visit enum init when the enum variable doesn't have any parameters

[lyming2007]

this fix #101208

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @petrochenkov (or someone else) soon.

Please see the contribution instructions for more information.

[petrochenkov]

This diagnostic is already reported in a very similar case

struct S(u8, u8);

fn main() {
    let _: S = S;
}

error[[E0308]](https://doc.rust-lang.org/nightly/error-index.html#E0308): mismatched types
 --> src/main.rs:4:16
  |
1 | struct S(u8, u8);
  | -------- fn(u8, u8) -> S {S} defined here
...
4 |     let _: S = S;
  |            -   ^ expected struct `S`, found fn item
  |            |
  |            expected due to this
  |
  = note: expected struct `S`
            found fn item `fn(u8, u8) -> S {S}`
help: use parentheses to construct this tuple struct
  |
4 |     let _: S = S(/* u8 */, /* u8 */);
  |                 ++++++++++++++++++++

, and it's not specific to tuple structs, it works for any function calls returning the expected type.

But for some reason it's not reported in cases like this

struct S(u8, u8);

fn main() {
    let S(_, _) = S;
}

error[[E0308]](https://doc.rust-lang.org/nightly/error-index.html#E0308): mismatched types
 --> src/main.rs:4:9
  |
4 |     let S(_, _) = S;
  |         ^^^^^^^   - this expression has type `fn(u8, u8) -> S {S}`
  |         |
  |         expected fn item, found struct `S`
  |
  = note: expected fn item `fn(u8, u8) -> S {S}`
              found struct `S`

I think we need to figure out why this S(_, _) doesn't correctly propagate the expected type in the old diagnostic instead of introducing a new diagnostic.
@rustbot author

[bors]

☔ The latest upstream changes (presumably #103636) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnCSimon]

@lyming2007
ping from triage - can you post your status on this PR? There hasn't been an update in a few months. Thanks!

FYI: when a PR is ready for review, send a message containing
@rustbot ready to switch to S-waiting-on-review so the PR is in the reviewer's backlog.

[lyming2007]

@lyming2007 ping from triage - can you post your status on this PR? There hasn't been an update in a few months. Thanks!

FYI: when a PR is ready for review, send a message containing @rustbot ready to switch to S-waiting-on-review so the PR is in the reviewer's backlog.

petrochenkov was right about his comment. There was a similar case. But they're actually very different in terms of implementation. My fix was straight forward but maybe not a perfect one. So I still need more work on this. I'll update it later

[JohnCSimon]

@lyming2007

Ping from triage: I'm closing this due to inactivity, Please reopen when you are ready to continue with this.
Note: if you are going to continue please open the PR BEFORE you push to it, else you won't be able to reopen - this is a quirk of github.
Thanks for your contribution.

@rustbot label: +S-inactive