Do not allocate attributes entry if there are no attributes

This saves 8mb.

crates/hir-def/src/item_tree/lower.rs

@@ -198,6 +198,7 @@ impl<'a> Ctx<'a> {
     }
 
     fn add_attrs(&mut self, item: AttrOwner, attrs: RawAttrs) {
+        if !attrs.is_empty() {
             match self.tree.attrs.entry(item) {
                 Entry::Occupied(mut entry) => {
                     *entry.get_mut() = entry.get().merge(attrs);
@@ -207,6 +208,7 @@ impl<'a> Ctx<'a> {
                 }
             }
         }
+    }
 
     fn lower_assoc_item(&mut self, item_node: &ast::AssocItem) -> Option<AssocItem> {
         let item: AssocItem = match item_node {

crates/hir-expand/src/attrs.rs

@@ -26,6 +26,7 @@ use crate::{
 /// Syntactical attributes, without filtering of `cfg_attr`s.
 #[derive(Default, Debug, Clone, PartialEq, Eq)]
 pub struct RawAttrs {
+    // FIXME: This can become `Box<[Attr]>` if https://internals.rust-lang.org/t/layout-of-dst-box/21728?u=chrefr is accepted.
     entries: Option<ThinArc<(), Attr>>,
 }
 
@@ -169,6 +170,10 @@ impl RawAttrs {
         };
         RawAttrs { entries }
     }
+
+    pub fn is_empty(&self) -> bool {
+        self.entries.is_none()
+    }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]