Match Ergonomics Issue C: Semantic Convention Enforcement

main.rs

#![allow(dead_code, unused, unreachable_patterns)]

enum State {
    CaseCopy { v: i32 },
    CaseSingle { v: Vec<i32> },
    CaseMulti { v1: Vec<i32>, v2: Vec<i32> },
    CaseComplex { flag: bool, v1: Vec<i32>, v2: Vec<i32> },
}

fn handle(state: &mut State) {

    // cases following convention
    match *state {
        State::CaseCopy { v } => println!("copied something out"),
        State::CaseSingle { ref v } => println!("not mutating the v"),
        State::CaseSingle { ref mut v } => println!("actually mutatint the v"),
        State::CaseMulti { ref mut v1, ref v2 } => println!("only mutating one of them"),
        State::CaseComplex { flag: true, ref mut v1, ref v2 } => println!("mut first"),
        State::CaseComplex { flag: false, ref v1, ref mut v2 } => println!("mut second"),
    }

    // not following convention
    // accidental mutation is hard to discover
    match state {
        State::CaseSingle { v } => println!("not actually copying out"),
        State::CaseMulti { v1, v2 } => println!("also not copying out"),
        _ => (),
    }

    // following convention with accidental mutation
    match state {
        // if you only expected one mutation, the second one is easier to notice
        State::CaseMulti { ref mut v1, ref mut v2 } => println!("accidentally mutate both"),
        _ => (),
    }
}

fn main() {
    let mut state = State::CaseCopy { v: 23 };
    handle(&mut state);
}