errors: document and export ErrorKind

ErrorKind is safe for us to export (unlike ErrorImpl) because we can
easily change the concrete types we use for our errors without causing
downstream breakages. Most users almost certainly would prefer something
that looks like std::io::ErrorKind anyway (which matches how we check
for libpathrs errors inside libpathrs as well).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

CHANGELOG.md

@@ -30,6 +30,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   efficient in some scenarios (especially with the openat2-based resolver or
   for C FFI users where function calls are expensive) as it saves one file
   descriptor allocation and extra function calls.
+- Error: `ErrorKind` is now exported, allowing you to programmatically handle
+  errors returned by libpathrs. This interface may change in the future (in
+  particular, `ErrorKind::OsError` might change its representation of `errno`
+  values).
 - capi: errors that are returned by libpathrs itself (such as invalid arguments
   being passed by users) will now contain a `saved_errno` value that makes
   sense for the error type (so `ErrorKind::InvalidArgument` will result in an

src/error.rs

@@ -36,6 +36,10 @@ use std::{borrow::Cow, io::Error as IOError};
 //       export the crate types here without std::backtrace::Backtrace.
 // MSRV(1.65): Use std::backtrace::Backtrace.
 
+/// Opaque error type for libpathrs.
+///
+/// If you wish to do non-trivial error handling with libpathrs errors, use
+/// [`Error::kind`] to get an [`ErrorKind`] you can handle programmatically.
 #[derive(thiserror::Error, Debug)]
 #[error(transparent)]
 pub struct Error(#[from] Box<ErrorImpl>);
@@ -49,7 +53,7 @@ impl<E: Into<ErrorImpl>> From<E> for Error {
 }
 
 impl Error {
-    pub(crate) fn kind(&self) -> ErrorKind {
+    pub fn kind(&self) -> ErrorKind {
         self.0.kind()
     }
 }
@@ -100,15 +104,28 @@ pub(crate) enum ErrorImpl {
     },
 }
 
-// TODO: Export this?
+/// Underlying error class for libpathrs errors.
+///
+/// This is similar in concept to [`std::io::ErrorKind`]. Note that the
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
 #[non_exhaustive]
-pub(crate) enum ErrorKind {
+pub enum ErrorKind {
+    /// The requested feature is not implemented in libpathrs.
     NotImplemented,
+    /// The requested feature is not supported by the system.
     NotSupported,
+    /// The provided arguments to libpathrs were invalid.
     InvalidArgument,
+    /// libpaths encountered a state where the safety of the operation could not
+    /// be guaranteeed. This is usually the result of an attack by a malicious
+    /// program.
     SafetyViolation,
+    /// Some internal error occurred. For more information, see the string
+    /// description of the original [`Error`].
     InternalError,
+    /// The underlying error came from a system call. The provided
+    /// [`std::io::RawOsError`] is the numerical value of the `errno` number, if
+    /// available.
     // TODO: We might want to use Option<std::io::ErrorKind>?
     OsError(Option<i32>),
 }