suggested fix for undeclared hrtb lifetimes in where clauses in produces error "[E0316] nested quantification of lifetimes" when specified just before the trait

Code

trait Trait<T>
  where T: for<'a> Trait<T> + 'b { }

Current output

error[E0261]: use of undeclared lifetime name `'b`
 --> src/lib.rs:2:31
  |
2 |   where T: for<'a> Trait<T> + 'b { }
  |                               ^^ undeclared lifetime
  |
  = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider making the bound lifetime-generic with a new `'b` lifetime
  |
2 |   where for<'b> T: for<'a> Trait<T> + 'b { }
  |         +++++++
help: consider introducing lifetime `'b` here
  |
1 | trait Trait<'b, T>
  |             +++

Desired output

error[E0261]: use of undeclared lifetime name `'b`
 --> src/lib.rs:2:31
  |
2 |   where T: for<'a> Trait<T> + 'b { }
  |                               ^^ undeclared lifetime
  |
  = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider making the bound lifetime-generic with a new `'b` lifetime
  |
2 |   where T: for<'a, 'b> Trait<T> + 'b { }
  |                  ++++
help: consider introducing lifetime `'b` here
  |
1 | trait Trait<'b, T>
  |             +++

Rationale and extra context

HRTB lifetimes can be put not only before the constrained type (for<...> T: ...), but also just before the constraining trait (T: for<...> Trait ...). When in this position, compiler suggests code which produces error "[E0316] nested quantification of lifetimes" as it ignores the existing HRTB just before the trait and only looks at constraint HRTB.

Other cases

No response

Rust Version

$ rustc --version --verbose
rustc 1.76.0 (07dca489a 2024-02-04)
binary: rustc
commit-hash: 07dca489ac2d933c78d3c5158e3f43beefeb02ce
commit-date: 2024-02-04
host: aarch64-apple-darwin
release: 1.76.0
LLVM version: 17.0.6

Anything else?

No response

I think this should suggest :

trait Trait<'b, T>
  where T: for<'a> Trait<'b, T> + 'b { }

Because 'b is bound for T

@rustbot

@rustbot claim

I think this should suggest :
trait Trait<'b, T>
  where T: for<'a> Trait<'b, T> + 'b { }
Because 'b is bound for T

It already does suggest that, I think. There are two suggestions.

help: consider making the bound lifetime-generic with a new `'b` lifetime
  |
2 |   where T: for<'a, 'b> Trait<T> + 'b { }
  |                  ++++
help: consider introducing lifetime `'b` here
  |
1 | trait Trait<'b, T>
  |             +++

Desired output

2 |   where T: for<'a, 'b> Trait<T> + 'b { }
  |                  ++++

where T: for<'a, 'b> Trait<T> + 'b isn't valid either tho b/c for<> after : only universally quantifies (i.e., introduces universal parameters for) the trait ref immediately following the binder meaning 'b in + 'b is still undefined / unbound.

We should keep the preexisting trait Trait<'b, T> suggestion (which can lead to follow-up errors unfortunately) and maybe suggest:

- where T: for<'a> Trait<T> + 'b
+ where for<'a, 'b> T: Trait<T> + 'b

removing the inner binder (which might be a bit harder to do). The outer binder to the left of the : quantifies the entire bound.

Fix incorrect suggestion for undeclared hrtb lifetimes in where clauses.

Fpoly_trait_refix incorrect suggestion for undeclared hrtb lifetimes …

Fix incorrect suggestion for undeclared hrtb lifetimes in where clauses.

Rollup merge of rust-lang#123122 - surechen:fix_122714, r=fmease

Unrolled build for rust-lang#123122

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suggested fix for undeclared hrtb lifetimes in where clauses in produces error "[E0316] nested quantification of lifetimes" when specified just before the trait #122714

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Rationale and extra context

Other cases

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suggested fix for undeclared hrtb lifetimes in where clauses in produces error "[E0316] nested quantification of lifetimes" when specified just before the trait #122714

Description

Code

Current output

Desired output

Rationale and extra context

Other cases

Rust Version

Anything else?

Activity

surechen commented on Mar 19, 2024

surechen commented on Mar 19, 2024

polina4096 commented on Mar 19, 2024

fmease commented on Mar 19, 2024

Desired output

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