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

Variants

Permitted

Fields

marker: bool

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

These impls are always allowed to overlap.

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

Formats the value using the given formatter. Read more

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

This method tests for !=.

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.

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
  • Issue33140: 0 bytes