Question about RTN MVP

help
1

Hey folks

I've seen the new blog post about RTN, there you can read:

Currently, RTN is only allowed for trait associated functions and methods with lifetime generics (not const or type generics) that use:

async fn in traits (AFIT) or
return-position impl Trait in traits (RPITIT) where the impl Trait is the outermost return type, i.e. -> impl Trait, but not -> Box<impl Trait>.

These restrictions are described in further detail in RFC 3654.

I understand the point in the RFC why you can only use it currently with AFIT and RPITIT but I cannot find any real explaination why RTN cannot be used with functions/methods that have const or type generics. The RFC says at least:

The method named in an RTN type may have generic parameters (e.g., fn widgets<'a>(&'a self) has a lifetime parameter 'a). Because RTN locations are limited to where-clauses and trait bounds in this RFC, these parameters can always be captured in a for to form a higher-ranked trait bound.

So is the generic lifetime only support here a design decision to make sure that RTNs - always - work with HRTBs or are there any other reasons for this restriction?

Regards
keks

PS: I ask this just out of curiosity because I want to understand it. :slight_smile:

2

Ok, I think I might understand it now. It seems as if generic parameters on a function have to desugar always to a higher-ranked generic on a RTN bound and as this is not yet possible for types you can only use functions with generic lifetimes for now.

3

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