Weird compiler error for type annotations

[olegnn]

use std::thread;

fn main() {
    let result = {
        let spawned_result = thread::spawn(|| 3).join();
        let result = spawned_result.map_err(|e| e.into());
        let handler = |a| Ok(a + 1);
        result.and_then(handler)
    };

    println!("{:?}", result);
}

(Playground)

Error will be

error[E0282]: type annotations needed for `std::result::Result<i32, E>`
 --> src/main.rs:6:37
  |
4 |     let result = {
  |         ------ consider giving `result` the explicit type `std::result::Result<i32, E>`, where the type parameter `F` is specified
5 |         let spawned_result = thread::spawn(|| 3).join();
6 |         let result = spawned_result.map_err(|e| e.into());
  |                                     ^^^^^^^ cannot infer type for `F`

Which is a little bit confusing because there's no type parameter F in std::result::Result<i32, E>.

On the other hand, if we place let handler = |a| Ok(a + 1); before let result = spawned_result.map_err(|e| e.into());, we will receive correct message:

error[E0282]: type annotations needed for `std::result::Result<i32, E>`
 --> src/main.rs:6:27
  |
4 |     let result = {
  |         ------ consider giving `result` the explicit type `std::result::Result<i32, E>`, where the type parameter `E` is specified
5 |         let spawned_result = thread::spawn(|| 3).join();
6 |         let handler = |a| Ok(a + 1);
  |                           ^^ cannot infer type for `E`

[estebank]

I believe the F it's referring to here is the one in thread::spawn<F, T>(f: F), while the E it should be talking about is the one in Result<T, E>. I believe we're mixing the type argument name from the return and the input.

[Spoonbender]

Triage: no change

[estebank]

Current output:

error[E0282]: type annotations needed for `Result<i32, _>`
 --> f71.rs:6:13
  |
6 |         let result = spawned_result.map_err(|e| e.into());
  |             ^^^^^^
  |
help: consider giving `result` an explicit type, where the type for type parameter `F` is specified
  |
6 |         let result: Result<i32, _> = spawned_result.map_err(|e| e.into());
  |                   ++++++++++++++++

After #114811:

error[E0283]: type annotations needed for `Result<i32, _>`
 --> f71.rs:6:13
  |
6 |         let result = spawned_result.map_err(|e| e.into());
  |             ^^^^^^                                ---- type must be known at this point
  |
  = note: cannot satisfy `_: From<Box<dyn Any + Send>>`
  = note: required for `Box<dyn Any + Send>` to implement `Into<_>`
help: consider giving `result` an explicit type, where the type for type parameter `U` is specified
  |
6 |         let result: Result<i32, _> = spawned_result.map_err(|e| e.into());
  |                   ++++++++++++++++