Struct rustc_middle::ty::UniverseIndex

pub struct UniverseIndex {
    pub(crate) private: u32,
}

“Universes” are used during type- and trait-checking in the presence of for<..> binders to control what sets of names are visible. Universes are arranged into a tree: the root universe contains names that are always visible. Each child then adds a new set of names that are visible, in addition to those of its parent. We say that the child universe “extends” the parent universe with new names.

To make this more concrete, consider this program:

struct Foo { }
fn bar<T>(x: T) {
  let y: for<'a> fn(&'a u8, Foo) = ...;
}

The struct name Foo is in the root universe U0. But the type parameter T, introduced on bar, is in an extended universe U1 – i.e., within bar, we can name both T and Foo, but outside of bar, we cannot name T. Then, within the type of y, the region 'a is in a universe U2 that extends U1, because we can name it inside the fn type but not outside.

Universes are used to do type- and trait-checking around these “forall” binders (also called universal quantification). The idea is that when, in the body of bar, we refer to T as a type, we aren’t referring to any type in particular, but rather a kind of “fresh” type that is distinct from all other types we have actually declared. This is called a placeholder type, and we use universes to talk about this. In other words, a type name in universe 0 always corresponds to some “ground” type that the user declared, but a type name in a non-zero universe is a placeholder type – an idealized representative of “types in general” that we use for checking generic functions.

Fields

private: u32

Trait Implementations

impl<'tcx> Lift<'tcx> for UniverseIndex

type Lifted = UniverseIndex

fn lift_to_tcx(self, _: TyCtxt<'tcx>) -> Option<Self>

impl<'tcx> TypeFoldable<'tcx> for UniverseIndex

fn try_fold_with<F: FallibleTypeFolder<'tcx>>(
    self,
    _: &mut F
) -> Result<Self, F::Error>

The entry point for folding. To fold a value t with a folder f call: t.try_fold_with(f).

fn fold_with<F: TypeFolder<'tcx>>(self, _: &mut F) -> Self

A convenient alternative to try_fold_with for use with infallible folders. Do not override this method, to ensure coherence with try_fold_with.

impl<'tcx> TypeVisitable<'tcx> for UniverseIndex

fn visit_with<F: TypeVisitor<'tcx>>(&self, _: &mut F) -> ControlFlow<F::BreakTy>

The entry point for visiting. To visit a value t with a visitor v call: t.visit_with(v).

fn has_vars_bound_at_or_above(&self, binder: DebruijnIndex) -> bool

Returns true if self has any late-bound regions that are either bound by binder or bound by some binder outside of binder. If binder is ty::INNERMOST, this indicates whether there are any late-bound regions that appear free.

fn has_vars_bound_above(&self, binder: DebruijnIndex) -> bool

Returns true if this type has any regions that escape binder (and hence are not bound by it).

fn has_escaping_bound_vars(&self) -> bool

Return true if this type has regions that are not a part of the type. For example, for<'a> fn(&'a i32) return false, while fn(&'a i32) would return true. The latter can occur when traversing through the former.

fn has_type_flags(&self, flags: TypeFlags) -> bool

fn has_projections(&self) -> bool

fn has_opaque_types(&self) -> bool

fn references_error(&self) -> bool

fn error_reported(&self) -> Result<(), ErrorGuaranteed>

fn has_non_region_param(&self) -> bool

fn has_infer_regions(&self) -> bool

fn has_infer_types(&self) -> bool

fn has_non_region_infer(&self) -> bool

fn needs_infer(&self) -> bool

fn has_placeholders(&self) -> bool

fn needs_subst(&self) -> bool

fn has_free_regions(&self) -> bool

“Free” regions in this context means that it has any region that is not (a) erased or (b) late-bound.

fn has_erased_regions(&self) -> bool

fn has_erasable_regions(&self) -> bool

True if there are any un-erased free regions.

fn is_global(&self) -> bool

Indicates whether this value references only ‘global’ generic parameters that are the same regardless of what fn we are in. This is used for caching.

fn has_late_bound_regions(&self) -> bool

True if there are any late-bound regions

fn still_further_specializable(&self) -> bool

Indicates whether this value still has parameters/placeholders/inference variables which could be replaced later, in a way that would change the results of impl specialization.

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: 4 bytes