feat(Defult Values): adds support for default values in args

Closes #418

Author: kbknapp

src/app/parser.rs

@@ -535,6 +535,7 @@ impl<'a, 'b> Parser<'a, 'b> where 'a: 'b {
             }
         }
 
+        try!(self.add_defaults(matcher));
         try!(self.validate_blacklist(matcher));
         try!(self.validate_num_args(matcher));
         matcher.usage(self.create_usage(&[]));
@@ -1518,4 +1519,22 @@ impl<'a, 'b> Parser<'a, 'b> where 'a: 'b {
         }
         w.flush().map_err(Error::from)
     }
+
+    fn add_defaults(&mut self, matcher: &mut ArgMatcher<'a>) -> ClapResult<()> {
+        macro_rules! add_val {
+            ($_self:ident, $a:ident, $m:ident) => {
+                if $m.get($a.name).is_none() {
+                    try!($_self.add_val_to_arg($a, OsStr::new($a.default_val.as_ref().unwrap()), $m));
+                    arg_post_processing!($_self, $a, $m);
+                }
+            };
+        }
+        for o in self.opts.iter().filter(|o| o.default_val.is_some()) {
+            add_val!(self, o, matcher);
+        }
+        for p in self.positionals.values().filter(|p| p.default_val.is_some()) {
+            add_val!(self, p, matcher);
+        }
+        Ok(())
+    }
 }

src/args/arg.rs

@@ -66,6 +66,8 @@ pub struct Arg<'a, 'b> where 'a: 'b {
     pub settings: ArgFlags,
     #[doc(hidden)]
     pub val_delim: Option<char>,
+    #[doc(hidden)]
+    pub default_val: Option<&'a str>,
 }
 
 impl<'a, 'b> Default for Arg<'a, 'b> {
@@ -88,6 +90,7 @@ impl<'a, 'b> Default for Arg<'a, 'b> {
             overrides: None,
             settings: ArgFlags::new(),
             val_delim: Some(','),
+            default_val: None,
         }
     }
 }
@@ -1087,14 +1090,23 @@ impl<'a, 'b> Arg<'a, 'b> {
         self
     }
 
-    #[doc(hidden)]
-    pub fn setb(&mut self, s: ArgSettings) {
-        self.settings.set(s);
-    }
-
-    #[doc(hidden)]
-    pub fn unsetb(&mut self, s: ArgSettings) {
-        self.settings.unset(s);
+    /// Specifies the value of the argument when *not* used at runtime.
+    ///
+    /// **NOTE:** implicitly sets `Arg::takes_value(true)`
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use clap::{App, Arg};
+    /// Arg::with_name("input")
+    ///     .long("option")
+    ///     .default_value("myval")
+    /// # ;
+    /// ```
+    pub fn default_value(mut self, val: &'a str) -> Self {
+        self.setb(ArgSettings::TakesValue);
+        self.default_val = Some(val);
+        self
     }
 
     /// Checks if one of the `ArgSettings` settings is set for the argument
@@ -1113,6 +1125,16 @@ impl<'a, 'b> Arg<'a, 'b> {
         self.unsetb(s);
         self
     }
+
+    #[doc(hidden)]
+    pub fn setb(&mut self, s: ArgSettings) {
+        self.settings.set(s);
+    }
+
+    #[doc(hidden)]
+    pub fn unsetb(&mut self, s: ArgSettings) {
+        self.settings.unset(s);
+    }
 }
 
 impl<'a, 'b, 'z> From<&'z Arg<'a, 'b>>
@@ -1136,6 +1158,7 @@ impl<'a, 'b, 'z> From<&'z Arg<'a, 'b>>
             overrides: a.overrides.clone(),
             settings: a.settings.clone(),
             val_delim: a.val_delim,
+            default_val: a.default_val,
         }
     }
 }

src/args/arg_builder/option.rs

@@ -26,6 +26,7 @@ pub struct OptBuilder<'n, 'e> {
     pub overrides: Option<Vec<&'e str>>,
     pub settings: ArgFlags,
     pub val_delim: Option<char>,
+    pub default_val: Option<&'n str>,
 }
 
 impl<'n, 'e> Default for OptBuilder<'n, 'e> {
@@ -46,6 +47,7 @@ impl<'n, 'e> Default for OptBuilder<'n, 'e> {
             overrides: None,
             settings: ArgFlags::new(),
             val_delim: Some(','),
+            default_val: None,
         }
     }
 }
@@ -79,6 +81,7 @@ impl<'n, 'e> OptBuilder<'n, 'e> {
             requires: a.requires.clone(),
             possible_vals: a.possible_vals.clone(),
             settings: a.settings.clone(),
+            default_val: a.default_val,
             ..Default::default()
         };
         if let Some(ref vec) = ob.val_names {

src/args/arg_builder/positional.rs

@@ -26,6 +26,7 @@ pub struct PosBuilder<'n, 'e> {
     pub overrides: Option<Vec<&'e str>>,
     pub settings: ArgFlags,
     pub val_delim: Option<char>,
+    pub default_val: Option<&'n str>,
 }
 
 impl<'n, 'e> Default for PosBuilder<'n, 'e> {
@@ -45,6 +46,7 @@ impl<'n, 'e> Default for PosBuilder<'n, 'e> {
             overrides: None,
             settings: ArgFlags::new(),
             val_delim: Some(','),
+            default_val: None,
         }
     }
 }
@@ -80,6 +82,7 @@ impl<'n, 'e> PosBuilder<'n, 'e> {
             help: a.help,
             val_delim: a.val_delim,
             settings: a.settings.clone(),
+            default_val: a.default_val,
             ..Default::default()
         };
         if a.max_vals.is_some()