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    //! Deserialization of a `Value<T>` type which tracks where it was deserialized
    //! from.
    //!
    //! Often Cargo wants to report semantic error information or other sorts of
    //! error information about configuration keys but it also may wish to indicate
    //! as an error context where the key was defined as well (to help user
    //! debugging). The `Value<T>` type here can be used to deserialize a `T` value
    //! from configuration, but also record where it was deserialized from when it
    //! was read.

    use crate::util::config::Config;
    use serde::de;
    use std::fmt;
    use std::marker;
    use std::mem;
    use std::path::{Path, PathBuf};

    /// A type which can be deserialized as a configuration value which records
    /// where it was deserialized from.
    #[derive(Debug, PartialEq, Clone)]
    pub struct Value<T> {
        /// The inner value that was deserialized.
        pub val: T,
        /// The location where `val` was defined in configuration (e.g. file it was
        /// defined in, env var etc).
        pub definition: Definition,
    }

    pub type OptValue<T> = Option<Value<T>>;

    // Deserializing `Value<T>` is pretty special, and serde doesn't have built-in
    // support for this operation. To implement this we extend serde's "data model"
    // a bit. We configure deserialization of `Value<T>` to basically only work with
    // our one deserializer using configuration.
    //
    // We define that `Value<T>` deserialization asks the deserializer for a very
    // special struct name and struct field names. In doing so the deserializer will
    // recognize this and synthesize a magical value for the `definition` field when
    // we deserialize it. This protocol is how we're able to have a channel of
    // information flowing from the configuration deserializer into the
    // deserialization implementation here.
    //
    // You'll want to also check out the implementation of `ValueDeserializer` in
    // `de.rs`. Also note that the names below are intended to be invalid Rust
    // identifiers to avoid how they might conflict with other valid structures.
    // Finally the `definition` field is transmitted as a tuple of i32/string, which
    // is effectively a tagged union of `Definition` itself.

    pub(crate) const VALUE_FIELD: &str = "$__cargo_private_value";
    pub(crate) const DEFINITION_FIELD: &str = "$__cargo_private_definition";
    pub(crate) const NAME: &str = "$__cargo_private_Value";
    pub(crate) static FIELDS: [&str; 2] = [VALUE_FIELD, DEFINITION_FIELD];

    /// Location where a config value is defined.
    #[derive(Clone, Debug, Eq)]
    pub enum Definition {
        /// Defined in a `.cargo/config`, includes the path to the file.
        Path(PathBuf),
        /// Defined in an environment variable, includes the environment key.
        Environment(String),
        /// Passed in on the command line.
        /// A path is attached when the config value is a path to a config file.
        Cli(Option<PathBuf>),
    }

    impl Definition {
        /// Root directory where this is defined.
        ///
        /// If from a file, it is the directory above `.cargo/config`.
        /// CLI and env are the current working directory.
        pub fn root<'a>(&'a self, config: &'a Config) -> &'a Path {
            match self {
                Definition::Path(p) | Definition::Cli(Some(p)) => p.parent().unwrap().parent().unwrap(),
                Definition::Environment(_) | Definition::Cli(None) => config.cwd(),
            }
        }

        /// Returns true if self is a higher priority to other.
        ///
        /// CLI is preferred over environment, which is preferred over files.
        pub fn is_higher_priority(&self, other: &Definition) -> bool {
            matches!(
                (self, other),
                (Definition::Cli(_), Definition::Environment(_))
                    | (Definition::Cli(_), Definition::Path(_))
                    | (Definition::Environment(_), Definition::Path(_))
            )
        }
    }

    impl PartialEq for Definition {
        fn eq(&self, other: &Definition) -> bool {
            // configuration values are equivalent no matter where they're defined,
            // but they need to be defined in the same location. For example if
            // they're defined in the environment that's different than being
            // defined in a file due to path interpretations.
            mem::discriminant(self) == mem::discriminant(other)
        }
    }

    impl fmt::Display for Definition {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match self {
                Definition::Path(p) | Definition::Cli(Some(p)) => p.display().fmt(f),
                Definition::Environment(key) => write!(f, "environment variable `{}`", key),
                Definition::Cli(None) => write!(f, "--config cli option"),
            }
        }
    }

    impl<'de, T> de::Deserialize<'de> for Value<T>
    where
        T: de::Deserialize<'de>,
    {
        fn deserialize<D>(deserializer: D) -> Result<Value<T>, D::Error>
        where
            D: de::Deserializer<'de>,
        {
            struct ValueVisitor<T> {
                _marker: marker::PhantomData<T>,
            }

            impl<'de, T> de::Visitor<'de> for ValueVisitor<T>
            where
                T: de::Deserialize<'de>,
            {
                type Value = Value<T>;

                fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
                    formatter.write_str("a value")
                }

                fn visit_map<V>(self, mut visitor: V) -> Result<Value<T>, V::Error>
                where
                    V: de::MapAccess<'de>,
                {
                    let value = visitor.next_key::<ValueKey>()?;
                    if value.is_none() {
                        return Err(de::Error::custom("value not found"));
                    }
                    let val: T = visitor.next_value()?;

                    let definition = visitor.next_key::<DefinitionKey>()?;
                    if definition.is_none() {
                        return Err(de::Error::custom("definition not found"));
                    }
                    let definition: Definition = visitor.next_value()?;
                    Ok(Value { val, definition })
                }
            }

            deserializer.deserialize_struct(
                NAME,
                &FIELDS,
                ValueVisitor {
                    _marker: marker::PhantomData,
                },
            )
        }
    }

    struct FieldVisitor {
        expected: &'static str,
    }

    impl<'de> de::Visitor<'de> for FieldVisitor {
        type Value = ();

        fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
            formatter.write_str("a valid value field")
        }

        fn visit_str<E>(self, s: &str) -> Result<(), E>
        where
            E: de::Error,
        {
            if s == self.expected {
                Ok(())
            } else {
                Err(de::Error::custom("expected field with custom name"))
            }
        }
    }

    struct ValueKey;

    impl<'de> de::Deserialize<'de> for ValueKey {
        fn deserialize<D>(deserializer: D) -> Result<ValueKey, D::Error>
        where
            D: de::Deserializer<'de>,
        {
            deserializer.deserialize_identifier(FieldVisitor {
                expected: VALUE_FIELD,
            })?;
            Ok(ValueKey)
        }
    }

    struct DefinitionKey;

    impl<'de> de::Deserialize<'de> for DefinitionKey {
        fn deserialize<D>(deserializer: D) -> Result<DefinitionKey, D::Error>
        where
            D: de::Deserializer<'de>,
        {
            deserializer.deserialize_identifier(FieldVisitor {
                expected: DEFINITION_FIELD,
            })?;
            Ok(DefinitionKey)
        }
    }

    impl<'de> de::Deserialize<'de> for Definition {
        fn deserialize<D>(deserializer: D) -> Result<Definition, D::Error>
        where
            D: de::Deserializer<'de>,
        {
            let (discr, value) = <(u32, String)>::deserialize(deserializer)?;
            match discr {
                0 => Ok(Definition::Path(value.into())),
                1 => Ok(Definition::Environment(value)),
                2 => {
                    let path = (value.len() > 0).then_some(value.into());
                    Ok(Definition::Cli(path))
                }
                _ => panic!("unexpected discriminant {discr} value {value}"),
            }
        }
    }