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Fix mod_inv precondition: any x is invertible modulo 1 #622

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Fix mod_inv precondition: any x is invertible modulo 1 #622

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5 changes: 3 additions & 2 deletions library/core/src/ptr/mod.rs
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Expand Up @@ -2238,14 +2238,15 @@ pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
///
/// * `m` is a power-of-two;
/// * `x < m`; (if `x ≥ m`, pass in `x % m` instead)
/// * `x` is odd, unless `m == 1` (any `x` is an inverse modulo 1)
///
/// Implementation of this function shall not panic. Ever.
#[safety::requires(m.is_power_of_two())]
#[safety::requires(x < m)]
#[safety::requires(x % 2 != 0)]
#[safety::requires(m == 1 || x % 2 != 0)]
// for Kani (v0.65.0), the below multiplication is too costly to prove
#[cfg_attr(not(kani),
safety::ensures(|result| wrapping_mul(*result, x) % m == 1))]
safety::ensures(|result| wrapping_mul(*result, x) % m == 1 % m))]
#[inline]
const unsafe fn mod_inv(x: usize, m: usize) -> usize {
/// Multiplicative modular inverse table modulo 2⁴ = 16.
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