Enum ImplOverlapKind

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

Variants

Permitted

These impls are always allowed to overlap.

Fields

marker: bool

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

FutureCompatOrderDepTraitObjects

These impls are allowed to overlap, but that raises an issue #33140 future-compatibility warning (tracked in #56484).

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 on that difference, doing 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 for ImplOverlapKind

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

Tests for self and other values to be equal, and is used by ==.

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

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Eq for ImplOverlapKind

impl StructuralPartialEq for ImplOverlapKind

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 1 byte

Size for each variant:

• Permitted: 1 byte
• FutureCompatOrderDepTraitObjects: 0 bytes