Suggest code to use enum type in return type when accidentally using variant instance

[estebank]

When encountering an enum variant instance in a return type, make a better effort to suggest appropriate code.

Right now, if you accidentally write Some(Span) when you meant Option<Span>, we mention the existence of the enum, but we don't suggest code:

error[E0573]: expected type, found variant `Some`
    --> src/librustc_typeck/check/mod.rs:5198:10
     |
5198 |     ) -> Some(Span) {
     |          ^^^^^^^^^^ not a type
     |
     = help: there is an enum variant `rustc::middle::cstore::LibSource::Some`, try using `rustc::middle::cstore::LibSource`?
     = help: there is an enum variant `rustc::session::config::Passes::Some`, try using `rustc::session::config::Passes`?
     = help: there is an enum variant `std::prelude::v1::Option::Some`, try using `std::prelude::v1::Option`?
     = help: there is an enum variant `std::prelude::v1::Some`, try using `std::prelude::v1`?

It'd be nice if we could suggest the appropriate code.

[zackmdavis]

@estebank Try nightly? I thought I addressed this in #56188?

[estebank]

@zackmdavis you're right. rustc hasn't updated to a recent enough nightly, which is why I didn't have it.

Trying it out in the playground, it doesn't seem to account for type arguments, locally accessible path (part of a larger issue) nor for reexported variants:

error[E0573]: expected type, found variant `Some`
 --> src/main.rs:5:13
  |
5 | fn foo() -> Some(usize) {
  |             ^^^^^^^^^^^ not a type
help: try using the variant's enum
  |
5 | fn foo() -> core::prelude::v1::Option {
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^
5 | fn foo() -> core::prelude::v1 {
  |             ^^^^^^^^^^^^^^^^^
5 | fn foo() -> lzw::Bits {
  |             ^^^^^^^^^
5 | fn foo() -> serde::export::Option {
  |             ^^^^^^^^^^^^^^^^^^^^^
and 4 other candidates

[zackmdavis]

it doesn't seem to account for type arguments

Oh, that's right. (We'd want the substitution to be Option<usize>, not Option.)

[estebank]

Triage: no change:

error[E0573]: expected type, found variant `Some`
 --> src/main.rs:5:13
  |
5 | fn foo() -> Some(usize) {
  |             ^^^^^^^^^^^ not a type
  |
help: try using the variant's enum
  |
5 - fn foo() -> Some(usize) {
5 + fn foo() -> core::option::Option {
  |
5 - fn foo() -> Some(usize) {
5 + fn foo() -> futures_util::__private {
  |
5 - fn foo() -> Some(usize) {
5 + fn foo() -> nom::lib::std::option::Option {
  |
5 - fn foo() -> Some(usize) {
5 + fn foo() -> pin_utils::core_reexport::option::Option {
  |
  = and 9 other candidates