How to implement TryFrom and TryInto

Implement TryFrom for your target type and TryInto is automatically provided by the standard library. Define the type Error associated type and implement the try_from method to return Ok on success or Err on failure.

use std::convert::TryFrom;

struct PositiveInt(u32);

impl TryFrom<i32> for PositiveInt {
    type Error = &'static str;

    fn try_from(value: i32) -> Result<Self, Self::Error> {
        if value > 0 {
            Ok(PositiveInt(value as u32))
        } else {
            Err("Value must be positive")
        }
    }
}

// TryInto is automatically implemented for PositiveInt
let result: Result<PositiveInt, _> = 5.try_into();

TryFrom is a trait that lets you convert one type into another, but only if the conversion is safe. If the conversion fails, it returns an error instead of crashing. TryInto is the reverse direction that the compiler automatically generates for you once you implement TryFrom.