winver 1.0.0

winver is a tiny crate to detect real OS version on Windows depending on windows crate only. There are several ways to get Windows OS version and each of them has its pitfall. This crate provides API to get the version more easily and safely avoiding the pitfalls.
Documentation 
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use std::alloc::LayoutError;
use std::error::Error as StdError;
use std::fmt;
use std::string::FromUtf16Error;
use windows::core::Error as WinError;

/// Kinds of errors returned from `WindowsVersion` methods.
#[derive(Debug)]
pub enum ErrorKind {
    /// Error on calling Win32 APIs with windows-rs crate.
    Windows(WinError),
    /// Layout error on allocating a memory buffer on heap.
    Layout(LayoutError),
    /// Version information is not found in kernel32.dll file.
    Kernel32VerNotFound,
    /// `RtlGetVersion` symbol is not found in ntdll.dll when dynamically loading the DLL file.
    NoRtlGetVersion,
    /// Failure with error code returned from `RtlGetVersion` call.
    RtlGetVersionFailure(i32),
    /// UTF-16 to UTF-8 conversion failure when converting `BSTR` into `String`.
    Utf16ToUtf8(FromUtf16Error),
    /// No version information is found in WMI's Win32 OS system provider.
    WmiNotFound,
    /// Version string obtained from WMI's Win32 OS system provider is in unexpected format.
    WmiUnexpectedVersion(String),
}

/// The single error type for all errors returned from this crate.
#[derive(Debug)]
pub struct Error(Box<ErrorKind>);

impl Error {
    /// Return the kind of the error to know the details and to get the underlying errors.
    pub fn kind(&self) -> &ErrorKind {
        &self.0
    }
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self.kind() {
            ErrorKind::Windows(err) => write!(f, "Error while calling Windows API: {}", err),
            ErrorKind::Layout(err) => write!(f, "Error while calculating memory layout: {}", err),
            ErrorKind::Kernel32VerNotFound => {
                write!(f, "Version information is not found in kernel32.dll")
            }
            ErrorKind::NoRtlGetVersion => {
                write!(f, "RtlGetVersion function does not exist in ntdll.dll")
            }
            ErrorKind::RtlGetVersionFailure(status) => write!(
                f,
                "RtlGetVersion function call failed with status {:x}",
                *status,
            ),
            ErrorKind::Utf16ToUtf8(err) => write!(
                f,
                "Error while converting UTF-16 string into UTF-8: {}",
                err,
            ),
            ErrorKind::WmiNotFound => write!(
                f,
                "No 'Version' property is found in 'Win32_OperatingSystem' class queried via WQL",
            ),
            ErrorKind::WmiUnexpectedVersion(ver) => write!(
                f,
                "Unexpected version string at 'Version' property of 'Win32_OperatingSystem' WMI provider: {:?}",
                ver,
            ),
        }
    }
}

impl StdError for Error {
    fn source(&self) -> Option<&(dyn StdError + 'static)> {
        match self.kind() {
            ErrorKind::Windows(err) => Some(err),
            ErrorKind::Layout(err) => Some(err),
            ErrorKind::Utf16ToUtf8(err) => Some(err),
            _ => None,
        }
    }
}

impl From<ErrorKind> for Error {
    fn from(kind: ErrorKind) -> Self {
        Self(Box::new(kind))
    }
}

impl From<WinError> for Error {
    fn from(err: WinError) -> Self {
        ErrorKind::Windows(err).into()
    }
}

impl From<LayoutError> for Error {
    fn from(err: LayoutError) -> Self {
        ErrorKind::Layout(err).into()
    }
}

impl From<FromUtf16Error> for Error {
    fn from(err: FromUtf16Error) -> Self {
        ErrorKind::Utf16ToUtf8(err).into()
    }
}

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

    #[test]
    fn test_format() {
        let err: Error = WinError::OK.into();
        let msg = format!("{}", err);
        assert!(msg.contains("Error while calling Windows API"), "{:?}", msg);
    }

    #[test]
    fn test_kind() {
        let err: Error = ErrorKind::Kernel32VerNotFound.into();
        assert!(
            matches!(err.kind(), ErrorKind::Kernel32VerNotFound),
            "{:?}",
            err,
        );
    }

    #[test]
    fn test_source() {
        let err: Error = WinError::OK.into();
        let inner = err.source().unwrap();
        let msg = format!("{}", inner);
        assert!(msg.contains("(0x00000000)"), "{:?}", msg);
    }
}