Automatic error conversion using the '?' operator for custom types

Question

I'm struggling to understand the nuances of the ? operator. Take the following code:

use std::{error::Error as StdError, fmt};

#[derive(Debug)]
struct MyError(Box<dyn StdError>);

impl fmt::Display for MyError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(self, f)
    }
}

impl StdError for MyError{}

impl From<Box<dyn StdError>> for MyError {
    fn from(err: Box<dyn StdError>) -> Self {
        MyError(err)
    }
}

#[derive(Debug)]
struct RandomErr(String);

impl fmt::Display for RandomErr {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(self, f)
    }
}

impl StdError for RandomErr{}

fn no_custom() -> Result<(), Box<dyn StdError>> {
    Err(RandomErr("hello there".to_owned()))?
}

// This fails to compile
fn custom() -> Result<(), MyError> {
    Err(RandomErr("hello there".to_owned()))?
}

I would think that custom() should compile. RandomError is a StdError, so RandomErr should be convertable to MyError since there's an impl for converting from StdError, no?

Masklinn · Accepted Answer · 2021-02-23 06:40:24Z

3

I would think that custom() should compile. RandomError is a StdError, so RandomErr should be convertable to MyError since there's an impl for converting from StdError, no?

Nope. There is no transitivity in From (or any trait, as far as I know). Rustc generally does what you tell it and no more to avoid problems like combinatory explosions in trait resolution.

So that C: From<B> and B: From<A> does not imply / translate to C: From<A>, you can write that reduced case and will hit E0277 (trait not satisfied):

struct A;
struct B;
struct C;

impl From<A> for B {
    fn from(a: A) -> Self { B }
}
impl From<B> for C {
    fn from(b: B) -> Self { C }
}

fn main() {
    let _: C = From::from(A);
}

answered Feb 23, 2021 at 6:40

Masklinn

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