windows_result/
error.rs

1use super::*;
2use core::num::NonZeroI32;
3
4#[allow(unused_imports)]
5use core::mem::size_of;
6
7/// An error object consists of both an error code and optional detailed error information for debugging.
8///
9/// # Extended error info and the `windows_slim_errors` configuration option
10///
11/// `Error` contains an [`HRESULT`] value that describes the error, as well as an optional
12/// `IErrorInfo` COM object. The `IErrorInfo` object is a COM object that can provide detailed information
13/// about an error, such as a text string, a `ProgID` of the originator, etc. If the error object
14/// was originated in an WinRT component, then additional information such as a stack track may be
15/// captured.
16///
17/// However, many systems based on COM do not use `IErrorInfo`. For these systems, the optional error
18/// info within `Error` has no benefits, but has substantial costs because it increases the size of
19/// the `Error` object, which also increases the size of `Result<T>`.
20///
21/// This error information can be disabled at compile time by setting `RUSTFLAGS=--cfg=windows_slim_errors`.
22/// This removes the `IErrorInfo` support within the [`Error`] type, which has these benefits:
23///
24/// * It reduces the size of [`Error`] to 4 bytes (the size of [`HRESULT`]).
25///
26/// * It reduces the size of `Result<(), Error>` to 4 bytes, allowing it to be returned in a single
27///   machine register.
28///
29/// * The `Error` (and `Result<T, Error>`) types no longer have a [`Drop`] impl. This removes the need
30///   for lifetime checking and running drop code when [`Error`] and [`Result`] go out of scope. This
31///   significantly reduces code size for codebase that make extensive use of [`Error`].
32///
33/// Of course, these benefits come with a cost; you lose extended error information for those
34/// COM objects that support it.
35///
36/// This is controlled by a `--cfg` option rather than a Cargo feature because this compilation
37/// option sets a policy that applies to an entire graph of crates. Individual crates that take a
38/// dependency on the `windows-result` crate are not in a good position to decide whether they want
39/// slim errors or full errors.  Cargo features are meant to be additive, but specifying the size
40/// and contents of `Error` is not a feature so much as a whole-program policy decision.
41///
42/// # References
43///
44/// * [`IErrorInfo`](https://learn.microsoft.com/en-us/windows/win32/api/oaidl/nn-oaidl-ierrorinfo)
45#[derive(Clone)]
46pub struct Error {
47    /// The `HRESULT` error code, but represented using [`NonZeroI32`]. [`NonZeroI32`] provides
48    /// a "niche" to the Rust compiler, which is a space-saving optimization. This allows the
49    /// compiler to use more compact representation for enum variants (such as [`Result`]) that
50    /// contain instances of [`Error`].
51    code: NonZeroI32,
52
53    /// Contains details about the error, such as error text.
54    info: ErrorInfo,
55}
56
57/// We remap S_OK to this error because the S_OK representation (zero) is reserved for niche
58/// optimizations.
59const S_EMPTY_ERROR: NonZeroI32 = const_nonzero_i32(u32::from_be_bytes(*b"S_OK") as i32);
60
61/// Converts an HRESULT into a NonZeroI32. If the input is S_OK (zero), then this is converted to
62/// S_EMPTY_ERROR. This is necessary because NonZeroI32, as the name implies, cannot represent the
63/// value zero. So we remap it to a value no one should be using, during storage.
64const fn const_nonzero_i32(i: i32) -> NonZeroI32 {
65    if let Some(nz) = NonZeroI32::new(i) {
66        nz
67    } else {
68        panic!();
69    }
70}
71
72fn nonzero_hresult(hr: HRESULT) -> NonZeroI32 {
73    if let Some(nz) = NonZeroI32::new(hr.0) {
74        nz
75    } else {
76        S_EMPTY_ERROR
77    }
78}
79
80impl Error {
81    /// Creates an error object without any failure information.
82    pub const fn empty() -> Self {
83        Self {
84            code: S_EMPTY_ERROR,
85            info: ErrorInfo::empty(),
86        }
87    }
88
89    /// Creates a new error object, capturing the stack and other information about the
90    /// point of failure.
91    pub fn new<T: AsRef<str>>(code: HRESULT, message: T) -> Self {
92        #[cfg(windows)]
93        {
94            let message: &str = message.as_ref();
95            if message.is_empty() {
96                Self::from_hresult(code)
97            } else {
98                ErrorInfo::originate_error(code, message);
99                code.into()
100            }
101        }
102        #[cfg(not(windows))]
103        {
104            let _ = message;
105            Self::from_hresult(code)
106        }
107    }
108
109    /// Creates a new error object with an error code, but without additional error information.
110    pub fn from_hresult(code: HRESULT) -> Self {
111        Self {
112            code: nonzero_hresult(code),
113            info: ErrorInfo::empty(),
114        }
115    }
116
117    /// Creates a new `Error` from the Win32 error code returned by `GetLastError()`.
118    pub fn from_win32() -> Self {
119        #[cfg(windows)]
120        {
121            let error = unsafe { GetLastError() };
122            Self::from_hresult(HRESULT::from_win32(error))
123        }
124        #[cfg(not(windows))]
125        {
126            unimplemented!()
127        }
128    }
129
130    /// The error code describing the error.
131    pub const fn code(&self) -> HRESULT {
132        if self.code.get() == S_EMPTY_ERROR.get() {
133            HRESULT(0)
134        } else {
135            HRESULT(self.code.get())
136        }
137    }
138
139    /// The error message describing the error.
140    pub fn message(&self) -> String {
141        if let Some(message) = self.info.message() {
142            return message;
143        }
144
145        // Otherwise fallback to a generic error code description.
146        self.code().message()
147    }
148
149    /// The error object describing the error.
150    #[cfg(windows)]
151    pub fn as_ptr(&self) -> *mut core::ffi::c_void {
152        self.info.as_ptr()
153    }
154}
155
156#[cfg(feature = "std")]
157impl std::error::Error for Error {}
158
159impl From<Error> for HRESULT {
160    fn from(error: Error) -> Self {
161        let code = error.code();
162        error.info.into_thread();
163        code
164    }
165}
166
167impl From<HRESULT> for Error {
168    fn from(code: HRESULT) -> Self {
169        Self {
170            code: nonzero_hresult(code),
171            info: ErrorInfo::from_thread(),
172        }
173    }
174}
175
176#[cfg(feature = "std")]
177impl From<Error> for std::io::Error {
178    fn from(from: Error) -> Self {
179        Self::from_raw_os_error(from.code().0)
180    }
181}
182
183#[cfg(feature = "std")]
184impl From<std::io::Error> for Error {
185    fn from(from: std::io::Error) -> Self {
186        match from.raw_os_error() {
187            Some(status) => HRESULT::from_win32(status as u32).into(),
188            None => HRESULT(E_UNEXPECTED).into(),
189        }
190    }
191}
192
193impl From<alloc::string::FromUtf16Error> for Error {
194    fn from(_: alloc::string::FromUtf16Error) -> Self {
195        Self::from_hresult(HRESULT::from_win32(ERROR_NO_UNICODE_TRANSLATION))
196    }
197}
198
199impl From<alloc::string::FromUtf8Error> for Error {
200    fn from(_: alloc::string::FromUtf8Error) -> Self {
201        Self::from_hresult(HRESULT::from_win32(ERROR_NO_UNICODE_TRANSLATION))
202    }
203}
204
205impl From<core::num::TryFromIntError> for Error {
206    fn from(_: core::num::TryFromIntError) -> Self {
207        Self::from_hresult(HRESULT::from_win32(ERROR_INVALID_DATA))
208    }
209}
210
211impl core::fmt::Debug for Error {
212    fn fmt(&self, fmt: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
213        let mut debug = fmt.debug_struct("Error");
214        debug
215            .field("code", &self.code())
216            .field("message", &self.message())
217            .finish()
218    }
219}
220
221impl core::fmt::Display for Error {
222    fn fmt(&self, fmt: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
223        let message = self.message();
224        if message.is_empty() {
225            core::write!(fmt, "{}", self.code())
226        } else {
227            core::write!(fmt, "{} ({})", self.message(), self.code())
228        }
229    }
230}
231
232impl core::hash::Hash for Error {
233    fn hash<H: core::hash::Hasher>(&self, state: &mut H) {
234        self.code.hash(state);
235        // We do not hash the error info.
236    }
237}
238
239// Equality tests only the HRESULT, not the error info (if any).
240impl PartialEq for Error {
241    fn eq(&self, other: &Self) -> bool {
242        self.code == other.code
243    }
244}
245
246impl Eq for Error {}
247
248impl PartialOrd for Error {
249    fn partial_cmp(&self, other: &Self) -> Option<core::cmp::Ordering> {
250        Some(self.cmp(other))
251    }
252}
253
254impl Ord for Error {
255    fn cmp(&self, other: &Self) -> core::cmp::Ordering {
256        self.code.cmp(&other.code)
257    }
258}
259
260use error_info::*;
261
262#[cfg(all(windows, not(windows_slim_errors)))]
263mod error_info {
264    use super::*;
265    use crate::com::ComPtr;
266
267    /// This type stores error detail, represented by a COM `IErrorInfo` object.
268    ///
269    /// # References
270    ///
271    /// * [`IErrorInfo`](https://learn.microsoft.com/en-us/windows/win32/api/oaidl/nn-oaidl-ierrorinfo)
272    #[derive(Clone, Default)]
273    pub(crate) struct ErrorInfo {
274        pub(super) ptr: Option<ComPtr>,
275    }
276
277    impl ErrorInfo {
278        pub(crate) const fn empty() -> Self {
279            Self { ptr: None }
280        }
281
282        pub(crate) fn from_thread() -> Self {
283            unsafe {
284                let mut ptr = core::mem::MaybeUninit::zeroed();
285                crate::bindings::GetErrorInfo(0, ptr.as_mut_ptr() as *mut _);
286                Self {
287                    ptr: ptr.assume_init(),
288                }
289            }
290        }
291
292        pub(crate) fn into_thread(self) {
293            if let Some(ptr) = self.ptr {
294                unsafe {
295                    crate::bindings::SetErrorInfo(0, ptr.as_raw());
296                }
297            }
298        }
299
300        pub(crate) fn originate_error(code: HRESULT, message: &str) {
301            let message: Vec<_> = message.encode_utf16().collect();
302            unsafe {
303                RoOriginateErrorW(code.0, message.len() as u32, message.as_ptr());
304            }
305        }
306
307        pub(crate) fn message(&self) -> Option<String> {
308            use crate::bstr::BasicString;
309
310            let ptr = self.ptr.as_ref()?;
311
312            let mut message = BasicString::default();
313
314            // First attempt to retrieve the restricted error information.
315            if let Some(info) = ptr.cast(&IID_IRestrictedErrorInfo) {
316                let mut fallback = BasicString::default();
317                let mut code = 0;
318
319                unsafe {
320                    com_call!(
321                        IRestrictedErrorInfo_Vtbl,
322                        info.GetErrorDetails(
323                            &mut fallback as *mut _ as _,
324                            &mut code,
325                            &mut message as *mut _ as _,
326                            &mut BasicString::default() as *mut _ as _
327                        )
328                    );
329                }
330
331                if message.is_empty() {
332                    message = fallback
333                };
334            }
335
336            // Next attempt to retrieve the regular error information.
337            if message.is_empty() {
338                unsafe {
339                    com_call!(
340                        IErrorInfo_Vtbl,
341                        ptr.GetDescription(&mut message as *mut _ as _)
342                    );
343                }
344            }
345
346            Some(String::from_utf16_lossy(wide_trim_end(&message)))
347        }
348
349        pub(crate) fn as_ptr(&self) -> *mut core::ffi::c_void {
350            if let Some(info) = self.ptr.as_ref() {
351                info.as_raw()
352            } else {
353                core::ptr::null_mut()
354            }
355        }
356    }
357
358    unsafe impl Send for ErrorInfo {}
359    unsafe impl Sync for ErrorInfo {}
360}
361
362#[cfg(not(all(windows, not(windows_slim_errors))))]
363mod error_info {
364    use super::*;
365
366    // We use this name so that the NatVis <Type> element for ErrorInfo does *not* match this type.
367    // This prevents the NatVis description from failing to load.
368    #[derive(Clone, Default)]
369    pub(crate) struct EmptyErrorInfo;
370
371    pub(crate) use EmptyErrorInfo as ErrorInfo;
372
373    impl EmptyErrorInfo {
374        pub(crate) const fn empty() -> Self {
375            Self
376        }
377
378        pub(crate) fn from_thread() -> Self {
379            Self
380        }
381
382        pub(crate) fn into_thread(self) {}
383
384        #[cfg(windows)]
385        pub(crate) fn originate_error(_code: HRESULT, _message: &str) {}
386
387        pub(crate) fn message(&self) -> Option<String> {
388            None
389        }
390
391        #[cfg(windows)]
392        pub(crate) fn as_ptr(&self) -> *mut core::ffi::c_void {
393            core::ptr::null_mut()
394        }
395    }
396}
windows_result/error.rs

windows_result/
error.rs
