Auto merge of #30872 - pitdicker:expand_open_options, r=alexcrichton

Tracking issue: #30014

This implements the RFC and makes a few other changes.
I have added a few extra tests, and made the Windows and
Unix code as similar as possible.

Part of the RFC mentions the unstable OpenOptionsExt trait
on Windows (see #27720). I have added a few extra methods
to future-proof it for CreateFile2.

src/libstd/sys/unix/ext/fs.rs

@@ -118,8 +118,38 @@ pub trait OpenOptionsExt {
     ///
     /// If a new file is created as part of a `File::open_opts` call then this
     /// specified `mode` will be used as the permission bits for the new file.
+    /// If no `mode` is set, the default of `0o666` will be used.
+    /// The operating system masks out bits with the systems `umask`, to produce
+    /// the final permissions.
     #[stable(feature = "fs_ext", since = "1.1.0")]
     fn mode(&mut self, mode: raw::mode_t) -> &mut Self;
+
+    /// Pass custom flags to the `flags` agument of `open`.
+    ///
+    /// The bits that define the access mode are masked out with `O_ACCMODE`, to
+    /// ensure they do not interfere with the access mode set by Rusts options.
+    ///
+    /// Custom flags can only set flags, not remove flags set by Rusts options.
+    /// This options overwrites any previously set custom flags.
+    ///
+    /// # Examples
+    ///
+    /// ```rust,ignore
+    /// extern crate libc;
+    /// use std::fs::OpenOptions;
+    /// use std::os::unix::fs::OpenOptionsExt;
+    ///
+    /// let mut options = OpenOptions::new();
+    /// options.write(true);
+    /// if cfg!(unix) {
+    ///     options.custom_flags(libc::O_NOFOLLOW);
+    /// }
+    /// let file = options.open("foo.txt");
+    /// ```
+    #[unstable(feature = "expand_open_options",
+               reason = "recently added",
+               issue = "30014")]
+    fn custom_flags(&mut self, flags: i32) -> &mut Self;
 }
 
 #[stable(feature = "fs_ext", since = "1.1.0")]
@@ -127,6 +157,10 @@ impl OpenOptionsExt for OpenOptions {
     fn mode(&mut self, mode: raw::mode_t) -> &mut OpenOptions {
         self.as_inner_mut().mode(mode); self
     }
+
+    fn custom_flags(&mut self, flags: i32) -> &mut OpenOptions {
+        self.as_inner_mut().custom_flags(flags); self
+    }
 }
 
 // Hm, why are there casts here to the returned type, shouldn't the types always
@@ -281,4 +315,3 @@ impl DirBuilderExt for fs::DirBuilder {
         self
     }
 }
-