// [^extern_fn]: this remains true for `unsafe` variants, any argument/return
// types, and any other ABI: `[unsafe] extern "abi" fn` (_e.g._, `extern
// "system" fn`)
const _: () = unsafe {
#[cfg(feature = "alloc")]
unsafe_impl!(
Expand Down Expand Up @@ -345,19 +346,31 @@ const _: () = unsafe {
A, B, C, D, E, F, G, H, I, J, K, L -> M => TryFromBytes for opt_fn!(...);
|c| pointer::is_zeroed(c)
);
unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => FromZeros for opt_unsafe_fn!(...));
unsafe_impl_for_power_set!(
A, B, C, D, E, F, G, H, I, J, K, L -> M => TryFromBytes for opt_unsafe_fn!(...);
|c| pointer::is_zeroed(c)
);
unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => FromZeros for opt_extern_c_fn!(...));
unsafe_impl_for_power_set!(
A, B, C, D, E, F, G, H, I, J, K, L -> M => TryFromBytes for opt_extern_c_fn!(...);
|c| pointer::is_zeroed(c)
);
unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => FromZeros for opt_unsafe_extern_c_fn!(...));
unsafe_impl_for_power_set!(
A, B, C, D, E, F, G, H, I, J, K, L -> M => TryFromBytes for opt_unsafe_extern_c_fn!(...);
|c| pointer::is_zeroed(c)
);
};

// SAFETY: `fn()` and `extern "C" fn()` self-evidently do not contain
// SAFETY: `[unsafe] [extern "C"] fn()` self-evidently do not contain
// `UnsafeCell`s. This is not a proof, but we are accepting this as a known risk
// per #1358.
const _: () = unsafe {
unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => Immutable for opt_fn!(...));
unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => Immutable for opt_unsafe_fn!(...));
unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => Immutable for opt_extern_c_fn!(...));
unsafe_impl_for_power_set!(A, B, C, D, E, F, G, H, I, J, K, L -> M => Immutable for opt_unsafe_extern_c_fn!(...));
};

#[cfg(all(
Expand Down
14 changes: 13 additions & 1 deletion src/util/macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -307,12 +307,24 @@ macro_rules! opt_extern_c_fn {
($($args:ident),* -> $ret:ident) => { Option<extern "C" fn($($args),*) -> $ret> };
}

/// Expands to a `Option<fn>` type with the given argument types and return
/// Expands to an `Option<unsafe extern "C" fn>` type with the given argument
/// types and return type. Designed for use with `unsafe_impl_for_power_set`.
macro_rules! opt_unsafe_extern_c_fn {
($($args:ident),* -> $ret:ident) => { Option<unsafe extern "C" fn($($args),*) -> $ret> };
}

/// Expands to an `Option<fn>` type with the given argument types and return
/// type. Designed for use with `unsafe_impl_for_power_set`.
macro_rules! opt_fn {
($($args:ident),* -> $ret:ident) => { Option<fn($($args),*) -> $ret> };
}

/// Expands to an `Option<unsafe fn>` type with the given argument types and
/// return type. Designed for use with `unsafe_impl_for_power_set`.
macro_rules! opt_unsafe_fn {
($($args:ident),* -> $ret:ident) => { Option<unsafe fn($($args),*) -> $ret> };
}

/// Implements trait(s) for a type or verifies the given implementation by
/// referencing an existing (derived) implementation.
///
Expand Down
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