Auto merge of #43883 - frewsxcv:frewsxcv-set-readonly-clarification, r=QuietMisdreavus

Clarify writable behavior of readonly-named `Permissions` methods.

Opened primarily to fix https://github.com/rust-lang/rust/issues/41984.

src/libstd/fs.rs

@@ -916,7 +916,7 @@ impl AsInner<fs_imp::FileAttr> for Metadata {
 }
 
 impl Permissions {
-    /// Returns whether these permissions describe a readonly file.
+    /// Returns whether these permissions describe a readonly (unwritable) file.
     ///
     /// # Examples
     ///
@@ -934,7 +934,11 @@ impl Permissions {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn readonly(&self) -> bool { self.0.readonly() }
 
-    /// Modifies the readonly flag for this set of permissions.
+    /// Modifies the readonly flag for this set of permissions. If the
+    /// `readonly` argument is `true`, using the resulting `Permission` will
+    /// update file permissions to forbid writing. Conversely, if it's `false`,
+    /// using the resulting `Permission` will update file permissions to allow
+    /// writing.
     ///
     /// This operation does **not** modify the filesystem. To modify the
     /// filesystem use the `fs::set_permissions` function.

src/libstd/sys/unix/fs.rs

@@ -170,11 +170,17 @@ impl AsInner<stat64> for FileAttr {
 }
 
 impl FilePermissions {
-    pub fn readonly(&self) -> bool { self.mode & 0o222 == 0 }
+    pub fn readonly(&self) -> bool {
+        // check if any class (owner, group, others) has write permission
+        self.mode & 0o222 == 0
+    }
+
     pub fn set_readonly(&mut self, readonly: bool) {
         if readonly {
+            // remove write permission for all classes; equivalent to `chmod a-w <file>`
             self.mode &= !0o222;
         } else {
+            // add write permission for all classes; equivalent to `chmod a+w <file>`
             self.mode |= 0o222;
         }
     }