E0700 does not point to where the captured lifetime is

[Nokel81]

The error message

error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds

does not point to where in the block the captured lifetime is (which makes debugging it difficult).

It does, however, point to the scope:

note: hidden type `impl futures::Future` captures the scope of call-site for function at 48:40
  --> rs/agent-updater/src/fetcher.rs:48:40
   |
48 |       ) -> Result<PathBuf, FetcherError> {
   |  ________________________________________^
49 | |         let checksum = m.get_valid_checksum()?;
51 | |
...  |
72 | |         Ok(folder)
73 | |     }
   | |_____^

[Centril]

@Nokel81 Do you have / could you make a small self-contained example we could use when trying to improve the diagnostics?

[Nokel81]

Yes, here is a minimal example. (cannot be a playground link since the playground doesn't have slog.

Example:

use std::{
    collections::HashMap, 
    io,
    sync::Mutex, 
    path::PathBuf
};
use reqwest::Response;
use hyper;
use slog::Logger;

pub struct Module;
pub struct DownloaderError;

pub trait Downloader {
    fn get<'life0, 'life1, 'async_trait>(
        &'life0 self,
        url: &'life1 str,
    ) -> ::core::pin::Pin<
        Box<
            dyn ::core::future::Future<Output = Result<Response, DownloaderError>>
                + ::core::marker::Send
                + 'async_trait,
        >,
    >
    where
        'life0: 'async_trait,
        'life1: 'async_trait,
        Self: 'async_trait;
}

pub struct Fetcher<T>
where
    T: Downloader,
{
    downloader: T,
    storage_folder: PathBuf,
    download_cache: Mutex<HashMap<[u8; 32], Vec<u8>>>,
}

impl<T> Fetcher<T>
where
    T: Downloader,
{
    pub fn new(downloader: T, storage_folder: PathBuf) -> Fetcher<T> {
        Self {
            downloader,
            storage_folder,
            download_cache: Mutex::new(HashMap::new()),
        }
    }

    pub async fn fetch_module(
        &self,
        logger: Logger,
        m: &Module,
    ) -> Result<PathBuf, DownloaderError> {
        Ok(PathBuf::new())
    }
}

This is compiled on stable 1.40.

If you remove the line logger: Logger, from the fetch_module method then the compiler error goes away.

[skeet70]

This may be related to #63033.

[tmandry]

Marking as triaged. This issue would benefit from a more minimized example.

[Nokel81]

I try and get a smaller one tomorrow.

[kadiwa4]

Minimized:

trait Trait {}
async fn f(_: &(), _: Box<dyn Trait>, _: &()) {}

This compiles successfully since 1.69.0.
@rustbot label: -E-needs-mcve +S-has-mcve