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    #![allow(dead_code)]
    #![unstable(feature = "process_internals", issue = "0")]

    use crate::ffi::{OsStr, OsString};
    use crate::env;
    use crate::collections::BTreeMap;
    use crate::borrow::Borrow;

    pub trait EnvKey:
        From<OsString> + Into<OsString> +
        Borrow<OsStr> + Borrow<Self> + AsRef<OsStr> +
        Ord + Clone {}

    // Implement a case-sensitive environment variable key
    #[derive(Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
    pub struct DefaultEnvKey(OsString);

    impl From<OsString> for DefaultEnvKey {
        fn from(k: OsString) -> Self { DefaultEnvKey(k) }
    }

    impl From<DefaultEnvKey> for OsString {
        fn from(k: DefaultEnvKey) -> Self { k.0 }
    }

    impl Borrow<OsStr> for DefaultEnvKey {
        fn borrow(&self) -> &OsStr { &self.0 }
    }

    impl AsRef<OsStr> for DefaultEnvKey {
        fn as_ref(&self) -> &OsStr { &self.0 }
    }

    impl EnvKey for DefaultEnvKey {}

    // Stores a set of changes to an environment
    #[derive(Clone, Debug)]
    pub struct CommandEnv<K> {
        clear: bool,
        saw_path: bool,
        vars: BTreeMap<K, Option<OsString>>
    }

    impl<K: EnvKey> Default for CommandEnv<K> {
        fn default() -> Self {
            CommandEnv {
                clear: false,
                saw_path: false,
                vars: Default::default()
            }
        }
    }

    impl<K: EnvKey> CommandEnv<K> {
        // Capture the current environment with these changes applied
        pub fn capture(&self) -> BTreeMap<K, OsString> {
            let mut result = BTreeMap::<K, OsString>::new();
            if !self.clear {
                for (k, v) in env::vars_os() {
                    result.insert(k.into(), v);
                }
            }
            for (k, maybe_v) in &self.vars {
                if let &Some(ref v) = maybe_v {
                    result.insert(k.clone(), v.clone());
                } else {
                    result.remove(k);
                }
            }
            result
        }

        // Apply these changes directly to the current environment
        pub fn apply(&self) {
            if self.clear {
                for (k, _) in env::vars_os() {
                    env::remove_var(k);
                }
            }
            for (key, maybe_val) in self.vars.iter() {
                if let &Some(ref val) = maybe_val {
                    env::set_var(key, val);
                } else {
                    env::remove_var(key);
                }
            }
        }

        pub fn is_unchanged(&self) -> bool {
            !self.clear && self.vars.is_empty()
        }

        pub fn capture_if_changed(&self) -> Option<BTreeMap<K, OsString>> {
            if self.is_unchanged() {
                None
            } else {
                Some(self.capture())
            }
        }

        // The following functions build up changes
        pub fn set(&mut self, key: &OsStr, value: &OsStr) {
            self.maybe_saw_path(&key);
            self.vars.insert(key.to_owned().into(), Some(value.to_owned()));
        }
        pub fn remove(&mut self, key: &OsStr) {
            self.maybe_saw_path(&key);
            if self.clear {
                self.vars.remove(key);
            } else {
                self.vars.insert(key.to_owned().into(), None);
            }
        }
        pub fn clear(&mut self) {
            self.clear = true;
            self.vars.clear();
        }
        pub fn have_changed_path(&self) -> bool {
            self.saw_path || self.clear
        }
        fn maybe_saw_path(&mut self, key: &OsStr) {
            if !self.saw_path && key == "PATH" {
                self.saw_path = true;
            }
        }
    }

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