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This is now a safe interface!

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nora 2022-10-09 17:31:20 +02:00 • committed by nils
parent 7d95a7dfaf
commit 22aaf91cc1
3 changed files with 10 additions and 14 deletions
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4
src/backend.rs
View file

@ -54,6 +54,7 @@ mod backend_size_asserts {
const _: () = assert_same_size::<usize, <usize as Backend>::Stored>(); const _: () = assert_same_size::<usize, <usize as Backend>::Stored>();
} }
// SAFETY: We are careful around provenance
unsafe impl Backend for usize { unsafe impl Backend for usize {
type Stored = *mut (); type Stored = *mut ();
@ -72,6 +73,7 @@ unsafe impl Backend for usize {
#[cfg(target_pointer_width = "64")] #[cfg(target_pointer_width = "64")]
/// on 64 bit, we can just treat u64/usize interchangeably, because uintptr_t == size_t in Rust /// on 64 bit, we can just treat u64/usize interchangeably, because uintptr_t == size_t in Rust
// SAFETY: We are careful around provenance
unsafe impl Backend for u64 { unsafe impl Backend for u64 {
type Stored = *mut (); type Stored = *mut ();
@ -90,6 +92,7 @@ unsafe impl Backend for u64 {
macro_rules! impl_backend_2_tuple { macro_rules! impl_backend_2_tuple {
(impl for $ty:ty { (*mut (), $int:ident), $num:expr }) => { (impl for $ty:ty { (*mut (), $int:ident), $num:expr }) => {
// SAFETY: We are careful around provenance
unsafe impl Backend for $ty { unsafe impl Backend for $ty {
// this one keeps the MSB in the pointer address, and the LSB in the integer // this one keeps the MSB in the pointer address, and the LSB in the integer
@ -117,6 +120,7 @@ macro_rules! impl_backend_2_tuple {
#[cfg_attr(target_pointer_width = "64", allow(unused))] // not required on 64 bit #[cfg_attr(target_pointer_width = "64", allow(unused))] // not required on 64 bit
macro_rules! impl_backend_3_tuple { macro_rules! impl_backend_3_tuple {
(impl for $ty:ty { (*mut (), $int1:ident, $int2:ident), $num1:expr, $num2:expr }) => { (impl for $ty:ty { (*mut (), $int1:ident, $int2:ident), $num1:expr, $num2:expr }) => {
// SAFETY: We are careful around provenance
unsafe impl Backend for $ty { unsafe impl Backend for $ty {
// this one keeps the MSB in the pointer address, ISB in int1 and the LSB in the int2 // this one keeps the MSB in the pointer address, ISB in int1 and the LSB in the int2

2
src/lib.rs
View file

@ -42,7 +42,7 @@
//! // implementation detail of NaN boxing, the sign bit of an f64 //! // implementation detail of NaN boxing, the sign bit of an f64
//! const SIGN_BIT: u64 = 0x8000000000000000; //! const SIGN_BIT: u64 = 0x8000000000000000;
//! //!
//! unsafe impl StuffingStrategy<u64> for NanBoxStrategy { //! impl StuffingStrategy<u64> for NanBoxStrategy {
//! type Other = f64; //! type Other = f64;
//! //!
//! fn stuff_other(inner: Self::Other) -> u64 { //! fn stuff_other(inner: Self::Other) -> u64 {

18
src/strategy.rs
View file

@ -10,15 +10,7 @@ use crate::{Backend, Unstuffed};
/// if possible. /// if possible.
/// ///
/// The generic parameter `B` stands for the [`Backend`](`crate::Backend`) used by the strategy. /// The generic parameter `B` stands for the [`Backend`](`crate::Backend`) used by the strategy.
/// pub trait StuffingStrategy<B> {
/// # Safety
///
/// If [`StuffingStrategy::is_other`] returns true for a value, then
/// [`StuffingStrategy::extract_other`] *must* return a valid `Other` for that same value.
///
/// For [`StuffingStrategy::stuff_ptr`] and [`StuffingStrategy::extract_ptr`],
/// `ptr == extract_ptr(stuff_ptr(ptr))` *must* hold true.
pub unsafe trait StuffingStrategy<B> {
/// The type of the other. /// The type of the other.
type Other: Copy; type Other: Copy;
@ -40,7 +32,7 @@ pub unsafe trait StuffingStrategy<B> {
fn stuff_ptr(addr: usize) -> B; fn stuff_ptr(addr: usize) -> B;
} }
unsafe impl<B> StuffingStrategy<B> for () impl<B> StuffingStrategy<B> for ()
where where
B: Backend + Default + TryInto<usize>, B: Backend + Default + TryInto<usize>,
usize: TryInto<B>, usize: TryInto<B>,
@ -75,7 +67,7 @@ pub(crate) mod test_strategies {
macro_rules! impl_usize_max_zst { macro_rules! impl_usize_max_zst {
($ty:ident) => { ($ty:ident) => {
// this one lives in usize::MAX // this one lives in usize::MAX
unsafe impl StuffingStrategy<usize> for $ty { impl StuffingStrategy<usize> for $ty {
type Other = Self; type Other = Self;
#[allow(clippy::forget_copy)] #[allow(clippy::forget_copy)]
@ -96,7 +88,7 @@ pub(crate) mod test_strategies {
} }
} }
unsafe impl StuffingStrategy<u64> for $ty { impl StuffingStrategy<u64> for $ty {
type Other = Self; type Other = Self;
#[allow(clippy::forget_copy)] #[allow(clippy::forget_copy)]
@ -117,7 +109,7 @@ pub(crate) mod test_strategies {
} }
} }
unsafe impl StuffingStrategy<u128> for $ty { impl StuffingStrategy<u128> for $ty {
type Other = Self; type Other = Self;
#[allow(clippy::forget_copy)] #[allow(clippy::forget_copy)]