suggest Ok-wrapping for ?-originating type mismatch when (and only when) applicable

[zackmdavis]

Sometimes E0308 on a try-expression can be fixed by wrapping with Ok. This is a somewhat common mistake in the form of a ?-expression appearing in tail position of a function that is expected to Result—

fn foo() -> Result<isize, ()> {
     Ok(1)
}
 
fn bar() -> Result<isize, ()> {
    foo()? // `Ok(foo()?)` would work
}

fn main() {}

However, not all type mismatches on ? are like this. Here's a negative example—

fn maybe_numbers() -> Result<Vec<i32>, ()> {
    Ok(vec![1, 2, 3])
}

fn try_it() -> Result<String, ()> {
    let n: i32 = maybe_numbers()?; // `Ok(maybe_numbers()?)` is not the right move here
    Ok(format!("{:?}", n))
}

fn main() {}

We want to suggest Ok-wrapping when that's the correct fix. (This issue is being filed because a previous attempt at this suggestion had too many false positives and is likely to be removed.)

See discussion on #52537 for one idea on how to proceed.

[estebank]

Current (correct) output for the first case:

error[E0308]: try expression alternatives have incompatible types
 --> src/main.rs:6:5
  |
6 |     foo()?
  |     ^^^^^-
  |     |    |
  |     |    help: try removing this `?`
  |     expected enum `std::result::Result`, found isize
  |
  = note: expected type `std::result::Result<isize, ()>`
             found type `isize`

[estebank]

Current output:

error[[E0308]](https://doc.rust-lang.org/stable/error_codes/E0308.html): `?` operator has incompatible types
 --> src/main.rs:6:5
  |
6 |     foo()?
  |     ^^^^^^ expected `Result<isize, ()>`, found `isize`
  |
  = note: `?` operator cannot convert from `isize` to `Result<isize, ()>`
  = note: expected enum `Result<isize, ()>`
             found type `isize`
help: try removing this `?`
  |
6 -     foo()?
6 +     foo()
  |
help: try wrapping the expression in `Ok`
  |
6 |     Ok(foo()?)
  |     +++      +

error[[E0308]](https://doc.rust-lang.org/stable/error_codes/E0308.html): `?` operator has incompatible types
 --> src/main.rs:6:18
  |
6 |     let n: i32 = maybe_numbers()?; // `Ok(maybe_numbers()?)` is not the right move here
  |                  ^^^^^^^^^^^^^^^^ expected `i32`, found `Vec<i32>`
  |
  = note: `?` operator cannot convert from `Vec<i32>` to `i32`
  = note: expected type `i32`
           found struct `Vec<i32>`