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    use std::fmt;

    #[derive(Debug)]
    pub struct ParseError {
        kind: ParseErrorKind,
        orig: String,
    }

    #[non_exhaustive]
    #[derive(Debug)]
    pub enum ParseErrorKind {
        UnterminatedString,
        UnexpectedChar(char),
        UnexpectedToken {
            expected: &'static str,
            found: &'static str,
        },
        IncompleteExpr(&'static str),
        UnterminatedExpression(String),
        InvalidTarget(String),
    }

    impl fmt::Display for ParseError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(
                f,
                "failed to parse `{}` as a cfg expression: {}",
                self.orig, self.kind
            )
        }
    }

    impl fmt::Display for ParseErrorKind {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            use ParseErrorKind::*;
            match self {
                UnterminatedString => write!(f, "unterminated string in cfg"),
                UnexpectedChar(ch) => write!(
                    f,
                    "unexpected character `{}` in cfg, expected parens, a comma, \
                     an identifier, or a string",
                    ch
                ),
                UnexpectedToken { expected, found } => {
                    write!(f, "expected {}, found {}", expected, found)
                }
                IncompleteExpr(expected) => {
                    write!(f, "expected {}, but cfg expression ended", expected)
                }
                UnterminatedExpression(s) => {
                    write!(f, "unexpected content `{}` found after cfg expression", s)
                }
                InvalidTarget(s) => write!(f, "invalid target specifier: {}", s),
            }
        }
    }

    impl std::error::Error for ParseError {}

    impl ParseError {
        pub fn new(orig: &str, kind: ParseErrorKind) -> ParseError {
            ParseError {
                kind,
                orig: orig.to_string(),
            }
        }
    }