How to understand why Box<&'longer T> is a subtype of Box<&'shorter T>>

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I would like to share about my intuition for SubTrait: SuperTrait.

Let's think this way:
the definition of
subset A = { types required to live for 'long lifetime } and
superset B = { types required to live for 'short lifetime }
is correct because that's to say
any type living for 'long is a type living for 'short[1].
A variable living for 'long can naturally shorten -- you can ask a variable living for 'long to just live for 'short.[2]

We don't think the case that any type living for 'short is a type living for 'long is correct:
obviously you cannot ask a variable only living for 'short to live for 'long.

The point here is to always stand in the perspective of types. Not in the perspective of lifetime itself -- the region where a value/reference is alive.

And the key concept for subtyping I learnt from Jonhoo's video, which has been posted above, is "usability".
Subtypes are more usable than supertypes for various reasons. Like for 'static: 'any, we don't manually and explicitly cast a 'static variable to a new one with a shorter lifetime, so 'static is more usable (and easy to use).

  1. equivalent to the definition in the link I share: all elements of A are also elements of B ↩︎

  2. Well, I might misuse the term variable and type. But a variable must have its specific type. ↩︎

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