Provide suggestion for error when `if let` used with enum variant whose values are not initialized

[Rageking8]

Given the following code: link

enum E {
    One(i32, i32)
}

fn main() {
    let var = E::One;
    if let E::One(var1, var2) = var {
        println!("{var1} {var2}");
    }
}

The current output is:

Compiling playground v0.0.1 (/playground)
error[[E0308]](https://doc.rust-lang.org/nightly/error-index.html#E0308): mismatched types
 --> src/main.rs:7:12
  |
7 |     if let E::One(var1, var2) = var {
  |            ^^^^^^^^^^^^^^^^^^   --- this expression has type `fn(i32, i32) -> E {E::One}`
  |            |
  |            expected fn item, found enum `E`
  |
  = note: expected fn item `fn(i32, i32) -> E {E::One}`
                found enum `E`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

Ideally the output should suggest to the user that they need to initialize the enum variant's value in order to use the if let.

[lyming2007]

Can you give the output of what it should look like?

[fmease]

It should probably look similar to the suggestion given for

• let x: Option<i32> = Some; and
• let some = Some; let x: Option<i32> = some;

So I guess sth. like:

  error[E0308]: mismatched types
   --> src/main.rs:7:12
    |
  7 |     if let E::One(var1, var2) = var {
    |            ^^^^^^^^^^^^^^^^^^   --- this expression has type `fn(i32, i32) -> E {E::One}`
    |            |
    |            expected fn item, found enum `E`
    |
    = note: expected fn item `fn(i32, i32) -> E {E::One}`
                  found enum `E`
+ help: use parentheses to instantiate this tuple variant
+   |
+ 7 |     if let E::One(var1, var2) = var(_, _) {
+   |                                    ++++++

Edit: Or instead of saying “to instantiate this tuple variant” we could say “to call this function” like it's done for the erroneous snippet let x: bool = make; (where make is a fn(i32) -> bool).

[lyming2007]

@rustbot claim

[lyming2007]

commit the fix #102062