Panic if more than one attribute found

In stratisd, we always expect exactly one. If that changes, we'll
probably have to enrich this method somehow.

Signed-off-by: mulhern <[email protected]>

stratisd_proc_macros/src/lib.rs

@@ -228,16 +228,23 @@ fn get_attr_level(attrs: &mut Vec<Attribute>) -> Option<Ident> {
 
 /// Determine whether a method has the given attribute.
 fn has_attribute(attrs: &mut Vec<Attribute>, attribute: &str) -> bool {
-    if let Some((index, _)) = attrs
+    let matching_attrs = attrs
         .iter()
         .enumerate()
         .filter(|(_, a)| a.path().is_ident(attribute))
-        .last()
-    {
-        attrs.remove(index);
-        true
-    } else {
-        false
+        .collect::<Vec<_>>();
+
+    match matching_attrs.len() {
+        0 => false,
+        1 => {
+            let (index, _) = matching_attrs[0];
+            attrs.remove(index);
+            true
+        }
+        _ => panic!(
+            "More than 1 {} attribute specified for the same syntactic entity.",
+            attribute
+        ),
     }
 }