Lifetime for (async) closure args / Problem with workaround: expected X found (same) X

[Yandros]

The trait idea is quite neat , although it would break for callers used to writing .boxed() or .boxed_local() over Box::pin. Hence why I personally find that that "discarded argument" approach with a PhantomData or an empty array can be less surprising.

@jbe

Very nice! Maybe using a PhantomData would be more idiomatic though?

As was written in the linked post, yeah you could even have a PhantomBound or something to express the intent even clearer.

@Yandros

This quirk may be more readable by defining a

type PhantomBound<'big, 'small> = PhantomData<&'small &'big ()>;

and then write the signature as:

for<'enc>
    F : FnMut(&mut RenderPas<'enc>, PhantomBound<'upper_bound, 'enc>)
,

In general, PhantomData<T> is a bit superior to [T; 0] insofar it doesn't raise the alignment of the data structure. But for a zero-sized parameter passed by value, the alignment doesn't matter, and writing [] is easier than writing PhantomData, hence my going for [], here.

Finally, in your case, in order to improve the ergonomics, you could instead wrap the parameter s, and use, for instance, a &'a &'max mut str^[1], or some &'a Str<'max> for some helper type Str<'max> that is Deref<Target = str>. I still think I'd just go for the discarded parameter.

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1. It is sound to transmute &'a &'a T to &'a &'_ mut T ↩︎