fix(parser): Rely on default_missing_value for flag actions

In the short term, this just provides a back door to custom actions.
Longer term, we can explore a `SetConst` action that relies on this
behavior.  Really, `SetTrue` and `SetFalse` are shortcuts for such an
action but shortcuts can be helpful for usability.

Apparently, this also reduced `.text` size by 1k

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Removed `PartialEq` and `Eq` from `Command`
 - `number_of_values(0)` no longer implies `takes_value(true).multiple_values(true)`
 - `number_of_values(1)` no longer implies `multiple_values(true)`
+- `ArgAction::SetTrue` and `ArgAction::SetFalse` now prioritize `Arg::default_missing_value` over their standard behavior
 - *(help)* Make `DeriveDisplayOrder` the default and removed the setting.  To sort help, set `next_display_order(None)` (#2808)
 - *(help)* Subcommand display order respects `Command::next_display_order` instead of `DeriveDisplayOrder` and using its own initial display order value (#2808)
 - *(env)* Parse `--help` and `--version` like any `ArgAction::SetTrue` flag (#3776)

src/builder/action.rs

@@ -260,6 +260,18 @@ impl ArgAction {
         }
     }
 
+    pub(crate) fn default_missing_value(&self) -> Option<&'static std::ffi::OsStr> {
+        match self {
+            Self::Set => None,
+            Self::Append => None,
+            Self::SetTrue => Some(std::ffi::OsStr::new("true")),
+            Self::SetFalse => Some(std::ffi::OsStr::new("false")),
+            Self::Count => None,
+            Self::Help => None,
+            Self::Version => None,
+        }
+    }
+
     pub(crate) fn default_value_parser(&self) -> Option<super::ValueParser> {
         match self {
             Self::Set => None,

src/builder/arg.rs

@@ -4303,6 +4303,11 @@ impl<'help> Arg<'help> {
                     self.default_vals = vec![default_value];
                 }
             }
+            if let Some(default_value) = action.default_missing_value() {
+                if self.default_missing_vals.is_empty() {
+                    self.default_missing_vals = vec![default_value];
+                }
+            }
             if action.takes_values() {
                 self.settings.set(ArgSettings::TakesValue);
             } else {