Better Error Messages for Partially Implemented Traits

[eyolfson]

Specifically, for types that have a trait implemented, but require the types they use to have the trait implemented as well. For instance, Result. Consider the following example:

#[derive(Debug)]
struct X;

struct Y;

fn main() {
    let x: Result<X, Y> = Ok(X);
    println!("{:?}", x);
}

Produces the following error message:

issue.rs:8:22: 8:23 error: the trait `core::fmt::Debug` is not implemented for the type `core::result::Result<X, Y>` [E0277]

This error message is confusing to someone beginning Rust, because according to the documentation, Result does implement Debug. In this case a clearer error message would be something like:

issue.rs:8:22: 8:23 error: the trait `core::fmt::Debug` is not implemented for the type `core::result::Result<X, Y>` because type `Y` does not implement `core::fmt::Debug`

Perhaps the error message could be tweaked, but in this case it should clearly point to Y as being the problem. Of course, if X and Y don't implement core::fmt::Debug the error message should reflect that was well.

[nagisa]

according to the documentation, Result does implement Debug.

Documentation clearly states there’s only impl<T, E> Debug for Result<T, E> where T: Debug, E: Debug, which does not apply to Result<A, B>∀A, B, which your sentence implies is the case.

[eyolfson]

I stated, for a beginner. So someone that just scans that it exists, at all. The main point is to just improve the error message to state which specific type T or E is missing an implementation of the Debug trait that's causing Results implementation of Debug not to exist.

[gsingh93]

Related: #20941

[apasel422]

This is essentially fixed:

> rustc --version
rustc 1.3.0-nightly (e4e93196e 2015-07-14)
> rustc foo.rs 
foo.rs:8:22: 8:23 error: the trait `core::fmt::Debug` is not implemented for the type `Y` [E0277]
foo.rs:8     println!("{:?}", x);

[Gankra]

... which is just #21793

[birkenfeld]

New error, but seems still fixed:

<anon>:8:22: 8:23 error: the trait bound `Y: std::fmt::Debug` is not satisfied [E0277]
<anon>:8     println!("{:?}", x);
                              ^
<anon>:8:22: 8:23 help: see the detailed explanation for E0277
<anon>:8:22: 8:23 note: `Y` cannot be formatted using `:?`; if it is defined in your crate, add `#[derive(Debug)]` or manually implement it

[arielb1]

Yeah. Fixed.