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    use std::fmt;

    use serde::de;

    use crate::{AsResult, Error, Parse};

    /// The number of workers that the server should start.
    ///
    /// By default the number of available logical cpu cores is used. Takes a string value: Either
    /// "default", or an integer N > 0 e.g. "6".
    #[derive(Debug, Clone, PartialEq, Eq, Hash)]
    pub enum NumWorkers {
        /// The default number of workers. See struct docs.
        Default,

        /// A specific number of workers.
        Manual(usize),
    }

    impl Parse for NumWorkers {
        fn parse(string: &str) -> AsResult<Self> {
            match string {
                "default" => Ok(NumWorkers::Default),
                string => match string.parse::<usize>() {
                    Ok(val) => Ok(NumWorkers::Manual(val)),
                    Err(_) => Err(InvalidValue! {
                        expected: "a positive integer",
                        got: string,
                    }),
                },
            }
        }
    }

    impl<'de> de::Deserialize<'de> for NumWorkers {
        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        where
            D: de::Deserializer<'de>,
        {
            struct NumWorkersVisitor;

            impl<'de> de::Visitor<'de> for NumWorkersVisitor {
                type Value = NumWorkers;

                fn expecting(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                    let msg = "Either \"default\" or a string containing an integer > 0";
                    f.write_str(msg)
                }

                fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
                where
                    E: de::Error,
                {
                    match NumWorkers::parse(value) {
                        Ok(num_workers) => Ok(num_workers),
                        Err(Error::InvalidValue { expected, got, .. }) => Err(
                            de::Error::invalid_value(de::Unexpected::Str(&got), &expected),
                        ),
                        Err(_) => unreachable!(),
                    }
                }
            }

            deserializer.deserialize_string(NumWorkersVisitor)
        }
    }