Suggest a trailing comma if a 1-tuple is expected and a parenthesized expression is found

[FabianWolff]

This pull request fixes #86100. The following code:

fn main() {
    let t: (i32,) = (1);
}

currently produces:

warning: unnecessary parentheses around assigned value
 --> test.rs:2:21
  |
2 |     let t: (i32,) = (1);
  |                     ^^^ help: remove these parentheses
  |
  = note: `#[warn(unused_parens)]` on by default

error[E0308]: mismatched types
 --> test.rs:2:21
  |
2 |     let t: (i32,) = (1);
  |            ------   ^^^ expected tuple, found integer
  |            |
  |            expected due to this
  |
  = note: expected tuple `(i32,)`
              found type `{integer}`

error: aborting due to previous error; 1 warning emitted

With my changes, I get the same warning and the following error:

error[E0308]: mismatched types
 --> test.rs:2:21
  |
2 |     let t: (i32,) = (1);
  |            ------   ^^^ expected tuple, found integer
  |            |
  |            expected due to this
  |
  = note: expected tuple `(i32,)`
              found type `{integer}`
help: use a trailing comma to create a tuple with one element
  |
2 |     let t: (i32,) = (1,);
  |                     ^^^^

i.e. I have added a suggestion to add a trailing comma to create a 1-tuple. This suggestion is only issued if a 1-tuple is expected and the expression ((1) in the example above) is surrounded by parentheses and does not already have a tuple type. In this situation, I'd say that it is pretty likely that the user meant to create a tuple.

[rust-highfive]

r? @varkor

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[varkor]

Looks good, thanks!

@bors r+

[bors]

📌 Commit 6206247 has been approved by varkor

[asquared31415]

This looks great, you picked this up really quick! It also has a much better wording than I could come up with for the help.

[bors]

⌛ Testing commit 6206247 with merge dddebf9...

[bors]

☀️ Test successful - checks-actions
Approved by: varkor
Pushing dddebf9 to master...