Provide better compiler output when using ? on Option in fn returning Result or vice-versa?

[alvinhochun]

It seems that rustc should provide a better error message or hint when one tries to use the ? operator on an Option in a function returning a Result or vice-versa.

Currently this is the error you get:

error[E0277]: `?` couldn't convert the error to `MyErrorType`
  --> src/main.rs:10:19
   |
10 |     let a = foo(i)?;
   |                   ^ the trait `std::convert::From<std::option::NoneError>` is not implemented for `MyErrorType`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = note: required by `std::convert::From::from`

However, it would be better to suggest the use of Option::ok_or or Option::ok_or_else, or Result::ok, instead of referencing something dependent on the try_trait feature (NoneError).

This issue has been assigned to @Duddino via this comment.

[estebank]

This error is emitted here:

rust/src/librustc_trait_selection/traits/error_reporting/mod.rs

Lines 278 to 313 in 8e18e26

	let is_try = self
	.tcx
	.sess
	.source_map()
	.span_to_snippet(span)
	.map(|s| &s == "?")
	.unwrap_or(false);
	let is_from = format!("{}", trait_ref.print_only_trait_path())
	.starts_with("std::convert::From<");
	let (message, note) = if is_try && is_from {
	(
	Some(format!(
	"`?` couldn't convert the error to `{}`",
	trait_ref.self_ty(),
	)),
	Some(
	"the question mark operation (`?`) implicitly performs a \
	conversion on the error value using the `From` trait"
	.to_owned(),
	),
	)
	} else {
	(message, note)
	};
	let mut err = struct_span_err!(
	self.tcx.sess,
	span,
	E0277,
	"{}",
	message.unwrap_or_else(|| format!(
	"the trait bound `{}` is not satisfied{}",
	trait_ref.without_const().to_predicate(),
	post_message,
	))
	);

We could look at whether the trait_ref is std::convert::From<std::option::NoneError>, and if so, provide a more targeted error, including a structured suggestion for ok_or_else.

[Duddino]

Can I claim this issue?

[Duddino]

@rustbot claim