Allow clippy::needless_lifetimes on tracked method getters

Previously, this would necessitate having to manually add an allow for this
clippy lint, since an extra `'db` lifetime was added to the signature.

components/salsa-2022-macros/src/tracked_fn.rs

@@ -234,6 +234,12 @@ fn tracked_method(
 
     let (config_ty, fn_struct) = crate::tracked_fn::fn_struct(&args, &item_fn)?;
 
+    // we generate a `'db` lifetime that clippy
+    // sometimes doesn't like
+    item_method
+        .attrs
+        .push(syn::parse_quote! {#[allow(clippy::needless_lifetimes)]});
+
     item_method.block = getter_fn(
         &args,
         &mut item_method.sig,

salsa-2022-tests/tests/warnings/main.rs

@@ -3,4 +3,5 @@
 #![deny(warnings)]
 
 mod needless_borrow;
+mod needless_lifetimes;
 mod unused_variable_db;

salsa-2022-tests/tests/warnings/needless_lifetimes.rs

@@ -0,0 +1,27 @@
+pub trait Db: salsa::DbWithJar<Jar> {}
+
+#[salsa::jar(db = Db)]
+pub struct Jar(SourceTree, SourceTree_all_items, use_tree);
+
+#[derive(Debug, PartialEq, Eq, Hash)]
+pub struct Item {}
+
+#[salsa::tracked(jar = Jar)]
+pub struct SourceTree {}
+
+#[salsa::tracked(jar = Jar)]
+impl SourceTree {
+    #[salsa::tracked(return_ref)]
+    pub fn all_items(self, _db: &dyn Db) -> Vec<Item> {
+        todo!()
+    }
+}
+
+#[salsa::tracked(jar = Jar, return_ref)]
+fn use_tree(_db: &dyn Db, _tree: SourceTree) {}
+
+#[allow(unused)]
+fn use_it(db: &dyn Db, tree: SourceTree) {
+    tree.all_items(db);
+    use_tree(db, tree);
+}