Why is the failure::Fail trait bound not satisfied by my Result type alias?

Question

I'm trying to implement event hooks as demonstrated by "simple event hooks in Rust" while also using the Error + ErrorKind pattern of the failure crate.

This is a stripped down version of my code:

#[macro_use]
extern crate failure;

use failure::{Backtrace, Context, Error, Fail};
use std::fmt;

#[derive(Debug)]
pub struct PortalError {
    inner: Context<PortalErrorKind>,
}

impl Fail for PortalError {
    fn cause(&self) -> Option<&Fail> {
        self.inner.cause()
    }

    fn backtrace(&self) -> Option<&Backtrace> {
        self.inner.backtrace()
    }
}

impl fmt::Display for PortalError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Display::fmt(&self.inner, f)
    }
}

#[derive(Copy, Clone, PartialEq, Debug, Fail)]
pub enum PortalErrorKind {
    #[fail(display = "Unknown Error")]
    Unknown,
}

//----------------------------------------------------------

pub type PortalResult<T> = Result<PortalError, T>;
pub trait Portal {
    fn get_something(&self) -> PortalResult<Vec<u32>>;
}

//----------------------------------------------------------

pub trait FeedApi<'a> {
    type T: FeedApi<'a>;

    fn new<P: Portal + 'a>(portal: P) -> Result<Self::T, Error>;
}

//----------------------------------------------------------

pub struct Feedly<'a> {
    portal: Box<Portal + 'a>,
}

impl<'a> FeedApi<'a> for Feedly<'a> {
    type T = Feedly<'a>;

    fn new<P: Portal + 'a>(portal: P) -> Result<Self::T, Error> {
        Ok(Feedly {
            portal: Box::new(portal),
        })
    }
}

impl<'a> Feedly<'a> {
    pub fn demo_function(&self) -> Result<(), Error> {
        let _ = self.portal.get_something().context(PortalErrorKind::Unknown)?;
        Ok(())
    }
}

fn main() {
    println!("Hello, world!");
}

[dependencies]
failure = "0.1.1"

In a method of 'Feedly' I want to use the portal:

self.portal.get_something().context(PortalErrorKind::Unknown)?

But I get the following error:

error[E0599]: no method named `context` found for type `std::result::Result<PortalError, std::vec::Vec<u32>>` in the current scope
  --> src/main.rs:67:45
   |
67 |         let _ = self.portal.get_something().context(PortalErrorKind::Unknown)?;
   |                                             ^^^^^^^
   |
   = note: the method `context` exists but the following trait bounds were not satisfied:
           `std::result::Result<PortalError, std::vec::Vec<u32>> : failure::Fail`
           `&std::result::Result<PortalError, std::vec::Vec<u32>> : failure::Fail`
           `&mut std::result::Result<PortalError, std::vec::Vec<u32>> : failure::Fail`

Looking through the docs the failure::Fail trait has a bound 'static . And the method context has a bound Self: Sized .

I'm not sure which trait is not satisfied here. The boxed Portal is neither Sized nor 'static , but the returned result should be, right?

This is the first time I'm handling boxes and lifetimes in Rust.

Answer 1

 pub type PortalResult<T> = Result<PortalError, T>; 

Result has two type parameters: the success type and the error type. You have transposed them; you want:

pub type PortalResult<T> = Result<T, PortalError>;