diff --git a/library/std/src/io/error.rs b/library/std/src/io/error.rs
index 829ef3d98bbc9..51666c0a3c7f1 100644
--- a/library/std/src/io/error.rs
+++ b/library/std/src/io/error.rs
@@ -261,19 +261,15 @@ pub enum ErrorKind {
     #[stable(feature = "rust1", since = "1.0.0")]
     Interrupted,
 
-    /// A custom error that does not fall under any other I/O error kind.
-    ///
-    /// This can be used to construct your own [`Error`]s that do not match any
-    /// [`ErrorKind`].
-    ///
-    /// This [`ErrorKind`] is not used by the standard library.
+    /// This operation is unsupported on this platform.
     ///
-    /// Errors from the standard library that do not fall under any of the I/O
-    /// error kinds cannot be `match`ed on, and will only match a wildcard (`_`) pattern.
-    /// New [`ErrorKind`]s might be added in the future for some of those.
-    #[stable(feature = "rust1", since = "1.0.0")]
-    Other,
+    /// This means that the operation can never succeed.
+    #[stable(feature = "unsupported_error", since = "1.53.0")]
+    Unsupported,
 
+    // ErrorKinds which are primarily categorisations for OS error
+    // codes should be added above.
+    //
     /// An error returned when an operation could not be completed because an
     /// "end of file" was reached prematurely.
     ///
@@ -283,17 +279,28 @@ pub enum ErrorKind {
     #[stable(feature = "read_exact", since = "1.6.0")]
     UnexpectedEof,
 
-    /// This operation is unsupported on this platform.
-    ///
-    /// This means that the operation can never succeed.
-    #[stable(feature = "unsupported_error", since = "1.53.0")]
-    Unsupported,
-
     /// An operation could not be completed, because it failed
     /// to allocate enough memory.
     #[stable(feature = "out_of_memory_error", since = "1.54.0")]
     OutOfMemory,
 
+    // "Unusual" error kinds which do not correspond simply to (sets
+    // of) OS error codes, should be added just above this comment.
+    // `Other` and `Uncategorised` should remain at the end:
+    //
+    /// A custom error that does not fall under any other I/O error kind.
+    ///
+    /// This can be used to construct your own [`Error`]s that do not match any
+    /// [`ErrorKind`].
+    ///
+    /// This [`ErrorKind`] is not used by the standard library.
+    ///
+    /// Errors from the standard library that do not fall under any of the I/O
+    /// error kinds cannot be `match`ed on, and will only match a wildcard (`_`) pattern.
+    /// New [`ErrorKind`]s might be added in the future for some of those.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    Other,
+
     /// Any I/O error from the standard library that's not part of this list.
     ///
     /// Errors that are `Uncategorized` now may move to a different or a new
@@ -307,13 +314,13 @@ pub enum ErrorKind {
 impl ErrorKind {
     pub(crate) fn as_str(&self) -> &'static str {
         use ErrorKind::*;
+        // Strictly alphabetical, please.  (Sadly rustfmt cannot do this yet.)
         match *self {
             AddrInUse => "address in use",
             AddrNotAvailable => "address not available",
             AlreadyExists => "entity already exists",
             ArgumentListTooLong => "argument list too long",
             BrokenPipe => "broken pipe",
-            ResourceBusy => "resource busy",
             ConnectionAborted => "connection aborted",
             ConnectionRefused => "connection refused",
             ConnectionReset => "connection reset",
@@ -321,9 +328,10 @@ impl ErrorKind {
             Deadlock => "deadlock",
             DirectoryNotEmpty => "directory not empty",
             ExecutableFileBusy => "executable file busy",
+            FileTooLarge => "file too large",
             FilenameTooLong => "filename too long",
+            FilesystemLoop => "filesystem loop or indirection limit (e.g. symlink loop)",
             FilesystemQuotaExceeded => "filesystem quota exceeded",
-            FileTooLarge => "file too large",
             HostUnreachable => "host unreachable",
             Interrupted => "operation interrupted",
             InvalidData => "invalid data",
@@ -332,16 +340,16 @@ impl ErrorKind {
             NetworkDown => "network down",
             NetworkUnreachable => "network unreachable",
             NotADirectory => "not a directory",
-            StorageFull => "no storage space",
             NotConnected => "not connected",
             NotFound => "entity not found",
+            NotSeekable => "seek on unseekable file",
             Other => "other error",
             OutOfMemory => "out of memory",
             PermissionDenied => "permission denied",
             ReadOnlyFilesystem => "read-only filesystem or storage medium",
+            ResourceBusy => "resource busy",
             StaleNetworkFileHandle => "stale network file handle",
-            FilesystemLoop => "filesystem loop or indirection limit (e.g. symlink loop)",
-            NotSeekable => "seek on unseekable file",
+            StorageFull => "no storage space",
             TimedOut => "timed out",
             TooManyLinks => "too many links",
             Uncategorized => "uncategorized error",