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Weird compiler error for type annotations #64524
Description
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);
}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`
Activity
estebank commentedon Sep 16, 2019
I believe the
Fit's referring to here is the one inthread::spawn<F, T>(f: F), while theEit should be talking about is the one inResult<T, E>. I believe we're mixing the type argument name from the return and the input.Spoonbender commentedon Mar 22, 2022
Triage: no change
estebank commentedon Aug 14, 2023
Current output:
After #114811: