When a method chain ending in `?` causes an E0277 because the
expression's `Result::Err` variant doesn't have a type that can be
converted to the `Result<_, E>` type parameter in the return type,
provide additional context of which parts of the chain can and can't
support the `?` operator.
```
error[E0277]: `?` couldn't convert the error to `String`
--> $DIR/question-mark-result-err-mismatch.rs:28:25
|
LL | fn bar() -> Result<(), String> {
| ------------------ expected `String` because of this
LL | let x = foo();
| ----- this can be annotated with `?` because it has type `Result<String, String>`
LL | let one = x
LL | .map(|s| ())
| ----------- this can be annotated with `?` because it has type `Result<(), String>`
LL | .map_err(|_| ())?;
| ---------------^ the trait `From<()>` is not implemented for `String`
| |
| this can't be annotated with `?` because it has type `Result<(), ()>`
|
= note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
= help: the following other types implement trait `From<T>`:
<String as From<char>>
<String as From<Box<str>>>
<String as From<Cow<'a, str>>>
<String as From<&str>>
<String as From<&mut str>>
<String as From<&String>>
= note: required for `Result<(), String>` to implement `FromResidual<Result<Infallible, ()>>`
```
Fix#72124.
Esteban Küber