Allow name querying for derive helpers

crates/hir/src/lib.rs

@@ -2255,12 +2255,27 @@ impl Local {
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub struct DeriveHelper {
     pub(crate) derive: MacroId,
+    pub(crate) idx: usize,
 }
 
 impl DeriveHelper {
     pub fn derive(&self) -> Macro {
         Macro { id: self.derive.into() }
     }
+
+    pub fn name(&self, db: &dyn HirDatabase) -> Name {
+        match self.derive {
+            MacroId::Macro2Id(_) => None,
+            MacroId::MacroRulesId(_) => None,
+            MacroId::ProcMacroId(proc_macro) => db
+                .proc_macro_data(proc_macro)
+                .helpers
+                .as_ref()
+                .and_then(|it| it.get(self.idx))
+                .cloned(),
+        }
+        .unwrap_or_else(|| Name::missing())
+    }
 }
 
 // FIXME: Wrong name? This is could also be a registered attribute