rustc_trait_selection::infer::canonical::ir

Struct UniverseIndex

pub struct UniverseIndex {
    pub(crate) private_use_as_methods_instead: 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_use_as_methods_instead: u32

Implementations§

§

impl UniverseIndex

pub const MAX_AS_U32: u32 = 4_294_967_040u32

Maximum value the index can take, as a u32.

pub const MAX: UniverseIndex = _

Maximum value the index can take.

pub const ZERO: UniverseIndex = _

Zero value of the index.

pub const fn from_usize(value: usize) -> UniverseIndex

Creates a new index from a given usize.

§Panics

Will panic if value exceeds MAX.

pub const fn from_u32(value: u32) -> UniverseIndex

Creates a new index from a given u32.

§Panics

Will panic if value exceeds MAX.

pub const fn from_u16(value: u16) -> UniverseIndex

Creates a new index from a given u16.

§Panics

Will panic if value exceeds MAX.

pub const unsafe fn from_u32_unchecked(value: u32) -> UniverseIndex

Creates a new index from a given u32.

§Safety

The provided value must be less than or equal to the maximum value for the newtype. Providing a value outside this range is undefined due to layout restrictions.

Prefer using from_u32.

pub const fn index(self) -> usize

Extracts the value of this index as a usize.

pub const fn as_u32(self) -> u32

Extracts the value of this index as a u32.

pub const fn as_usize(self) -> usize

Extracts the value of this index as a usize.

§

impl UniverseIndex

pub const ROOT: UniverseIndex = UniverseIndex::ZERO

pub fn next_universe(self) -> UniverseIndex

Returns the “next” universe index in order – this new index is considered to extend all previous universes. This corresponds to entering a forall quantifier. So, for example, suppose we have this type in universe U:

for<'a> fn(&'a u32)

Once we “enter” into this for<'a> quantifier, we are in a new universe that extends U – in this new universe, we can name the region 'a, but that region was not nameable from U because it was not in scope there.

pub fn can_name(self, other: UniverseIndex) -> bool

Returns true if self can name a name from other – in other words, if the set of names in self is a superset of those in other (self >= other).

pub fn cannot_name(self, other: UniverseIndex) -> bool

Returns true if self cannot name some names from other – in other words, if the set of names in self is a strict subset of those in other (self < other).