Enum rustc_middle::ty::ImplOverlapKind

pub enum ImplOverlapKind {
    Permitted {
        marker: bool,
    },
    Issue33140,
}

Variants

Permitted

These impls are always allowed to overlap.

Fields of Permitted

marker: bool

Whether or not the impl is permitted due to the trait being a #[marker] trait

Issue33140

These impls are allowed to overlap, but that raises an issue #33140 future-compatibility warning.

Some background: in Rust 1.0, the trait-object types Send + Sync (today’s dyn Send + Sync) and Sync + Send (now dyn Sync + Send) were different.

The widely-used version 0.1.0 of the crate traitobject had accidentally relied that difference, making what reduces to the following set of impls:

trait Trait {}
impl Trait for dyn Send + Sync {}
impl Trait for dyn Sync + Send {}

Obviously, once we made these types be identical, that code causes a coherence error and a fairly big headache for us. However, luckily for us, the trait Trait used in this case is basically a marker trait, and therefore having overlapping impls for it is sound.

To handle this, we basically regard the trait as a marker trait, with an additional future-compatibility warning. To avoid accidentally “stabilizing” this feature, it has the following restrictions:

1. The trait must indeed be a marker-like trait (i.e., no items), and must be positive impls.
2. The trait-ref of both impls must be equal.
3. The trait-ref of both impls must be a trait object type consisting only of marker traits.
4. Neither of the impls can have any where-clauses.

Once traitobject 0.1.0 is no longer an active concern, this hack can be removed.

Trait Implementations

impl Debug for ImplOverlapKind

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl PartialEq<ImplOverlapKind> for ImplOverlapKind

fn eq(&self, other: &ImplOverlapKind) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &ImplOverlapKind) -> bool

This method tests for !=.

impl Eq for ImplOverlapKind

impl StructuralEq for ImplOverlapKind

impl StructuralPartialEq for ImplOverlapKind