Struct Id

pub struct Id<S: ?Sized, T: ?Sized>(/* private fields */);

A proof term that S and T are the same type (type identity). This type is only ever inhabited when S is nominally equivalent to T.

A note on variance and safety

S and T are invariant here. This means that for two, possibly disjoint, lifetimes 'a, 'b we cannot construct an Id<&'a T, &'b T>. If we could, which we would if we had covariance, we could define the following unsound function in safe Rust:

fn transmute_lifetime<'a, 'b, T: 'a + 'b>(r: &'a T) -> &'b T {
    refl().cast(r)
}

See

• https://doc.rust-lang.org/nightly/nomicon/phantom-data.html
• https://doc.rust-lang.org/beta/nomicon/subtyping.html

for more information on variance.

Implementations

impl<T: ?Sized> Id<T, T>

pub const REFL: Self

A proof witness of the fact that a type is equivalent to itself.

This is equivalent to refl().

impl<S: ?Sized, T: ?Sized> Id<S, T>

pub fn cast(self, value: S) -> T

where S: Sized, T: Sized,

Casts a value of type S to T.

This is safe because the Id type is always guaranteed to only be inhabited by Id<T, T> types by construction.

pub fn sym(self) -> Id<T, S>

Converts Id<S, T> into Id<T, S> since type equality is symmetric.

pub fn trans<U: ?Sized>(self, _: Id<T, U>) -> Id<S, U>

If you have proofs Id<S, T> and Id<T, U> you can conclude Id<S, U> since type equality is transitive.

pub fn cast_ref<'a>(self, value: &'a S) -> &'a T

Casts a value of type &S to &T where S == T.

use refl::*;

fn main() {
    let x: Box<u8> = Box::new(1);
    let refl: Id<Box<u8>, Box<u8>> = refl();
    assert_eq!(&x, refl.cast_ref(&x));
}

pub fn cast_ref_mut<'a>(self, value: &'a mut S) -> &'a mut T

Casts a value of type &S to &T where S == T.

use refl::*;

fn main() {
    let mut x: Box<u8> = Box::new(1);
    let refl: Id<Box<u8>, Box<u8>> = refl();
    **refl.cast_ref_mut(&mut x) += 1;
    assert_eq!(*x, 2);
}

Trait Implementations

impl<S: ?Sized, T: ?Sized> Clone for Id<S, T>

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, _: &Self)

Performs copy-assignment from source.

impl<S: ?Sized, T: ?Sized> Debug for Id<S, T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<X: ?Sized> Default for Id<X, X>

fn default() -> Self

Returns the “default value” for a type.

impl<S: ?Sized, T: ?Sized> Hash for Id<S, T>

fn hash<H: Hasher>(&self, _: &mut H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<S: ?Sized, T: ?Sized> Ord for Id<S, T>

fn cmp(&self, _other: &Self) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl<S: ?Sized, T: ?Sized> PartialEq for Id<S, T>

fn eq(&self, _: &Self) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, _: &Self) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<S: ?Sized, T: ?Sized> PartialOrd for Id<S, T>

fn partial_cmp(&self, _: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl<S: ?Sized, T: ?Sized> Copy for Id<S, T>

impl<S: ?Sized, T: ?Sized> Eq for Id<S, T>