Tracking Issue for #![feature(phantom_variance_markers)] · Issue #135806 · rust-lang/rust (original) (raw)

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Description

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

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

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

Steps / History

• ACP: Phantom variance ZSTs libs-team#488
• Implementation: Implement phantom variance markers #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?

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩