api-macro: util: add join and join_debug helper

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

proxmox-api-macro/src/api/structs.rs

@@ -188,14 +188,7 @@ fn handle_regular_struct(
 
     // now error out about all the fields not found in the struct:
     if !schema_fields.is_empty() {
-        let bad_fields = schema_fields.keys().fold(String::new(), |mut acc, key| {
+        let bad_fields = util::join(", ", schema_fields.keys());
-            if !acc.is_empty() {
-                acc.push_str(", ");
-                acc
-            } else {
-                key.to_owned()
-            }
-        });
         bail!(
             schema.span,
             "struct does not contain the following fields: {}",

proxmox-api-macro/src/util.rs

@@ -542,3 +542,31 @@ impl Parse for AttrArgs {
         })
     }
 }
+
+/// Join an iterator over `Display` values.
+pub fn join<T>(separator: &str, iter: impl Iterator<Item = T>) -> String
+where
+    T: std::fmt::Display,
+{
+    let mut text = String::new();
+    let mut sep = "";
+    for i in iter {
+        text = format!("{}{}{}", text, sep, i);
+        sep = separator;
+    }
+    text
+}
+
+/// Join an iterator over `Debug` values.
+pub fn join_debug<T>(separator: &str, iter: impl Iterator<Item = T>) -> String
+where
+    T: std::fmt::Debug,
+{
+    let mut text = String::new();
+    let mut sep = "";
+    for i in iter {
+        text = format!("{}{}{:?}", text, sep, i);
+        sep = separator;
+    }
+    text
+}