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Replace byte-inspection NonZero contracts with niche test and raw_eq #624

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Replace byte-inspection NonZero contracts with niche test and raw_eq #624

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34 changes: 13 additions & 21 deletions library/core/src/num/nonzero.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -419,21 +419,16 @@ where
#[must_use]
#[inline]
#[track_caller]
#[requires({
let size = core::mem::size_of::<T>();
let ptr = &n as *const T as *const u8;
let slice = unsafe { core::slice::from_raw_parts(ptr, size) };
!slice.iter().all(|&byte| byte == 0)
})]
#[ensures(|result: &Self|{
let size = core::mem::size_of::<T>();
let n_ptr: *const T = &n;
let result_inner: T = result.get();
let result_ptr: *const T = &result_inner;
let n_slice = unsafe { core::slice::from_raw_parts(n_ptr as *const u8, size) };
let result_slice = unsafe { core::slice::from_raw_parts(result_ptr as *const u8, size) };
n_slice == result_slice
})]
// `NonZero::new` performs the canonical zero test via the niche layout
// (`Option<NonZero<T>>` has the same layout as `T`, with 0 as `None`),
// and `raw_eq` compares the object representations directly. Both are
// single operations for the verifier. Spelling these clauses as raw
// byte-slice inspections instead (as done previously) makes every
// asserted occurrence of this contract pay for symbolic pointer
// indirection and iterator reasoning, which dominated verification time
// of harnesses whose call graph contains NonZero constructions.
#[requires(NonZero::new(n).is_some())]
#[ensures(|result: &Self| unsafe { core::intrinsics::raw_eq(&result.get(), &n) })]
pub const unsafe fn new_unchecked(n: T) -> Self {
match Self::new(n) {
Some(n) => n,
Expand Down Expand Up @@ -475,12 +470,9 @@ where
#[must_use]
#[inline]
#[track_caller]
#[requires({
let size = core::mem::size_of::<T>();
let ptr = n as *const T as *const u8;
let slice = unsafe { core::slice::from_raw_parts(ptr, size) };
!slice.iter().all(|&byte| byte == 0)
})]
// See new_unchecked: the niche-layout zero test is much cheaper for the
// verifier than inspecting the object representation byte by byte.
#[requires(NonZero::new(*n).is_some())]
pub unsafe fn from_mut_unchecked(n: &mut T) -> &mut Self {
match Self::from_mut(n) {
Some(n) => n,
Expand Down
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