pub struct UniverseIndex(u32);
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"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. But when you enter into
some subuniverse, then it may add names that are only visible
within that subtree (but it can still name the names of its
ancestor universes).
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 a subuniverse 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 subuniverse U2 of U1, because we can name it
inside the fn type but not outside.
Universes are related to skolemization -- which is a way of
doing 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 skolemized 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 skolemized type -- an
idealized representative of "types in general" that we use for
checking generic functions.
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The root universe, where things that the user defined are
visible.
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The "max universe" -- this isn't really a valid universe, but
it's useful sometimes as a "starting value" when you are
taking the minimum of a (non-empty!) set of universes.
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Creates a universe index from the given integer. Not to be
used lightly lest you pick a bad value. But sometimes we
convert universe indices into integers and back for various
reasons.
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A "subuniverse" corresponds to being inside 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
subuniverse of 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.
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True if the names in this universe are a subset of the names in other
.
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🔬 This is a nightly-only experimental API. (rustc_private
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Performs copy-assignment from source
. Read more
This method tests for self
and other
values to be equal, and is used by ==
. Read more
This method tests for !=
.
This method returns an ordering between self
and other
values if one exists. Read more
This method tests less than (for self
and other
) and is used by the <
operator. Read more
This method tests less than or equal to (for self
and other
) and is used by the <=
operator. Read more
This method tests greater than (for self
and other
) and is used by the >
operator. Read more
This method tests greater than or equal to (for self
and other
) and is used by the >=
operator. Read more
This method returns an Ordering
between self
and other
. Read more
fn max(self, other: Self) -> Self | 1.21.0 [src] |
Compares and returns the maximum of two values. Read more
fn min(self, other: Self) -> Self | 1.21.0 [src] |
Compares and returns the minimum of two values. Read more
Feeds this value into the given [Hasher
]. Read more
Feeds a slice of this type into the given [Hasher
]. Read more
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Formats the value using the given formatter. Read more
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🔬 This is a nightly-only experimental API. (rustc_private
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Creates owned data from borrowed data, usually by cloning. Read more
🔬 This is a nightly-only experimental API. (toowned_clone_into
)
recently added
Uses borrowed data to replace owned data, usually by cloning. Read more
🔬 This is a nightly-only experimental API. (try_from
)
The type returned in the event of a conversion error.
🔬 This is a nightly-only experimental API. (try_from
)
Immutably borrows from an owned value. Read more
type Error = <U as TryFrom<T>>::Error
🔬 This is a nightly-only experimental API. (try_from
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The type returned in the event of a conversion error.
🔬 This is a nightly-only experimental API. (try_from
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Mutably borrows from an owned value. Read more
🔬 This is a nightly-only experimental API. (get_type_id
)
this method will likely be replaced by an associated static
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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Create an error for a missing method specialization. Defaults to panicking with type, trait & method names. S
is the encoder/decoder state type, T
is the type being encoded/decoded, and the arguments are the names of the trait and method that should've been overridden. Read more