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This commit is contained in:
Michael Goulet 2023-07-30 21:46:04 +00:00
parent ca49a37390
commit 0e20155662
5 changed files with 65 additions and 49 deletions
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41
compiler/rustc_lint_defs/src/builtin.rs
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@ -4433,42 +4433,25 @@ declare_lint! {
/// ```rust,compile_fail
/// #![deny(coinductive_overlap_in_coherence)]
///
/// use std::borrow::Borrow;
/// use std::cmp::Ordering;
/// use std::marker::PhantomData;
/// trait CyclicTrait {}
/// impl<T: CyclicTrait> CyclicTrait for T {}
///
/// #[derive(PartialEq, Default)]
/// pub(crate) struct Interval<T>(T);
///
/// impl<T, Q> PartialEq<Q> for Interval<T>
/// where
/// Q: PartialOrd,
/// {
/// fn eq(&self, other: &Q) -> bool {
/// todo!()
/// }
/// }
///
/// impl<T, Q> PartialOrd<Q> for Interval<T>
/// where
/// Q: PartialOrd,
/// {
/// fn partial_cmp(&self, other: &Q) -> Option<Ordering> {
/// todo!()
/// }
/// }
/// trait Trait {}
/// impl<T: CyclicTrait> Trait for T {}
/// // conflicting impl with the above
/// impl Trait for u8 {}
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// The manual impl of `PartialEq` impl overlaps with the `derive`, since
/// if we replace `Q = Interval<T>`, then the second impl leads to a cycle:
/// `PartialOrd for Interval<T> where Interval<T>: PartialOrd`. This cycle
/// currently causes the compiler to consider `Interval<T>: PartialOrd` to not
/// hold, causing the two implementations to be disjoint. This will change in
/// a future release.
/// We have two choices for impl which satisfy `u8: Trait`: the blanket impl
/// for generic `T`, and the direct impl for `u8`. These two impls nominally
/// overlap, since we can infer `T = u8` in the former impl, but since the where
/// clause `u8: CyclicTrait` would end up resulting in a cycle (since it depends
/// on itself), the blanket impl is not considered to hold for `u8`. This will
/// change in a future release.
pub COINDUCTIVE_OVERLAP_IN_COHERENCE,
Warn,
"impls that are not considered to overlap may be considered to \