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Improve panic message and surrounding documentation for Ord violations

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The new sort implementations have the ability to
detect Ord violations in many cases. This commit
improves the message in a way that should help
users realize what went wrong in their program.
This commit is contained in:
Lukas Bergdoll 2024-07-27 16:47:10 +02:00
parent 3942254d00
commit 644550f3de
1 changed files with 19 additions and 4 deletions
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23
library/core/src/slice/sort/shared/smallsort.rs
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@ -834,9 +834,10 @@ unsafe fn bidirectional_merge<T: FreezeMarker, F: FnMut(&T, &T) -> bool>(
right = right.add((!left_nonempty) as usize);
}
// We now should have consumed the full input exactly once. This can
// only fail if the comparison operator fails to be Ord, in which case
// we will panic and never access the inconsistent state in dst.
// We now should have consumed the full input exactly once. This can only fail if the
// user-provided comparison operator fails implements a strict weak ordering as required by
// Ord incorrectly, in which case we will panic and never access the inconsistent state in
// dst.
if left != left_end || right != right_end {
panic_on_ord_violation();
}
@ -845,7 +846,21 @@ unsafe fn bidirectional_merge<T: FreezeMarker, F: FnMut(&T, &T) -> bool>(
#[inline(never)]
fn panic_on_ord_violation() -> ! {
panic!("Ord violation");
// This is indicative of a logic bug in the user-provided comparison function or Ord
// implementation. They are expected to implement a total order as explained in the Ord
// documentation.
//
// By raising this panic we inform the user, that they have a logic bug in their program. If a
// strict weak ordering is not given, the concept of comparison based sorting makes no sense.
// E.g.: a < b < c < a
//
// The Ord documentation requires users to implement a total order, arguably that's
// unnecessarily strict in the context of sorting. Issues only arise if the weaker requirement
// of a strict weak ordering is violated.
//
// The panic message talks about a total order because that's what the Ord documentation talks
// about and requires, so as to not confuse users.
panic!("user-provided comparison function does not correctly implement a total order");
}
#[must_use]