feat(builder): Allow flags to take num_args=0..=1

Author: epage

clap_builder/src/builder/action.rs

@@ -376,8 +376,8 @@ impl ArgAction {
         match self {
             Self::Set => ValueRange::FULL,
             Self::Append => ValueRange::FULL,
-            Self::SetTrue => ValueRange::EMPTY,
-            Self::SetFalse => ValueRange::EMPTY,
+            Self::SetTrue => ValueRange::OPTIONAL,
+            Self::SetFalse => ValueRange::OPTIONAL,
             Self::Count => ValueRange::EMPTY,
             Self::Help => ValueRange::EMPTY,
             Self::HelpShort => ValueRange::EMPTY,

clap_builder/src/builder/range.rs

@@ -18,6 +18,12 @@ impl ValueRange {
         end_inclusive: 1,
     };
 
+    #[cfg(debug_assertions)]
+    pub(crate) const OPTIONAL: Self = Self {
+        start_inclusive: 0,
+        end_inclusive: 1,
+    };
+
     pub(crate) const FULL: Self = Self {
         start_inclusive: 0,
         end_inclusive: usize::MAX,

tests/builder/flags.rs

@@ -225,19 +225,25 @@ fn leading_dash_stripped() {
 
 #[test]
 fn optional_value() {
-    let cmd = Command::new("flag").args([arg!(-f --flag "some flag").action(ArgAction::SetTrue)]);
+    let cmd = Command::new("flag").args([arg!(-f --flag "some flag")
+        .action(ArgAction::SetTrue)
+        .num_args(0..=1)]);
 
     let m = cmd.clone().try_get_matches_from(vec![""]).unwrap();
     assert!(!*m.get_one::<bool>("flag").expect("defaulted by clap"));
 
     let m = cmd.clone().try_get_matches_from(vec!["", "-f"]).unwrap();
     assert!(*m.get_one::<bool>("flag").expect("defaulted by clap"));
 
-    cmd.clone()
-        .try_get_matches_from(vec!["", "-f", "true"])
-        .unwrap_err();
-
-    cmd.clone()
+    let m = cmd
+        .clone()
         .try_get_matches_from(vec!["", "-f", "false"])
-        .unwrap_err();
+        .unwrap();
+    assert!(!*m.get_one::<bool>("flag").expect("defaulted by clap"));
+
+    let m = cmd
+        .clone()
+        .try_get_matches_from(vec!["", "-f", "true"])
+        .unwrap();
+    assert!(*m.get_one::<bool>("flag").expect("defaulted by clap"));
 }

tests/derive/flags.rs

@@ -331,7 +331,6 @@ fn unit_for_negation() {
 }
 
 #[test]
-#[should_panic = "Argument `alice`'s action SetTrue is incompatible with `num_args(0..=1)`"]
 fn optional_value_flag() {
     #[derive(Parser, PartialEq, Eq, Debug)]
     struct Opt {