Trait higher_kinded_types::extra_arities::CovariantForLt

source ·

pub trait CovariantForLt {
    type Of<'lt>
       where Self: 'lt;

    // Required method
    fn covariant_cast<'smol, 'humongous: 'smol>(
        it: Self::Of<'humongous>
    ) -> Self::Of<'smol>;
}

Available on advanced only.

Expand description

Same as crate::ForLifetime, but for enforcing covariance of Self::Of<'_> (over '_).

Example

use ::higher_kinded_types::extra_arities::*;

//                                👇
fn higher_kinded_api<'caller, T : CovariantForLt>(
    caller: T::Of<'caller>,
    from_str: impl FnOnce(&str) -> T::Of<'_>,
) -> bool
where
    for<'callee>
        T::Of<'callee> : PartialEq
    ,
{
    let local: String = ::std::fs::read_to_string(file!()).expect("demo");
    let callee: T::Of<'_> = from_str(&local);
    let comparison = {
        callee == T::covariant_cast(caller) // 👈
    };
    comparison
}

new_For_type! {
    type StrRef = For!(#![covariant]<'r> = &'r str);
}

higher_kinded_api::<StrRef>("…", |s| s);

Counter-example

use ::higher_kinded_types::extra_arities::*;

new_For_type! {
    type NotCov = For!(#![covariant]<'r> = &'r mut &'r str);
}

yields:

error: lifetime may not live long enough
 --> src/lib.rs:126:1
  |
6 | / new_For_type! {
7 | |     type NotCov = For!(#![covariant]<'r> = &'r mut &'r str);
8 | | }
  | | ^
  | | |
  | | lifetime `'if_you_are_getting_this_error` defined here
  | |_lifetime `'it_means_your_type_is_not_covariant` defined here
  |   associated function was supposed to return data with lifetime `'it_means_your_type_is_not_covariant` but it is returning data with lifetime `'if_you_are_getting_this_error`
  |
  = help: consider adding the following bound: `'if_you_are_getting_this_error: 'it_means_your_type_is_not_covariant`
  = note: requirement occurs because of a mutable reference to `&str`
  = note: mutable references are invariant over their type parameter
  = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
  = note: this error originates in the macro `$crate::ඞFor` which comes from the expansion of the macro `new_For_type` (in Nightly builds, run with -Z macro-backtrace for more info)

Required Associated Types§

source

type Of<'lt> where Self: 'lt

In order to help palliate WF-bounds, this trait carries such a bound.

Required Methods§