Provide more context on `?` where the error type can't be converted to the expected one

[estebank]

Code

struct E;

fn foo() -> Result<(), Box<dyn std::error::Error>> {
    Ok(bar()?)
}
fn bar() -> Result<(), E> {
    Err(E)
}
fn main() {}

Current output

error[E0277]: the trait bound `E: std::error::Error` is not satisfied
 --> bad.rs:4:13
  |
4 |     Ok(bar()?)
  |             ^ the trait `std::error::Error` is not implemented for `E`
  |
  = help: the trait `FromResidual<Result<Infallible, E>>` is implemented for `Result<T, F>`
  = note: required for `Box<dyn std::error::Error>` to implement `From<E>`

Desired output

error[E0277]: the trait bound `E: std::error::Error` is not satisfied
 --> bad.rs:4:13
  |
X | struct E;
  |        - doesn't implement trait `std::error::Error`
...
X | fn foo() -> Result<(), Box<dyn std::error::Error>> {
  |             -------------------------------------- requires that `Box<dyn std::error::Error>` implements `From<E>`
X |     Ok(bar()?)
  |             ^ the trait `From<E>` is not implemented for `Box<dyn std::error::Error>`
  |
  = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait

Rationale and extra context

As far as the (stable) end user is concerned, ? is a little more than a compiler intrinsic. We shouldn't mention FromResidual. We also should provide extra context on where the obligation comes from, and we should point at the return type of the enclosing function, and ideally also point at the type that couldn't be converted which is missing a From or Into impl.

Other cases

Rust Version

rustc 1.86.0-dev
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.86.0-dev
LLVM version: 19.1.7

Anything else?

No response

[mcclure]

Note: I believe that this change was inspired by me complaining on Mastodon about an error message I did not understand. In context on my machine, the true meaning of the error message was that my type had met the requirements for std::trait::Error but I needed to add an empty impl std::error:Error for MyError {} for it to actually have the trait. The original error message was inadequate for me to understand I needed this. On the other hand, I believe this altered error message would have been enough for me to work it out (in conjunction with the documentation I had).

[estebank]

@mcclure is there anything you'd want to add to the following?

error[E0277]: `?` couldn't convert the error: `E: std::error::Error` is not satisfied
  --> $DIR/bad-question-mark-on-trait-object.rs:7:13
   |
LL | fn foo() -> Result<(), Box<dyn std::error::Error>> {
   |             -------------------------------------- required `E: std::error::Error` because of this
LL |     Ok(bar()?)
   |        -----^ the trait `std::error::Error` is not implemented for `E`
   |        |
   |        this has type `Result<_, E>`
   |
note: `E` needs to implement `std::error::Error`
  --> $DIR/bad-question-mark-on-trait-object.rs:1:1
   |
LL | struct E;
   | ^^^^^^^^
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = note: required for `Box<dyn std::error::Error>` to implement `From<E>`

[mcclure]

@estebank , that seems fine to me