pub fn new(input: &str) -> Result<Self, Error> {
        let expr = Expression::parse(input)
            .map_err(|err| Error::InvalidExpression(ExpressionParseError::new(err)))?;
        Ok(Self {
            inner: Arc::new(expr),
        })
    }
Creates a new `TargetExpression` from a string beginning with `cfg(`.

Returns an error if the string could not be parsed, or if the string contains a predicate
that wasn't understood by `target-spec`.
target-spec/src/spec.rs