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error: X does not live long enough doesn't provide enough context #111852

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frederikhors opened this issue May 22, 2023 · 4 comments
Open

error: X does not live long enough doesn't provide enough context #111852

frederikhors opened this issue May 22, 2023 · 4 comments
Labels
A-async-await Area: Async & Await A-diagnostics Area: Messages for errors, warnings, and lints A-GATs Area: Generic associated types (GATs) A-lifetimes Area: Lifetimes / regions AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. C-bug Category: This is a bug. D-terse Diagnostics: An error or lint that doesn't give enough information about the problem at hand. S-has-mcve Status: A Minimal Complete and Verifiable Example has been found for this issue

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@frederikhors
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frederikhors commented May 22, 2023

REPRODUCTION: https://www.rustexplorer.com/b/oe3uh4

error:

error: `impl Executor` does not live long enough
   --> src/main.rs:136:5
    |
136 | /     async move {
137 | |         dbg!("I'm:", user);
138 | |
139 | |         let mut store = store.unwrap_or_default();
...   |
169 | |     }
170 | |     .boxed()
    | |____________^

Some code:

fn query_teams<'a>(
    user: &'a str,
    db: &'a mut impl Executor,
    store: Option<Store>,
    with_player: bool,
) -> BoxFuture<'a, Store> {
    async move {
        dbg!("I'm:", user);

        let mut store = store.unwrap_or_default();

        let mut query = sqlx::QueryBuilder::new(r#"SELECT "teams".*" FROM "teams"#);

        let teams = query
            .build_query_as::<Team>()
            .fetch_all(db.as_executor())
            .await
            .unwrap();

        for team in teams {
            store.teams.insert(team.id.to_string(), team);
        }

        if with_player {
            store = query_players(user, db, Some(store), false).await;
        }

        store
    }
    .boxed()
}
error: `impl Executor` does not live long enough
   --> src/main.rs:136:5
    |
136 | /     async move {
137 | |         dbg!("I'm:", user);
138 | |
139 | |         let mut store = store.unwrap_or_default();
...   |
169 | |     }
170 | |     .boxed()
    | |____________^

Meta

rustc 1.69.0 (84c898d65 2023-04-16)
binary: rustc
commit-hash: 84c898d65adf2f39a5a98507f1fe0ce10a2b8dbc
commit-date: 2023-04-16
host: x86_64-pc-windows-msvc
release: 1.69.0
LLVM version: 15.0.7
@frederikhors frederikhors added the C-bug Category: This is a bug. label May 22, 2023
@fee1-dead fee1-dead added the E-needs-mcve Call for participation: This issue has a repro, but needs a Minimal Complete and Verifiable Example label May 24, 2023
@fee1-dead
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simplified a bit using perses, still need to eliminate the dependency though.

use futures::future::{BoxFuture, FutureExt};

trait Executor: Send {
    type Executor<'this>: sqlx::PgExecutor<'this>
    where
        Self: 'this;
    fn as_executor(&mut self) -> Self::Executor<'_>;
}

struct Store {}

#[derive(sqlx::FromRow)]
struct Team {}

fn query_teams(user: &str, db: impl Executor, store: Store) -> BoxFuture<Store> {
    async move {
        let query = sqlx::QueryBuilder::new(r#"SELECT "teams".*" FROM "teams"#);
        query
            .build_query_as::<Team>()
            .fetch_all(db.as_executor())
            .await;
        store
    }
    .boxed()
}

@jyn514 jyn514 added A-diagnostics Area: Messages for errors, warnings, and lints A-lifetimes Area: Lifetimes / regions A-async-await Area: Async & Await labels May 26, 2023
@fee1-dead fee1-dead changed the title Maybe a bad-error-message? error: X does not live long enough doesn't provide enough context May 30, 2023
@WaffleLapkin
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Here is, what I believe is, the root cause/MCVE of this issue (play):

trait G: Send {
    type Gat<'l>: Send
    where
        Self: 'l;

    fn as_gat(&self) -> Self::Gat<'_>;
}

fn a(g: impl G) {
    let _: &dyn Send = &async move {
        let _gat = g.as_gat();
        async{}.await
    };
}

It looks like this is what happens:

  1. Proving async{}: Send requires that typeof(_gat): Send (because it lives across an .await point)
  2. typeof(_gat): Send requires Self: 'l
  3. impl G: 'l is the problem

@WaffleLapkin WaffleLapkin added S-has-mcve Status: A Minimal Complete and Verifiable Example has been found for this issue and removed E-needs-mcve Call for participation: This issue has a repro, but needs a Minimal Complete and Verifiable Example labels May 30, 2023
@tmandry tmandry added F-generic_associated_types `#![feature(generic_associated_types)]` a.k.a. GATs D-terse Diagnostics: An error or lint that doesn't give enough information about the problem at hand. labels Jun 5, 2023
@eholk eholk added the AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. label Jun 5, 2023
@QuineDot
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QuineDot commented Jun 9, 2023

Removing where Self: 'l from the GAT works around the issue.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1cb323c9dbbf99bd8cba8870286fc19d

@WaffleLapkin
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@QuineDot note that this is not always possible, sometimes such bound is required.

@fmease fmease added A-GATs Area: Generic associated types (GATs) and removed F-generic_associated_types `#![feature(generic_associated_types)]` a.k.a. GATs labels Sep 24, 2024
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Labels
A-async-await Area: Async & Await A-diagnostics Area: Messages for errors, warnings, and lints A-GATs Area: Generic associated types (GATs) A-lifetimes Area: Lifetimes / regions AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. C-bug Category: This is a bug. D-terse Diagnostics: An error or lint that doesn't give enough information about the problem at hand. S-has-mcve Status: A Minimal Complete and Verifiable Example has been found for this issue
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