Argument-position impl Trait requires a named lifetime

This function produces an "expected lifetime parameter" error:

fn foo(_: impl Iterator<Item = &u8>) {}

This code should instead be accepted and bound the impl Trait parameter by the elided lifetime.

cc #34511

In my opinion, we should accept '_, in-band lifetimes, and friends in where clauses as well. I'm not 100% sure whether this was controversial though, have to check.

Ran into this today. Providing the '_ lifetime gives a very fun error:

fn f(_: impl Iterator<Item = &'_ ()>) {}

error[E0106]: missing lifetime specifier
 --> src/lib.rs:1:31
  |
1 | fn f(_: impl Iterator<Item = &'_ ()>) {}
  |                               ^^ expected lifetime parameter

It's actually gotten slightly worse:

error[E0106]: missing lifetime specifier
 --> src/lib.rs:1:31
  |
1 | fn f(_: impl Iterator<Item = &'_ ()>) {}
  |                               ^^ expected named lifetime parameter
  |
help: consider introducing a named lifetime parameter
  |
1 | fn f(_: 'a, impl Iterator<Item = &'a ()>) {}
  |         ^^                       ^^^

#68583 will address that. After it gets merged the output will be

I feel that's going to be as good a diagnostic we're gonna have for this.

Account for `impl Trait`

@estebank

I feel that's going to be as good a diagnostic we're gonna have for this.

@estebank Does that mean that this will be the "final" state for this feature? As in, anonymous lifetimes will not be usable in the positions mentioned by @nikomatsakis in #49287 (comment)?

What I mean that until anonymous lifetimes are accepted in impl Trait we're going to have some diagnostic, and the one I posted is likely the best we can do given our constraints.

Rollup merge of rust-lang#68583 - estebank:hrlt, r=oli-obk

Account for `impl Trait`

The reason that lifetimes were not permitted here, as I recall, was some uncertainty about what they should mean (in particular, should impl Foo<'_> mean impl for<'a> Foo<'a> or what). I am actually still a bit unsure as to my opinion here -- I've seen people expect it to mean both things.

In this case, impl for<'a> Foo<'a> wouldn't work, but could you show me a case where it would? I feel that the only case where impl for<'a> X is common is for Fn() and in that case unnamed lifetimes usually work, and in the case where a named lifetime that hasn't been introduced is used produces the following now (not yet in the latest nightly):

error[E0261]: use of undeclared lifetime name `'a`
 --> file4.rs:1:27
  |
1 | fn foo(_compare: impl Fn(&'a u8)) {
  |                           ^^ undeclared lifetime
  |
  = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider introducing lifetime `'a` here
  |
1 | fn foo<'a>(_compare: impl Fn(&'a u8)) {
  |       ^^^^
help: consider making the bound lifetime-generic with a new `'a` lifetime
  |
1 | fn foo(_compare: impl for<'a> Fn(&'a u8)) {
  |                       ^^^^^^^

I'm not sure what you mean by "show you a case where it would work", I guess you mean a realistic example where that is what you would want? I don't really have one off the top of my head, but I agree with you it's probably quite a bit less common -- after all, I almost never write for bounds in practice.

So yeah, I pretty much agree it'd be the wrong meaning, and I think it'd even be somewhat counter-intuitive. We have a pretty strong precedent right now that '_ binds into the "innermost enclosing parentheses", in a sense, and I would be quite surprising to find impl Foo<'_> create a lifetime bound within the scope of impl keyword.

Auto merge of #68583 - estebank:hrlt, r=oli-obk

Current output:

error[E0658]: anonymous lifetimes in `impl Trait` are unstable
 --> src/lib.rs:1:33
  |
1 | fn foo(_: impl Iterator<Item = &u8>) {}
  |                                 ^ expected named lifetime parameter
  |
help: consider introducing a named lifetime parameter
  |
1 | fn foo<'a>(_: impl Iterator<Item = &'a u8>) {}
  |       ++++                          ++

For more information about this error, try `rustc --explain E0658`.

Navigation Menu

Repository navigation

Argument-position `impl Trait` requires a named lifetime #49287

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Argument-position impl Trait requires a named lifetime #49287

Description

Activity

nikomatsakis commented on Mar 23, 2018

alexreg commented on Jan 28, 2019

cramertj commented on Jan 29, 2019

alexreg commented on Jan 29, 2019

CAD97 commented on May 8, 2019

estebank commented on Jan 30, 2020

runiq commented on Jan 31, 2020

estebank commented on Jan 31, 2020

nikomatsakis commented on Feb 5, 2020

estebank commented on Feb 5, 2020

nikomatsakis commented on Feb 5, 2020

Dylan-DPC commented on Dec 12, 2024

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

Argument-position `impl Trait` requires a named lifetime #49287