feat(Help): adds ability to hide the possible values on a per argument basis

Previously one could only hide the possible values of an argument application or command wide, and
not on a per argument basis. Now one can use the `Arg::hide_possible_values(bool)` method to hide
only that arguments possible values.

Closes #640

src/app/help.rs

@@ -495,7 +495,7 @@ impl<'a> Help<'a> {
                            } else {
                                Format::None(pv)
                            },
-                           if self.hide_pv {
+                           if self.hide_pv || a.is_set(ArgSettings::HidePossibleValues) {
                                "".into()
                            } else {
                                if let Some(ref pv) = a.possible_vals() {
@@ -523,7 +523,7 @@ impl<'a> Help<'a> {
                            } else {
                                aliases.join(", ")
                            });
-        } else if !self.hide_pv {
+        } else if !self.hide_pv && !a.is_set(ArgSettings::HidePossibleValues) {
             debugln!("Writing values");
             if let Some(pv) = a.possible_vals() {
                 debugln!("Possible vals...{:?}", pv);

src/args/arg.rs

@@ -1081,6 +1081,42 @@ impl<'a, 'b> Arg<'a, 'b> {
         }
     }
 
+    /// Specifies if the possible values of an argument should be displayed in the help text or
+    /// not. Defaults to `false` (i.e. show possible values)
+    ///
+    /// This is useful for args with many values, or ones which are explained elsewhere in the
+    /// help text.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use clap::{App, Arg};
+    /// Arg::with_name("config")
+    ///     .hide_possible_values(true)
+    /// # ;
+    /// ```
+    ///
+    /// ```rust
+    /// # use clap::{App, Arg};
+    /// let m = App::new("hide_posvals")
+    ///     .arg(Arg::with_name("mode")
+    ///         .long("mode")
+    ///         .possible_values(&["fast", "slow"])
+    ///         .takes_value(true)
+    ///         .hide_possible_values(true));
+    ///
+    /// ```
+    ///
+    /// If we were to run the above program with `--help` the `[values: fast, slow]` portion of
+    /// the help text would be omitted.
+    pub fn hide_possible_values(self, hide: bool) -> Self {
+        if hide {
+            self.set(ArgSettings::HidePossibleValues)
+        } else {
+            self.unset(ArgSettings::HidePossibleValues)
+        }
+    }
+
     /// Specifies the index of a positional argument **starting at** 1.
     ///
     /// **NOTE:** The index refers to position according to **other positional argument**. It does

src/args/settings.rs

@@ -4,17 +4,18 @@ use std::str::FromStr;
 
 bitflags! {
     flags Flags: u16 {
-        const REQUIRED       = 0b00000000001,
-        const MULTIPLE       = 0b00000000010,
-        const EMPTY_VALS     = 0b00000000100,
-        const GLOBAL         = 0b00000001000,
-        const HIDDEN         = 0b00000010000,
-        const TAKES_VAL      = 0b00000100000,
-        const USE_DELIM      = 0b00001000000,
-        const NEXT_LINE_HELP = 0b00010000000,
-        const R_UNLESS_ALL   = 0b00100000000,
-        const REQ_DELIM      = 0b01000000000,
-        const DELIM_NOT_SET  = 0b10000000000,
+        const REQUIRED       = 0b000000000001,
+        const MULTIPLE       = 0b000000000010,
+        const EMPTY_VALS     = 0b000000000100,
+        const GLOBAL         = 0b000000001000,
+        const HIDDEN         = 0b000000010000,
+        const TAKES_VAL      = 0b000000100000,
+        const USE_DELIM      = 0b000001000000,
+        const NEXT_LINE_HELP = 0b000010000000,
+        const R_UNLESS_ALL   = 0b000100000000,
+        const REQ_DELIM      = 0b001000000000,
+        const DELIM_NOT_SET  = 0b010000000000,
+        const HIDE_POS_VALS  = 0b100000000000,
     }
 }
 
@@ -38,7 +39,8 @@ impl ArgFlags {
         NextLineHelp => NEXT_LINE_HELP,
         RequiredUnlessAll => R_UNLESS_ALL,
         RequireDelimiter => REQ_DELIM,
-        ValueDelimiterNotSet => DELIM_NOT_SET
+        ValueDelimiterNotSet => DELIM_NOT_SET,
+        HidePossibleValues => HIDE_POS_VALS
     }
 }
 
@@ -74,6 +76,8 @@ pub enum ArgSettings {
     NextLineHelp,
     /// Requires the use of a value delimiter for all multiple values
     RequireDelimiter,
+    /// Hides the possible values from the help string
+    HidePossibleValues,
     #[doc(hidden)]
     RequiredUnlessAll,
     #[doc(hidden)]
@@ -95,6 +99,7 @@ impl FromStr for ArgSettings {
             "requiredunlessall" => Ok(ArgSettings::RequiredUnlessAll),
             "requiredelimiter" => Ok(ArgSettings::RequireDelimiter),
             "valuedelimiternotset" => Ok(ArgSettings::ValueDelimiterNotSet),
+            "hidepossiblevalues" => Ok(ArgSettings::HidePossibleValues),
             _ => Err("unknown ArgSetting, cannot convert from str".to_owned()),
         }
     }