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

“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

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

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

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

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. Read more

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

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

True if there are any un-erased free regions.

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. Read more

True if there are any late-bound regions

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. Read more

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

This method turns the parameters of a DepNodeConstructor into an opaque Fingerprint to be used in DepNode. Not all DepNodeParams support being turned into a Fingerprint (they don’t need to if the corresponding DepNode is anonymous). Read more

This method tries to recover the query key from the given DepNode, something which is needed when forcing DepNodes during red-green evaluation. The query system will only call this method if fingerprint_style() is not FingerprintStyle::Opaque. It is always valid to return None here, in which case incremental compilation will treat the query as having changed instead of forcing it. 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 resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

Uses borrowed data to replace owned data, usually by cloning. Read more

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: 4 bytes