try_v2 0.9.1

Provides a derive macro & helper traits for `Try`
Documentation 
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#![feature(never_type)]
#![feature(try_trait_v2)]
#![feature(try_trait_v2_residual)]
#![feature(associated_type_defaults)]

use try_v2_derive::Try;

/// Make TryFrom able to return arbitrary Try types
trait TryFrom<T>: Sized {
    /// Must keep, otherwise would be a breaking change
    type Error = !;
    /// The specific Try-type to return
    /// Defaults to Result, to make this a non-breaking change
    type Return: std::ops::Try = Result<Self, Self::Error>;

    fn try_from(value: T) -> Self::Return;
}

#[derive(Try)]
#[FromResidual(Result<_, Self::Residual>)]
#[must_use]
enum Eightball<Y, N> {
    Yes(Y),
    TryAgain,
    No(N),
}

struct Even(i32);
struct Odd(i32);

/// Uses new API to return a _custom_ TryType
impl TryFrom<i32> for Even {
    type Return = Eightball<Self, Odd>;

    fn try_from(num: i32) -> Self::Return {
        if num % 2 == 0 {
            Eightball::Yes(Even(num))
        } else {
            Eightball::No(Odd(num))
        }
    }
}

struct Even2(i32);

/// Uses _current hack_ of wrapping custom Try-type's Residual in a Result
impl TryFrom<i32> for Even2 {
    type Error = Eightball<!, Odd>;

    fn try_from(num: i32) -> Result<Even2, Eightball<!, Odd>> {
        if num % 2 == 0 {
            Result::Ok(Even2(num))
        } else {
            Result::Err(Eightball::No(Odd(num)))
        }
    }
}

// Cannot instantiate a std::num::TryFromIntError
struct TryFromIntError;

/// Shows this is **non-breaking change**: this is identical (text) to std impl
impl TryFrom<i8> for u8 {
    type Error = TryFromIntError;

    fn try_from(u: i8) -> Result<Self, Self::Error> {
        if u >= 0 {
            Ok(u as Self)
        } else {
            Err(TryFromIntError)
        }
    }
}

/// Can call try_from in a function returning same try type, with different generics
fn even_string_own_try_type(num: i32) -> Eightball<String, Odd> {
    let n = <Even as TryFrom<i32>>::try_from(num)?;
    let s = format!("{}", n.0);
    Eightball::Yes(s)
}

/// Can call try_from in a function returning a _different_ try type (this goes via Result)
/// as long as a suitable FromResidual implementation exists
fn even_string_via_result(num: i32) -> Eightball<String, Odd> {
    let n = <Even2 as TryFrom<i32>>::try_from(num)?;
    let s = format!("{}", n.0);
    Eightball::Yes(s)
}

/// Current case not broken
fn unsigned(num: i8) -> Result<String, TryFromIntError> {
    let n = <u8 as TryFrom<i8>>::try_from(num)?;
    let s = format!("{}", n);
    Ok(s)
}

struct ThirdWord(String);

/// Could even return an Option
impl TryFrom<&str> for ThirdWord {
    type Return = Option<Self>;

    fn try_from(input: &str) -> Self::Return {
        input
            .split_whitespace()
            .nth(2)
            .map(|s| ThirdWord(s.to_string()))
    }
}

fn main() {
    assert!(matches!(even_string_own_try_type(2), Eightball::Yes(s) if s == "2"));
    assert!(matches!(even_string_own_try_type(1), Eightball::No(Odd(1))));

    assert!(matches!(even_string_via_result(2), Eightball::Yes(s) if s == "2"));
    assert!(matches!(even_string_via_result(1), Eightball::No(Odd(1))));

    assert!(matches!(unsigned(5), Ok(s) if s == "5"));
    assert!(matches!(unsigned(-1), Err(TryFromIntError)));

    assert!(
        matches!(<ThirdWord as TryFrom<&str>>::try_from("a lot of words"), Some(s) if s.0 == "of")
    );
    assert!(<ThirdWord as TryFrom<&str>>::try_from("two words").is_none());
}