Tracking Issue for `#![feature(phantom_variance_markers)]`

Feature gate: #![feature(phantom_variance_markers)]

This is a tracking issue for phantom variance markers, which are identical to PhantomData but provide a self-documenting variance rather than contrived types such as PhantomData<fn() -> T> to indicate contravariance.

Public API

// in core::marker

pub struct PhantomCovariant<T: ?Sized>(/* ... */);
pub struct PhantomInvariant<T: ?Sized>(/* ... */);
pub struct PhantomContravariant<T: ?Sized>(/* ... */);
pub struct PhantomCovariantLifetime<'a>(/* ... */);
pub struct PhantomInvariantLifetime<'a>(/* ... */);
pub struct PhantomContravariantLifetime<'a>(/* ... */);

pub trait Variance: Sealed {}
impl<T: ?Sized> Variance for PhantomCovariant<T> {}
impl<T: ?Sized> Variance for PhantomInvariant<T> {}
impl<T: ?Sized> Variance for PhantomContravaiant<T> {}
impl<T: ?Sized> Variance for PhantomCovariantLifetime<T> {}
impl<T: ?Sized> Variance for PhantomInvariantLifetime<T> {}
impl<T: ?Sized> Variance for PhantomContravaiantLifetime<T> {}

pub fn variance<T: Variance>() -> T {}

// also the trait impls you would expect; identical to PhantomData

Steps / History

• ACP: https://github.com/rust-lang/libs-team/issues/488
• Implementation: #135807
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• Should there be a single Lifetime type or one for each possible variance? The former would mean writing PhantomCovariant<Lifetime<'a>> instead of PhantomCovariantLifetime<'a>, with the API otherwise being identical.
• Should the values be directly constructable, eliminating the need for ::new()? This would presumably mean adding 4–6 lang items, depending on the previous bullet point.
• Are we happy exposing the (unstable) rustc_pub_transparent on these, allowing indirect inclusion in repr(transparent) types?

Start by reading the feature-gate description, ACP rust-lang/libs-team#488, and implementation PR #135807. Review the unresolved API questions and the stabilization process linked in the issue. Done means reaching a decision on the listed questions, completing the final comment period, and preparing or reviewing the stabilization PR.