CODEWITHSUNDEEP
rust
ysimonson
ysimonson

Reputation: 1102

Cannot find trait implementations for associated types

I have code that used to work on nightly-2016-11-15. When I upgraded to 1.15.1 stable, I started getting a bunch of errors about type implementations not being found. Here's an example:

error[E0277]: the trait bound `errors::Error: core::convert::From<r2d2_postgres::<unnamed>::error::Error>` is not satisfied
  --> src/pg/datastore.rs:79:23
   |
79 |         let results = conn.query("DELETE FROM accounts WHERE id=$1 RETURNING 1", &[&account_id])?;
   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `core::convert::From<r2d2_postgres::<unnamed>::error::Error>` is not implemented for `errors::Error`
   |
   = help: the following implementations were found:
   = help:   <errors::Error as core::convert::From<r2d2_postgres::Error>>
   = help:   <errors::Error as core::convert::From<postgres::error::Error>>
   = help:   <errors::Error as core::convert::From<r2d2::GetTimeout>>
   = help:   <errors::Error as core::convert::From<rocksdb::Error>>
   = help: and 3 others
   = note: required by `core::convert::From::from`

... even though there is a relevant From implementation. Here's a shortened version:

use std::error::Error as StdError;
use r2d2::GetTimeout;
use postgres::error::Error as PostgresError;
use r2d2_postgres::Error as R2D2PostgresError;
use super::fmt;

#[derive(Eq, PartialEq, Clone, Debug)]
pub enum Error {
    Unexpected(String),
    ...
}

impl StdError for Error {
    fn description(&self) -> &str {
        ...
    }

    fn cause(&self) -> Option<&StdError> {
        None
    }
}

impl From<R2D2PostgresError> for Error {
    fn from(err: R2D2PostgresError) -> Error {
        Error::Unexpected(format!("{}", err))
    }
}

impl From<PostgresError> for Error {
    fn from(err: PostgresError) -> Error {
        Error::Unexpected(pg_error_to_description(err))
    }
}

impl From<GetTimeout> for Error {
    fn from(err: GetTimeout) -> Error {
        Error::Unexpected(format!("Could not fetch connection: {}", err))
    }
}

I think this has something to do with the use of associated types, as it doesn't appear to happen in other contexts. Additionally, the namespace r2d2_postgres::<unnamed>::error::Error doesn't make sense - what is <unnamed>? Here is the relevant type association.

Upvotes: 1

Views: 293

Answers (1)

ysimonson
ysimonson

Reputation: 1102

This turned out to be due to version conflicts. I had switched to postgres's master branch to fix a separate version conflict, but r2d2_postgres was referencing a different version of postgres.

Luckily, as explained in this issue, Cargo.toml has a [replace] section that allows you to handle it like this: 

[replace]
"postgres:0.13.6" = { git = "https://github.com/sfackler/rust-postgres" }

Upvotes: 2

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