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Cake day: June 21st, 2023

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  • You can’t create a subset of an enum directly, but splitting this up into multiple types works. You can have FunctionAError with errors that function can produce and a variant for your common errors, and FunctionBError which is similar:

    #[derive(Debug, Error)]
    enum MyErrorCommon {
        #[error("bad value ({0})")]
        MyErrorCommon(String),
    }
    
    #[derive(Debug, Error)]
    enum FunctionAError {
        #[error("error a")]
        MyErrorA,
        Common(#[from] MyErrorCommon),
    }
    
    // and same for FunctionBError
    

    The try operator (?) will automatically use From impls to convert errors for you as well. If a function returns a result containing MyErrorCommon in your function and you use ? on it, it gets converted to that function’s error type for you. thiserror generates the From impl for you if you use #[from].