3 releases
0.1.2 | Jan 2, 2022 |
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0.1.1 | Jan 2, 2022 |
0.1.0 | Jan 2, 2022 |
#488 in Debugging
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Used in 21 crates
(5 directly)
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179 lines
assert-unchecked
Unsafe assertions that allow for optimizations in release mode.
These macros use core::hint::unreachable_unchecked
, which make it possible to write assertions that simultaneously clarify code as well as hint at optimizations to LLVM.
Usage
Add this to your Cargo.toml
[dependencies]
assert_unchecked = "0.1.2"
Examples
use assert_unchecked::{
assert_eq_unchecked, assert_ne_unchecked, assert_unchecked, unreachable_unchecked,
};
fn copy(from_arr: &[u8], to_arr: &mut [u8]) {
assert_eq!(from_arr.len(), to_arr.len());
for i in 0..to_arr.len() {
// SAFETY: bounds of to_arr is checked outside of loop
// Without this line, the compiler isn't smart enough to remove the bounds check
unsafe { assert_unchecked!(i <= to_arr.len()) };
to_arr[i] = from_arr[i];
}
}
fn get_last(len: usize) -> usize {
if len == 0 {
return 0;
}
let mut v = vec![0];
for i in 1..len {
v.push(i)
}
// SAFETY: `len` elements have been added to v at this point
// Without this line, the compiler isn't smart enough to remove the bounds check
unsafe { assert_eq_unchecked!(len, v.len()) };
v[len - 1]
}
// Modifies `a[0]` and `a[delta]`, and then returns `a[0]`.
// delta must be non-zero and delta < a.len().
unsafe fn modify_start_and_delta(a: &mut [u8], delta: usize) -> u8 {
// SAFETY: requirements are invariants of the unsafe function.
assert_unchecked!(delta < a.len());
// With this assertion, we know that a[delta] does not modify a[0],
// which means the function can optimize the return value to always be 0.
// This also means that all bounds checks can be removed.
assert_ne_unchecked!(delta, 0);
a[0] = 0;
a[delta] = 1;
a[0]
}
fn div_1(a: u32, b: u32) -> u32 {
// `b.saturating_add(1)` is always positive (not zero),
// hence `checked_div` will never return `None`.
// Therefore, the else branch is unreachable.
a.checked_div(b.saturating_add(1))
.unwrap_or_else(|| unsafe { unreachable_unchecked!("division by zero isn't possible") })
}