feat(builder): Add Extension API

Cargo.toml

@@ -168,6 +168,7 @@ string = ["clap_builder/string"]  # Allow runtime generated strings
 
 # In-work features
 unstable-v5 = ["clap_builder/unstable-v5", "clap_derive?/unstable-v5", "deprecated"]
+unstable-ext = []
 unstable-styles = ["clap_builder/unstable-styles"]  # deprecated
 
 [lib]

clap_builder/Cargo.toml

@@ -52,6 +52,7 @@ string = []  # Allow runtime generated strings
 
 # In-work features
 unstable-v5 = ["deprecated"]
+unstable-ext = []
 unstable-styles = ["color"]  # deprecated
 
 [lib]

clap_builder/src/builder/arg.rs

@@ -11,6 +11,10 @@ use std::{
 
 // Internal
 use super::{ArgFlags, ArgSettings};
+#[cfg(feature = "unstable-ext")]
+use crate::builder::ext::Extension;
+#[cfg(feature = "unstable-ext")]
+use crate::builder::ext::Extensions;
 use crate::builder::ArgPredicate;
 use crate::builder::IntoResettable;
 use crate::builder::OsStr;
@@ -86,6 +90,8 @@ pub struct Arg {
     pub(crate) index: Option<usize>,
     pub(crate) help_heading: Option<Option<Str>>,
     pub(crate) value_hint: Option<ValueHint>,
+    #[cfg(feature = "unstable-ext")]
+    pub(crate) ext: Extensions,
 }
 
 /// # Basic API
@@ -869,6 +875,18 @@ impl Arg {
         self.settings.unset(setting);
         self
     }
+
+    /// Extend [`Arg`] with [`ArgExt`] data
+    #[cfg(feature = "unstable-ext")]
+    pub fn add<T: ArgExt + Extension>(&mut self, tagged: T) -> bool {
+        self.ext.set(tagged)
+    }
+
+    /// Remove an [`ArgExt`]
+    #[cfg(feature = "unstable-ext")]
+    pub fn remove<T: ArgExt + Extension>(&mut self) -> Option<T> {
+        self.ext.remove::<T>()
+    }
 }
 
 /// # Value Handling
@@ -4210,6 +4228,12 @@ impl Arg {
     pub fn is_ignore_case_set(&self) -> bool {
         self.is_set(ArgSettings::IgnoreCase)
     }
+
+    /// Access an [`ArgExt`]
+    #[cfg(feature = "unstable-ext")]
+    pub fn get<T: ArgExt + Extension>(&self) -> Option<&T> {
+        self.ext.get::<T>()
+    }
 }
 
 /// # Internally used only
@@ -4492,10 +4516,19 @@ impl fmt::Debug for Arg {
             ds = ds.field("env", &self.env);
         }
 
+        #[cfg(feature = "unstable-ext")]
+        {
+            ds = ds.field("ext", &self.ext);
+        }
+
         ds.finish()
     }
 }
 
+/// User-provided data that can be attached to an [`Arg`]
+#[cfg(feature = "unstable-ext")]
+pub trait ArgExt: Extension {}
+
 // Flags
 #[cfg(test)]
 mod test {

clap_builder/src/builder/ext.rs

@@ -40,9 +40,7 @@ impl Extensions {
     }
 }
 
-pub(crate) trait Extension:
-    std::fmt::Debug + Clone + std::any::Any + Send + Sync + 'static
-{
-}
+#[allow(unreachable_pub)]
+pub trait Extension: std::fmt::Debug + Clone + std::any::Any + Send + Sync + 'static {}
 
 impl<T> Extension for T where T: std::fmt::Debug + Clone + std::any::Any + Send + Sync + 'static {}

clap_builder/src/builder/mod.rs

@@ -28,6 +28,8 @@ pub mod styling;
 pub use self::str::Str;
 pub use action::ArgAction;
 pub use arg::Arg;
+#[cfg(feature = "unstable-ext")]
+pub use arg::ArgExt;
 pub use arg_group::ArgGroup;
 pub use arg_predicate::ArgPredicate;
 pub use command::Command;