[rustc.git] / src / doc / unstable-book / src / language-features / re-rebalance-coherence.md

# `re_rebalance_coherence`

The tracking issue for this feature is: [#55437]

[#55437]: https://github.com/rust-lang/rust/issues/55437

------------------------

The `re_rebalance_coherence` feature tweaks the rules regarding which trait
impls are allowed in crates.
The following rule is used:

Given `impl<P1..=Pn> Trait<T1..=Tn> for T0`, an impl is valid only if at
least one of the following is true:
- `Trait` is a local trait
- All of
  - At least one of the types `T0..=Tn` must be a local type. Let `Ti` be the
    first such type.
  - No uncovered type parameters `P1..=Pn` may appear in `T0..Ti` (excluding
    `Ti`)


See the [RFC](https://github.com/rust-lang/rfcs/blob/master/text/2451-re-rebalancing-coherence.md) for details.
Commit	Line	Data
0731742a XL	1	# `re_rebalance_coherence`
	2
	3	The tracking issue for this feature is: [#55437]
	4
	5	[#55437]: https://github.com/rust-lang/rust/issues/55437
	6
	7	------------------------
	8
	9	The `re_rebalance_coherence` feature tweaks the rules regarding which trait
	10	impls are allowed in crates.
	11	The following rule is used:
	12
	13	Given `impl<P1..=Pn> Trait<T1..=Tn> for T0`, an impl is valid only if at
	14	least one of the following is true:
	15	- `Trait` is a local trait
	16	- All of
	17	- At least one of the types `T0..=Tn` must be a local type. Let `Ti` be the
	18	first such type.
	19	- No uncovered type parameters `P1..=Pn` may appear in `T0..Ti` (excluding
	20	`Ti`)
	21
	22
	23	See the [RFC](https://github.com/rust-lang/rfcs/blob/master/text/2451-re-rebalancing-coherence.md) for details.