Struct refl::Id

pub struct Id<S: ?Sized, T: ?Sized>(_);

A proof term that S and T are the same type (type identity). This type is only every 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 can not construct an Id<&'a T, &'b T>. If we could, which we would if we had covariance, we could define the following unsafe 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.

Methods

impl<T: ?Sized> Id<T, T>

const REFL: Self

REFL: Self = Id(PhantomData)

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

The benefit of this version compared to refl() is that this is usable in const contexts while refl() can't.

impl<S: ?Sized, T: ?Sized> Id<S, T>

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.

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

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

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.

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

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

extern crate refl;
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));
}

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

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

extern crate refl;
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: Copy + ?Sized, T: Copy + ?Sized> Copy for Id<S, T>

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

fn clone(&self) -> Id<S, T>

Returns a copy of the value.

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

Performs copy-assignment from source.

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

fn fmt(&self, __arg_0: &mut Formatter) -> Result

Formats the value using the given formatter.

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

fn hash<__HST: Hasher>(&self, __arg_0: &mut __HST)

Feeds this value into the given [Hasher].

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

Feeds a slice of this type into the given [Hasher].

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

fn eq(&self, __arg_0: &Id<S, T>) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, __arg_0: &Id<S, T>) -> bool

This method tests for !=.

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

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

fn partial_cmp(&self, __arg_0: &Id<S, T>) -> Option<Ordering>

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

fn lt(&self, __arg_0: &Id<S, T>) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, __arg_0: &Id<S, T>) -> bool

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

fn gt(&self, __arg_0: &Id<S, T>) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, __arg_0: &Id<S, T>) -> bool

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

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

fn cmp(&self, __arg_0: &Id<S, T>) -> Ordering

This method returns an Ordering between self and other.

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

Compares and returns the maximum of two values.

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

Compares and returns the minimum of two values.

impl<X: ?Sized> Default for Id<X, X>

fn default() -> Self

Returns the "default value" for a type.

impl<A: ?Sized, B: ?Sized> Sync for Id<A, B>

impl<A: ?Sized, B: ?Sized> Send for Id<A, B>