Error with nested async fn and anonymous lifetimes

The following breaks on nightly (see playground)

#![feature(async_await)]

struct Foo<'a>(&'a ());

impl Foo<'_> {
    fn test() {
        async fn test() {}
    }
}

With:

error[E0261]: use of undeclared lifetime name `'_`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0261`.
error: Could not compile `playground`.

To learn more, run the command again with --verbose.

CC: dtolnay/async-trait#18

Reduced:

#![feature(async_await)]

struct Foo<'a>(&'a ());

impl<'a> Foo<'a> {
    fn test() {
        async fn test() {}
    }
}

Forgot to mention in case this helps, giving the inner fn a matching lifetime avoids this issue for some reason:

#![feature(async_await)]

struct Foo<'a>(&'a ());

impl<'a> Foo<'a> {
    fn test() {
        async fn test<'a>() {}
    }
}

@nikomatsakis

The reason this is happening is that the desugared version of async fn applies all in-scope lifetimes to the returned existential type, which in this case is creating -> Fut<'_>, which is invalid. I'm not sure exactly what the best fix is for this, though, since elided lifetimes in the impl signature are intentionally unnameable. The closest idea I have would be to create some special new lifetime that is '_ but can refer to lifetimes originating from the impl header. cc @nikomatsakis who may have opinions.

@cramertj

Hmm @cramertj wouldn't you expect the nested async fn to not inherit anything from its enclosing scopes? Seems like a bug to me that it does? Am I confused about something?

@nikomatsakis

@nikomatsakis The function in the middle is actually a bit of a source of confusion here, although it does indicate that there's a second bug since we shouldn't be inheriting generics since the nested async fn isn't part of the impl.

The bug I was focusing on is better illustrated by this example:

#![feature(async_await)]

struct Foo<'a>(&'a u8);

impl Foo<'_> {
    async fn bar() {}
}

which produces this error:

error[E0261]: use of undeclared lifetime name `'_`

error: aborting due to previous error

This is because the code is transformed into

impl Foo<'_> {
    fn bar() -> OpaqueTy<'_> {}
}

and in that case '_ does not refer back to the usage from the impl header.

@cramertj

@cramertj I see. Indeed, I would expect that, internally, we'd have some way to refer back to the '_ from the impl header. I believe this should be possible, no? In particular, the '_ in impls expands (internally) to LifetimeName::Param(ParamName::Fresh(N)) for some integer N, which I believe is basically treated like any other named lifetime.

@nikomatsakis

@nikomatsakis Yep, I would've expected so as well-- I'm not sure why that isn't working, I'd have to take a look.

@cramertj

@cramertj I was looking and I think the bug lies here:

rust/src/librustc/hir/lowering.rs

Lines 2267 to 2272 in c01be67

    
           let lifetime_params: Vec<(Span, ParamName)> = 
        
               this.in_scope_lifetimes 
        
                   .iter().cloned() 
        
                   .map(|ident| (ident.span, ParamName::Plain(ident))) 
        
                   .chain(this.lifetimes_to_define.iter().cloned()) 
        
                   .collect();

Probably in_scope_lifetimes needs to be a Vec<ParamName> instead, or we need to use something else for this purpose.

OK, I have a fix for the async fn method issue, but not the nested async fn item issue.

I forked off #63500 for the method issue

@cramertj

@cramertj I didn't prep a fix for this one yet. It seems obvious that some internal state is not being reset when we enter a new item.

but if you have a chance to investigate, would be appreciated

Presumably the problem is that visit_item doesn't "reset" the in_scope_lifetimes vector -- it only seems to extend it...

rust/src/librustc/hir/lowering/item.rs

Line 70 in c01be67

self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {

Indeed, in-band lifetimes are broken in the same way. This fails to compile (playground):

#![feature(in_band_lifetimes)]

struct Foo<'a> {
    x: &'a u32
    
}

impl Foo<'a> {
    fn method(&self) {
        fn blah(f: Foo<'a>) { }
    }
}

Working on a possible fix

Yep, have a fix.

Inband lifetimes looks similar to #52532 fwiw

@Mark-Simulacrum

@Mark-Simulacrum thanks

Rollup merge of rust-lang#63501 - nikomatsakis:issue-63500-async-anon…

Rollup merge of rust-lang#63209 - Centril:stabilize-async-await, r=cr…

Navigation Menu

Repository navigation

Error with nested async fn and anonymous lifetimes #63225

2 remaining items

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

	let lifetime_params: Vec<(Span, ParamName)> =
	this.in_scope_lifetimes
	.iter().cloned()
	.map(\|ident\| (ident.span, ParamName::Plain(ident)))
	.chain(this.lifetimes_to_define.iter().cloned())
	.collect();

Error with nested async fn and anonymous lifetimes #63225

Description

Activity

Centril commented on Aug 2, 2019

udoprog commented on Aug 2, 2019

cramertj commented on Aug 6, 2019

nikomatsakis commented on Aug 7, 2019

cramertj commented on Aug 7, 2019

nikomatsakis commented on Aug 12, 2019

cramertj commented on Aug 12, 2019

nikomatsakis commented on Aug 12, 2019

2 remaining items

nikomatsakis commented on Aug 12, 2019

nikomatsakis commented on Aug 12, 2019

nikomatsakis commented on Aug 12, 2019

nikomatsakis commented on Aug 12, 2019

nikomatsakis commented on Aug 12, 2019

nikomatsakis commented on Aug 12, 2019

nikomatsakis commented on Aug 12, 2019

nikomatsakis commented on Aug 13, 2019

Mark-Simulacrum commented on Aug 13, 2019

nikomatsakis commented on Aug 13, 2019

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions