feat: add io_err module with classifiers.

This makes it easier to portably identify IO errors, especially while
the MSRV is not Rustc v1.83 which stabilizes the error kind variants.

Author: Byron

gix-fs/src/lib.rs

@@ -83,6 +83,18 @@ pub fn is_executable(metadata: &std::fs::Metadata) -> bool {
     (metadata.mode() & 0o100) != 0
 }
 
+/// Classifiers for IO-errors.
+pub mod io_err {
+    use std::io::ErrorKind;
+
+    /// Return `true` if `err` indicates that the entry doesn't exist on disk. `raw` is used as well
+    /// for additional checks while the variants are outside the MSRV.
+    pub fn is_not_found(err: ErrorKind, raw_err: Option<i32>) -> bool {
+        // TODO: use variant once MSRV is 1.83
+        err == ErrorKind::NotFound || raw_err == Some(20)
+    }
+}
+
 #[cfg(not(unix))]
 /// Returns whether a a file has the executable permission set.
 pub fn is_executable(_metadata: &std::fs::Metadata) -> bool {