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GAT lifetime error is unhelpful #91883

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vorot93 opened this issue Dec 14, 2021 · 3 comments · Fixed by #98538
Closed

GAT lifetime error is unhelpful #91883

vorot93 opened this issue Dec 14, 2021 · 3 comments · Fixed by #98538
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A-diagnostics Area: Messages for errors, warnings, and lints A-GATs Area: Generic associated types (GATs) D-papercut Diagnostics: An error or lint that needs small tweaks. D-terse Diagnostics: An error or lint that doesn't give enough information about the problem at hand. E-needs-test Call for participation: An issue has been fixed and does not reproduce, but no test has been added. F-generic_associated_types `#![feature(generic_associated_types)]` a.k.a. GATs GATs-triaged Issues using the `generic_associated_types` feature that have been triaged T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@vorot93
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vorot93 commented Dec 14, 2021

Given the following code:

#![feature(generic_associated_types)]

use std::fmt::Debug;
use std::marker::PhantomData;

#[derive(Debug)]
pub struct TransactionImpl<'db> {
    _marker: PhantomData<&'db ()>,
}

#[derive(Debug)]
pub struct CursorImpl<'txn> {
    _marker: PhantomData<&'txn ()>,
}

pub trait Cursor<'txn> {}

pub trait Transaction<'db>: Send + Sync + Debug + Sized {
    type Cursor<'tx>: Cursor<'tx>
    where
        'db: 'tx,
        Self: 'tx;

    fn cursor<'tx>(&'tx self) -> Result<Self::Cursor<'tx>, ()>
    where
        'db: 'tx;
}

impl<'tx> Cursor<'tx> for CursorImpl<'tx> {}

impl<'db> Transaction<'db> for TransactionImpl<'db> {
    type Cursor<'tx> = CursorImpl<'tx>;

    fn cursor<'tx>(&'tx self) -> Result<Self::Cursor<'tx>, ()>
    where
        'db: 'tx,
    {
        loop {}
    }
}

The current output is:

error[E0478]: lifetime bound not satisfied
  --> src/lib.rs:32:24
   |
32 |     type Cursor<'tx> = CursorImpl<'tx>;
   |                        ^^^^^^^^^^^^^^^
   |
note: lifetime parameter instantiated with the lifetime `'db` as defined here
  --> src/lib.rs:31:6
   |
31 | impl<'db> Transaction<'db> for TransactionImpl<'db> {
   |      ^^^
note: but lifetime parameter must outlive the lifetime `'tx` as defined here
  --> src/lib.rs:32:17
   |
32 |     type Cursor<'tx> = CursorImpl<'tx>;

Error is fixed by adding a where clause to associated type in impl, which is not immediately clear:

impl<'db> Transaction<'db> for TransactionImpl<'db> {
    type Cursor<'tx>
    where
        'db: 'tx,
    = CursorImpl<'tx>;
}
@vorot93 vorot93 added A-diagnostics Area: Messages for errors, warnings, and lints T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Dec 14, 2021
@jackh726 jackh726 added the F-generic_associated_types `#![feature(generic_associated_types)]` a.k.a. GATs label Dec 14, 2021
@jackh726
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GATs issue triage: not blocking. Poor diagnostic, but not a miscompile.

@jackh726 jackh726 added the GATs-triaged Issues using the `generic_associated_types` feature that have been triaged label Dec 14, 2021
@BGR360
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BGR360 commented Dec 25, 2021

@rustbot label +D-papercut +D-terse

@rustbot rustbot added D-papercut Diagnostics: An error or lint that needs small tweaks. D-terse Diagnostics: An error or lint that doesn't give enough information about the problem at hand. labels Dec 25, 2021
@jackh726
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jackh726 commented Mar 9, 2022

Current output is

help: try copying these clauses from the trait: `where 'db: 'tx, Self: 'tx`

I think that's pretty helpful. Going to mark as needs-test

@jackh726 jackh726 added the E-needs-test Call for participation: An issue has been fixed and does not reproduce, but no test has been added. label Mar 9, 2022
@TaKO8Ki TaKO8Ki self-assigned this Jun 26, 2022
TaKO8Ki added a commit to TaKO8Ki/rust that referenced this issue Jun 26, 2022
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Jun 26, 2022
bors added a commit to rust-lang-ci/rust that referenced this issue Jun 26, 2022
…askrgr

Rollup of 11 pull requests

Successful merges:

 - rust-lang#97140 (std: use an event-flag-based thread parker on SOLID)
 - rust-lang#97295 ([rustc_parse] Forbid `let`s in certain places)
 - rust-lang#97743 (make const_err show up in future breakage reports)
 - rust-lang#97908 (Stabilize NonZero* checked operations constness.)
 - rust-lang#98297 (Transform help popup into a pocket menu)
 - rust-lang#98428 (macros: use typed identifiers in diag and subdiag derive)
 - rust-lang#98528 (Respect --color when building rustbuild itself)
 - rust-lang#98535 (Add regression test for generic const in rustdoc)
 - rust-lang#98538 (Add a ui test for issue rust-lang#91883)
 - rust-lang#98540 (Add regression test for rust-lang#87558)
 - rust-lang#98541 (Update `std::alloc::System` doc example code style)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@fmease fmease added the A-GATs Area: Generic associated types (GATs) label Nov 2, 2024
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Labels
A-diagnostics Area: Messages for errors, warnings, and lints A-GATs Area: Generic associated types (GATs) D-papercut Diagnostics: An error or lint that needs small tweaks. D-terse Diagnostics: An error or lint that doesn't give enough information about the problem at hand. E-needs-test Call for participation: An issue has been fixed and does not reproduce, but no test has been added. F-generic_associated_types `#![feature(generic_associated_types)]` a.k.a. GATs GATs-triaged Issues using the `generic_associated_types` feature that have been triaged T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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