fix(derive): Add "id" attribute

Previously the Arg id was set with the "name" attribute. This allows use
of an "id" attribute to match the underlying struct.

A side effect of this is that the "id" attribute may also be used on
Commands. This isn't desired, but given the current architecture of the
attribute parser, it's hard to avoid.

Fixes: #3785

clap_derive/src/attrs.rs

@@ -400,7 +400,7 @@ impl Attrs {
     }
 
     fn push_method(&mut self, name: Ident, arg: impl ToTokens) {
-        if name == "name" {
+        if name == "name" || name == "id" {
             self.name = Name::Assigned(quote!(#arg));
         } else if name == "value_parser" {
             self.value_parser = Some(ValueParser::Explicit(Method::new(name, quote!(#arg))));

src/_derive/mod.rs

@@ -181,7 +181,7 @@
 //! - e.g. `#[clap(max_values(3))]` would translate to `arg.max_values(3)`
 //!
 //! **Magic attributes**:
-//! - `name = <expr>`: [`Arg::id`][crate::Arg::id]
+//! - `id = <expr>`: [`Arg::id`][crate::Arg::id]
 //!   - When not present: case-converted field name is used
 //! - `value_parser [= <expr>]`: [`Arg::value_parser`][crate::Arg::value_parser]
 //!   - When not present: will auto-select an implementation based on the field type using

tests/derive/naming.rs

@@ -57,6 +57,34 @@ fn test_standalone_long_ignores_afterwards_defined_custom_name() {
     );
 }
 
+#[test]
+fn test_standalone_long_uses_previous_defined_custom_id() {
+    #[derive(Parser, Debug, PartialEq)]
+    struct Opt {
+        #[clap(id = "foo", long)]
+        foo_option: bool,
+    }
+
+    assert_eq!(
+        Opt { foo_option: true },
+        Opt::try_parse_from(&["test", "--foo"]).unwrap()
+    );
+}
+
+#[test]
+fn test_standalone_long_ignores_afterwards_defined_custom_id() {
+    #[derive(Parser, Debug, PartialEq)]
+    struct Opt {
+        #[clap(long, id = "foo")]
+        foo_option: bool,
+    }
+
+    assert_eq!(
+        Opt { foo_option: true },
+        Opt::try_parse_from(&["test", "--foo-option"]).unwrap()
+    );
+}
+
 #[test]
 fn test_standalone_short_generates_kebab_case() {
     #[derive(Parser, Debug, PartialEq)]
@@ -114,6 +142,34 @@ fn test_standalone_short_ignores_afterwards_defined_custom_name() {
     );
 }
 
+#[test]
+fn test_standalone_short_uses_previous_defined_custom_id() {
+    #[derive(Parser, Debug, PartialEq)]
+    struct Opt {
+        #[clap(id = "option", short)]
+        foo_option: bool,
+    }
+
+    assert_eq!(
+        Opt { foo_option: true },
+        Opt::try_parse_from(&["test", "-o"]).unwrap()
+    );
+}
+
+#[test]
+fn test_standalone_short_ignores_afterwards_defined_custom_id() {
+    #[derive(Parser, Debug, PartialEq)]
+    struct Opt {
+        #[clap(short, id = "option")]
+        foo_option: bool,
+    }
+
+    assert_eq!(
+        Opt { foo_option: true },
+        Opt::try_parse_from(&["test", "-f"]).unwrap()
+    );
+}
+
 #[test]
 fn test_standalone_long_uses_previous_defined_casing() {
     #[derive(Parser, Debug, PartialEq)]