Add a few doc examples for std::ffi::OsStr.

* `std::ffi::OsStr::new`.
* `std::ffi::OsStr::is_empty`.
* `std::ffi::OsStr::len`.

src/libstd/ffi/os_str.rs

@@ -251,6 +251,14 @@ impl Hash for OsString {
 
 impl OsStr {
     /// Coerces into an `OsStr` slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsStr;
+    ///
+    /// let os_str = OsStr::new("foo");
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new<S: AsRef<OsStr> + ?Sized>(s: &S) -> &OsStr {
         s.as_ref()
@@ -283,6 +291,18 @@ impl OsStr {
     }
 
     /// Checks whether the `OsStr` is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsStr;
+    ///
+    /// let os_str = OsStr::new("");
+    /// assert!(os_str.is_empty());
+    ///
+    /// let os_str = OsStr::new("foo");
+    /// assert!(!os_str.is_empty());
+    /// ```
     #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
     pub fn is_empty(&self) -> bool {
         self.inner.inner.is_empty()
@@ -296,6 +316,18 @@ impl OsStr {
     /// other methods like `OsString::with_capacity` to avoid reallocations.
     ///
     /// See `OsStr` introduction for more information about encoding.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsStr;
+    ///
+    /// let os_str = OsStr::new("");
+    /// assert_eq!(os_str.len(), 0);
+    ///
+    /// let os_str = OsStr::new("foo");
+    /// assert_eq!(os_str.len(), 3);
+    /// ```
     #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
     pub fn len(&self) -> usize {
         self.inner.inner.len()