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> TypeFoldable<TyCtxt<'tcx>> for UniverseIndex

fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'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<TyCtxt<'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<TyCtxt<'tcx>> for UniverseIndex

fn visit_with<F: TypeVisitor<TyCtxt<'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).

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