Is `&'a Foo<'a>` a footgun?

[quinedot]

@simonbuchan

Hopefully it's at least implicit from general Rust usage that you can pass the longer lifetime in as a parameter, so the subtyping relationship is the shorter lifetime is the subtype, but then generalizing that to a (shared) reference with a lifetime parameter is as a field an output usage and therefore contravariant, but as a parameter type an input usage and therefore covariant, etc. is a bit much to expect someone to figure out without any hints.

You can coerce from subtypes to supertypes. So a &'long T is a subtype of &'short T.

Lifetime aren't types so it's all a bit of an abuse of terminology when we talk about their "subtypes". You actually have a choice between putting 'static at the top or the bottom of the hierarchy. So we could have said that the 'a in &'a T is contravariant -- and in fact, we use to! -- in which case shorter lifetimes are "subs" of longer lifetimes and 'static is at the top (and there is no bottom).

But (today) the terminology that has been settled on is to call it covariant. With that covariant terminology, the longer lifetime must be a "sub" of the shorter lifetime, and 'static is the bottom of the hierarchy (and there is no top).

Which is confusing to many. (But I guess having "contra" so prevalent was also confusing.)