Rollup merge of #130289 - intgr-forks:Permissions-readonly-vs-unix-root, r=ChrisDenton

docs: Permissions.readonly() also ignores root user special permissions

The root user can write to files without any (write) permission bits set. But this is not taken into account by `std::fs::Permissions.readonly()`.

The rustdoc for `readonly()` also mentions shortcomings later:

> On Unix-based platforms this checks if any of the owner, group or others write permission bits are set. It does not check if the current user is in the file’s assigned group. It also does not check ACLs.

But since this part already clarifies how it works -- it checks write permission bits -- I think it's not necessary to repeat the root user shortcomings here.

Author: matthiaskrgr

Diff for: library/std/src/fs.rs

@@ -1869,8 +1869,10 @@ impl Permissions {
     ///
     /// # Note
     ///
-    /// This function does not take Access Control Lists (ACLs) or Unix group
-    /// membership into account.
+    /// This function does not take Access Control Lists (ACLs), Unix group
+    /// membership and other nuances into account.
+    /// Therefore the return value of this function cannot be relied upon
+    /// to predict whether attempts to read or write the file will actually succeed.
     ///
     /// # Windows
     ///
@@ -1885,10 +1887,13 @@ impl Permissions {
     /// # Unix (including macOS)
     ///
     /// On Unix-based platforms this checks if *any* of the owner, group or others
-    /// write permission bits are set. It does not check if the current
-    /// user is in the file's assigned group. It also does not check ACLs.
-    /// Therefore the return value of this function cannot be relied upon
-    /// to predict whether attempts to read or write the file will actually succeed.
+    /// write permission bits are set. It does not consider anything else, including:
+    ///
+    /// * Whether the current user is in the file's assigned group.
+    /// * Permissions granted by ACL.
+    /// * That `root` user can write to files that do not have any write bits set.
+    /// * Writable files on a filesystem that is mounted read-only.
+    ///
     /// The [`PermissionsExt`] trait gives direct access to the permission bits but
     /// also does not read ACLs.
     ///