Difference in lifetime between passing trait and concrete struct as argument to function

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There is also this other very lengthy and detailed post I have written quite recently: Looking for a deeper understanding of PhantomData - #4 by Yandros

In there, I show a macro to test subtyping (and thus variance):

macro_rules! const_assert_subtypes {(
    for [<$($generics:tt)*]
    $T1:ty : $($T2:tt)*
) => (
    const _: () = {
        fn foo<$($generics)* (x: *const $T1)
          -> *const $($T2)*
        {
             x
        }
    };
)}

trait Trait<'lifetime> {}

const_assert_subtypes!(
    for [<'short, 'long : 'short>]
    dyn Trait<'long> : dyn Trait<'short>
);

// Same for dyn Trait<'short> : dyn Trait<'long>
// And for impl Trait
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