Rust understanding From trait [duplicate]

问题

This question already has answers here:

Using .into() when type inference is impossible (4 answers)

How to use std::convert::Into in statements in Rust? [duplicate] (1 answer)

Closed 3 months ago.

I'm trying to understand how the From trait works. I've seen it used a lot when you want to override an error from a library with your own custom error. I tried to come up with a contrived example to see that property in action:

struct Blah{

}

impl From<i32> for Blah {

    fn from(b:i32) -> Blah {
        return Blah {

        };
    }

}

fn main() {

}

fn get_result() -> Result<Blah, Box<dyn std::error::Error>> {

    return Ok(20);
}

Error:

return Ok(20);
   |               ^^ expected struct `Blah`, found integer
   |
   = note: expected type `Blah`
              found type `{integer}`

I understand the error, but I don't understand why the integer isn't being implicity converted into a Blah, given that I have implemented From<i32> for Blah

来源：https://stackoverflow.com/questions/57853235/rust-understanding-from-trait