Confusing error message when matching on an unnecessarily-unwrapped value

[mkadziolka]

Consider this code (playground):

pub fn test(input: &str) -> Result<serde_json::Value, String> {
    match serde_json::from_str(input).unwrap() {
        Ok(v) => Ok(v),
        Err(why) => Err(format!("JSON decode failed: {:?}", why)),
    }
}

This looks like perfectly reasonable code, but upon further inspection, you can see an .unwrap() as a leftover from refactoring. However, Rust chooses a quite confusing way of pointing this out:

error[E0282]: type annotations needed
 --> src/lib.rs:4:13
  |
4 |         Err(why) => Err(format!("JSON decode failed: {:?}", why)),
  |             ^^^ cannot infer type

As far as I am aware, there is no way to solve this problem with just a type annotation.

[jonas-schievink]

As far as I am aware, there is no way to solve this problem with just a type annotation.

Using from_str::<Result<_, ()>> compiles

[estebank]

Current output:

error[E0282]: type annotations needed
 --> src/lib.rs:4:13
  |
2 |     match serde_json::from_str(input).unwrap() {
  |           ------------------------------------ this method call resolves to `T`
3 |         Ok(v) => Ok(v),
4 |         Err(why) => Err(format!("JSON decode failed: {:?}", why)),
  |             ^^^ cannot infer type

[estebank]

Current output:

error[[E0282]](https://doc.rust-lang.org/nightly/error_codes/E0282.html): type annotations needed
 --> src/lib.rs:2:11
  |
2 |     match serde_json::from_str(input).unwrap() {
  |           ^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the function `from_str`
  |
help: consider specifying the generic argument
  |
2 |     match serde_json::from_str::<Result<Value, E>>(input).unwrap() {
  |                               ++++++++++++++++++++