Do not allocate attributes entry if there are no attributes

This saves 8mb.

src/tools/rust-analyzer/crates/hir-def/src/item_tree/lower.rs

@@ -198,12 +198,14 @@ impl<'a> Ctx<'a> {
     }
 
     fn add_attrs(&mut self, item: AttrOwner, attrs: RawAttrs) {
-        match self.tree.attrs.entry(item) {
-            Entry::Occupied(mut entry) => {
-                *entry.get_mut() = entry.get().merge(attrs);
-            }
-            Entry::Vacant(entry) => {
-                entry.insert(attrs);
+        if !attrs.is_empty() {
+            match self.tree.attrs.entry(item) {
+                Entry::Occupied(mut entry) => {
+                    *entry.get_mut() = entry.get().merge(attrs);
+                }
+                Entry::Vacant(entry) => {
+                    entry.insert(attrs);
+                }
             }
         }
     }

src/tools/rust-analyzer/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)]