rustdoc-search: prevent struct fields from blocking functions

This perf hack tweaks the inverted indexes so that fields appear
further down. This doesn't affect the results, because fields are
pushed to the bottom anyway, but it prevents us from having to
load the metadata (in order to actually figure out that it *is*
a field).

Author: notriddle

src/librustdoc/formats/item_type.rs

@@ -243,6 +243,10 @@ impl ItemType {
     pub(crate) fn is_adt(&self) -> bool {
         matches!(self, ItemType::Struct | ItemType::Union | ItemType::Enum)
     }
+    /// Keep this the same as isFnLikeTy in search.js
+    pub(crate) fn is_fn_like(&self) -> bool {
+        matches!(self, ItemType::Function | ItemType::Method | ItemType::TyMethod)
+    }
 }
 
 impl fmt::Display for ItemType {

src/librustdoc/html/render/search_index.rs

@@ -1732,7 +1732,15 @@ pub(crate) fn build_index(
                     );
                 }
             }
-            let search_type_size = search_type.size();
+            let search_type_size = search_type.size() +
+                // Artificially give struct fields a size of 8 instead of their real
+                // size of 2. This is because search.js sorts them to the end, so
+                // by pushing them down, we prevent them from blocking real 2-arity functions.
+                //
+                // The number 8 is arbitrary. We want it big, but not enormous,
+                // because the postings list has to fill in an empty array for each
+                // unoccupied size.
+                if item.ty.is_fn_like() { 0 } else { 16 };
             serialized_index.function_data[new_entry_id] = Some(FunctionData {
                 function_signature: {
                     let mut function_signature = String::new();

src/librustdoc/html/static/js/search.js

@@ -292,6 +292,8 @@ function isEndCharacter(c) {
 }
 
 /**
+ * Same thing as ItemType::is_fn_like in item_type.rs
+ *
  * @param {rustdoc.ItemType} ty
  * @returns
  */