Auto merge of #65429 - Timmmm:withoptions, r=Mark-Simulacrum

Add File::with_options

This provides a more fluent API to create files with options, and also avoids the need to import OpenOptions.

This implements @aldanor's [suggestion](https://github.com/rust-lang/rfcs/pull/2615#issuecomment-448591304) which was popular.

src/libstd/fs.rs

@@ -29,7 +29,7 @@ use crate::time::SystemTime;
 ///
 /// # Examples
 ///
-/// Creates a new file and write bytes to it:
+/// Creates a new file and write bytes to it (you can also use [`write`]):
 ///
 /// ```no_run
 /// use std::fs::File;
@@ -42,7 +42,7 @@ use crate::time::SystemTime;
 /// }
 /// ```
 ///
-/// Read the contents of a file into a [`String`]:
+/// Read the contents of a file into a [`String`] (you can also use [`read`]):
 ///
 /// ```no_run
 /// use std::fs::File;
@@ -89,6 +89,8 @@ use crate::time::SystemTime;
 /// [`Write`]: ../io/trait.Write.html
 /// [`BufReader<R>`]: ../io/struct.BufReader.html
 /// [`sync_all`]: struct.File.html#method.sync_all
+/// [`read`]: fn.read.html
+/// [`write`]: fn.write.html
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct File {
     inner: fs_imp::File,
@@ -397,6 +399,37 @@ impl File {
         OpenOptions::new().write(true).create(true).truncate(true).open(path.as_ref())
     }
 
+    /// Returns a new OpenOptions object.
+    ///
+    /// This function returns a new OpenOptions object that you can use to
+    /// open or create a file with specific options if `open()` or `create()`
+    /// are not appropriate.
+    ///
+    /// It is equivalent to `OpenOptions::new()` but allows you to write more
+    /// readable code. Instead of `OpenOptions::new().read(true).open("foo.txt")`
+    /// you can write `File::with_options().read(true).open("foo.txt"). This
+    /// also avoids the need to import `OpenOptions`.
+    ///
+    /// See the [`OpenOptions::new`] function for more details.
+    ///
+    /// [`OpenOptions::new`]: struct.OpenOptions.html#method.new
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(with_options)]
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut f = File::with_options().read(true).open("foo.txt")?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[unstable(feature = "with_options", issue = "65439")]
+    pub fn with_options() -> OpenOptions {
+        OpenOptions::new()
+    }
+
     /// Attempts to sync all OS-internal metadata to disk.
     ///
     /// This function will attempt to ensure that all in-memory data reaches the