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    use std::fmt;

    use crate::{lowercase, transform};

    /// This trait defines a snake case conversion.
    ///
    /// In snake_case, word boundaries are indicated by underscores.
    ///
    /// ## Example:
    ///
    /// ```rust
    /// use heck::ToSnakeCase;
    ///
    /// let sentence = "We carry a new world here, in our hearts.";
    /// assert_eq!(sentence.to_snake_case(), "we_carry_a_new_world_here_in_our_hearts");
    /// ```
    pub trait ToSnakeCase: ToOwned {
        /// Convert this type to snake case.
        fn to_snake_case(&self) -> Self::Owned;
    }

    /// Oh heck, SnekCase is an alias for ToSnakeCase. See ToSnakeCase for
    /// more documentation.
    pub trait ToSnekCase: ToOwned {
        /// Convert this type to snek case.
        fn to_snek_case(&self) -> Self::Owned;
    }

    impl<T: ?Sized + ToSnakeCase> ToSnekCase for T {
        fn to_snek_case(&self) -> Self::Owned {
            self.to_snake_case()
        }
    }

    impl ToSnakeCase for str {
        fn to_snake_case(&self) -> String {
            AsSnakeCase(self).to_string()
        }
    }

    /// This wrapper performs a snake case conversion in [`fmt::Display`].
    ///
    /// ## Example:
    ///
    /// ```
    /// use heck::AsSnakeCase;
    ///
    /// let sentence = "We carry a new world here, in our hearts.";
    /// assert_eq!(format!("{}", AsSnakeCase(sentence)), "we_carry_a_new_world_here_in_our_hearts");
    /// ```
    pub struct AsSnakeCase<T: AsRef<str>>(pub T);

    impl<T: AsRef<str>> fmt::Display for AsSnakeCase<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            transform(self.0.as_ref(), lowercase, |f| write!(f, "_"), f)
        }
    }

    #[cfg(test)]
    mod tests {
        use super::ToSnakeCase;

        macro_rules! t {
            ($t:ident : $s1:expr => $s2:expr) => {
                #[test]
                fn $t() {
                    assert_eq!($s1.to_snake_case(), $s2)
                }
            };
        }

        t!(test1: "CamelCase" => "camel_case");
        t!(test2: "This is Human case." => "this_is_human_case");
        t!(test3: "MixedUP CamelCase, with some Spaces" => "mixed_up_camel_case_with_some_spaces");
        t!(test4: "mixed_up_ snake_case with some _spaces" => "mixed_up_snake_case_with_some_spaces");
        t!(test5: "kebab-case" => "kebab_case");
        t!(test6: "SHOUTY_SNAKE_CASE" => "shouty_snake_case");
        t!(test7: "snake_case" => "snake_case");
        t!(test8: "this-contains_ ALLKinds OfWord_Boundaries" => "this_contains_all_kinds_of_word_boundaries");
        #[cfg(feature = "unicode")]
        t!(test9: "XΣXΣ baﬄe" => "xσxς_baﬄe");
        t!(test10: "XMLHttpRequest" => "xml_http_request");
        t!(test11: "FIELD_NAME11" => "field_name11");
        t!(test12: "99BOTTLES" => "99bottles");
        t!(test13: "FieldNamE11" => "field_nam_e11");

        t!(test14: "abc123def456" => "abc123def456");
        t!(test16: "abc123DEF456" => "abc123_def456");
        t!(test17: "abc123Def456" => "abc123_def456");
        t!(test18: "abc123DEf456" => "abc123_d_ef456");
        t!(test19: "ABC123def456" => "abc123def456");
        t!(test20: "ABC123DEF456" => "abc123def456");
        t!(test21: "ABC123Def456" => "abc123_def456");
        t!(test22: "ABC123DEf456" => "abc123d_ef456");
        t!(test23: "ABC123dEEf456FOO" => "abc123d_e_ef456_foo");
        t!(test24: "abcDEF" => "abc_def");
        t!(test25: "ABcDE" => "a_bc_de");
    }