fix(derive): preserve .0 in float default_value_t help

clap_derive/src/item.rs

@@ -584,12 +584,25 @@ impl Item {
                             s
                         })
                     } else {
-                        quote_spanned!(attr.name.span()=> {
-                            static DEFAULT_VALUE: ::std::sync::OnceLock<String> = ::std::sync::OnceLock::new();
-                            let s = DEFAULT_VALUE.get_or_init(|| {
+                        let default_string = if is_simple_ty(ty, "f32") || is_simple_ty(ty, "f64") {
+                            quote_spanned!(attr.name.span()=> {
+                                let val: #ty = #val;
+                                let s = val.to_string();
+                                if s.contains('.') || s.contains('e') || s.contains('E') {
+                                    s
+                                } else {
+                                    format!("{s}.0")
+                                }
+                            })
+                        } else {
+                            quote_spanned!(attr.name.span()=> {
                                 let val: #ty = #val;
                                 ::std::string::ToString::to_string(&val)
-                            });
+                            })
+                        };
+                        quote_spanned!(attr.name.span()=> {
+                            static DEFAULT_VALUE: ::std::sync::OnceLock<String> = ::std::sync::OnceLock::new();
+                            let s = DEFAULT_VALUE.get_or_init(|| #default_string);
                             let s: &'static str = &*s;
                             s
                         })

tests/derive/default_value.rs

@@ -57,6 +57,32 @@ Options:
 "#]].raw());
 }
 
+#[test]
+fn default_value_t_float() {
+    #[derive(Parser, PartialEq, Debug)]
+    struct Opt {
+        #[arg(short = 'a', default_value_t = 1.0)]
+        amplify: f32,
+    }
+    assert_eq!(
+        Opt { amplify: 1.0 },
+        Opt::try_parse_from(["test"]).unwrap()
+    );
+
+    let help = utils::get_long_help::<Opt>();
+    assert_data_eq!(help, str![[r#"
+Usage: clap [OPTIONS]
+
+Options:
+  -a <AMPLIFY>
+          [default: 1.0]
+
+  -h, --help
+          Print help
+
+"#]].raw());
+}
+
 #[test]
 fn auto_default_value_t() {
     #[derive(Parser, PartialEq, Debug)]