api(Arg): add default_value_os

Also add related tests, and reframe `default_value` in terms of
`defualt_value_os`.

src/args/arg.rs

@@ -3062,9 +3062,17 @@ impl<'a, 'b> Arg<'a, 'b> {
     /// [`Arg::takes_value(true)`]: ./struct.Arg.html#method.takes_value
     /// [`ArgMatches::is_present`]: ./struct.ArgMatches.html#method.is_present
     /// [`Arg::default_value_if`]: ./struct.Arg.html#method.default_value_if
-    pub fn default_value(mut self, val: &'a str) -> Self {
+    pub fn default_value(self, val: &'a str) -> Self {
+        self.default_value_os(OsStr::from_bytes(val.as_bytes()))
+    }
+
+    /// Provides a default value in the exact same manner as [`Arg::default_value`]
+    /// only using [`OsStr`]s instead.
+    /// [`Arg::default_value`]: ./struct.Arg.html#method.default_value
+    /// [`OsStr`]: https://doc.rust-lang.org/std/ffi/struct.OsStr.html
+    pub fn default_value_os(mut self, val: &'a OsStr) -> Self {
         self.setb(ArgSettings::TakesValue);
-        self.v.default_val = Some(OsStr::from_bytes(val.as_bytes()));
+        self.v.default_val = Some(val);
         self
     }
 

tests/default_vals.rs

@@ -53,6 +53,68 @@ fn positional_user_override() {
     assert_eq!(m.value_of("arg").unwrap(), "value");
 }
 
+// OsStr Default Values
+
+#[test]
+fn osstr_opts() {
+    use std::ffi::OsStr;
+    let expected = OsStr::new("default");
+
+    let r = App::new("df")
+        .arg( Arg::from_usage("-o [opt] 'some opt'")
+            .default_value_os(expected))
+        .get_matches_from_safe(vec![""]);
+    assert!(r.is_ok());
+    let m = r.unwrap();
+    assert!(m.is_present("o"));
+    assert_eq!(m.value_of("o").unwrap(), expected);
+}
+
+#[test]
+fn osstr_opt_user_override() {
+    use std::ffi::OsStr;
+    let default = OsStr::new("default");
+
+    let r = App::new("df")
+        .arg( Arg::from_usage("--opt [FILE] 'some arg'")
+            .default_value_os(default))
+        .get_matches_from_safe(vec!["", "--opt", "value"]);
+    assert!(r.is_ok());
+    let m = r.unwrap();
+    assert!(m.is_present("opt"));
+    assert_eq!(m.value_of("opt").unwrap(), "value");
+}
+
+#[test]
+fn osstr_positionals() {
+    use std::ffi::OsStr;
+    let expected = OsStr::new("default");
+
+    let r = App::new("df")
+        .arg( Arg::from_usage("[arg] 'some opt'")
+            .default_value_os(expected))
+        .get_matches_from_safe(vec![""]);
+    assert!(r.is_ok());
+    let m = r.unwrap();
+    assert!(m.is_present("arg"));
+    assert_eq!(m.value_of("arg").unwrap(), expected);
+}
+
+#[test]
+fn osstr_positional_user_override() {
+    use std::ffi::OsStr;
+    let default = OsStr::new("default");
+
+    let r = App::new("df")
+        .arg( Arg::from_usage("[arg] 'some arg'")
+            .default_value_os(default))
+        .get_matches_from_safe(vec!["", "value"]);
+    assert!(r.is_ok());
+    let m = r.unwrap();
+    assert!(m.is_present("arg"));
+    assert_eq!(m.value_of("arg").unwrap(), "value");
+}
+
 // --- Default if arg is present 
 
 #[test]
@@ -368,4 +430,4 @@ fn conditional_reqs_pass() {
     let m = m.unwrap();
     assert_eq!(m.value_of("output"), Some("other"));
     assert_eq!(m.value_of("input"), Some("some"));
-}
+}