more helpful error messages for repeat or flatten in wrong position

Author: cbeck88

conf_derive/src/proc_macro_options/field_item/parameter_item.rs

@@ -302,6 +302,10 @@ impl ParameterItem {
                         set_once(&path, &mut result.test, Some(ParameterTestItem::new(meta)?))
                     } else if path.is_ident("pos") {
                         set_once(&path, &mut result.is_positional, Some(path.span()))
+                    } else if path.is_ident("repeat") {
+                        Err(meta.error("`repeat` must be the first conf attribute on this field"))
+                    } else if path.is_ident("flatten") {
+                        Err(meta.error("`flatten` must be the first conf attribute on this field"))
                     } else {
                         Err(meta.error("unrecognized conf parameter option"))
                     }

tests/ui/flatten_not_first.rs

@@ -0,0 +1,15 @@
+use conf::Conf;
+
+#[derive(Conf, Debug)]
+pub struct Inner {
+    #[arg(long)]
+    pub value: String,
+}
+
+#[derive(Conf, Debug)]
+pub struct BadConfig {
+    #[conf(long, flatten)]
+    pub inner: Inner,
+}
+
+fn main() {}

tests/ui/flatten_not_first.stderr

@@ -0,0 +1,5 @@
+error: `flatten` must be the first conf attribute on this field
+  --> tests/ui/flatten_not_first.rs:11:18
+   |
+11 |     #[conf(long, flatten)]
+   |                  ^^^^^^^

tests/ui/repeat_not_first.rs

@@ -0,0 +1,9 @@
+use conf::Conf;
+
+#[derive(Conf, Debug)]
+pub struct BadConfig {
+    #[conf(long, repeat)]
+    pub values: Vec<String>,
+}
+
+fn main() {}

tests/ui/repeat_not_first.stderr

@@ -0,0 +1,5 @@
+error: `repeat` must be the first conf attribute on this field
+ --> tests/ui/repeat_not_first.rs:5:18
+  |
+5 |     #[conf(long, repeat)]
+  |                  ^^^^^^