rustdoc: a few micro-optimizations targeted at build_impl

compiler/rustc_resolve/src/rustdoc.rs

@@ -207,8 +207,10 @@ pub fn attrs_to_doc_fragments<'a, A: AttributeExt + Clone + 'a>(
     attrs: impl Iterator<Item = (&'a A, Option<DefId>)>,
     doc_only: bool,
 ) -> (Vec<DocFragment>, ThinVec<A>) {
-    let mut doc_fragments = Vec::new();
-    let mut other_attrs = ThinVec::<A>::new();
+    let (min_size, max_size) = attrs.size_hint();
+    let size_hint = max_size.unwrap_or(min_size);
+    let mut doc_fragments = Vec::with_capacity(size_hint);
+    let mut other_attrs = ThinVec::<A>::with_capacity(if doc_only { 0 } else { size_hint });
     for (attr, item_id) in attrs {
         if let Some((doc_str, comment_kind)) = attr.doc_str_and_comment_kind() {
             let doc = beautify_doc_string(doc_str, comment_kind);
@@ -230,6 +232,9 @@ pub fn attrs_to_doc_fragments<'a, A: AttributeExt + Clone + 'a>(
         }
     }
 
+    doc_fragments.shrink_to_fit();
+    other_attrs.shrink_to_fit();
+
     unindent_doc_fragments(&mut doc_fragments);
 
     (doc_fragments, other_attrs)

src/librustdoc/clean/inline.rs

@@ -572,30 +572,30 @@ pub(crate) fn build_impl(
         super::build_deref_target_impls(cx, &trait_items, ret);
     }
 
-    // Return if the trait itself or any types of the generic parameters are doc(hidden).
-    let mut stack: Vec<&Type> = vec![&for_];
+    if !document_hidden {
+        // Return if the trait itself or any types of the generic parameters are doc(hidden).
+        let mut stack: Vec<&Type> = vec![&for_];
 
-    if let Some(did) = trait_.as_ref().map(|t| t.def_id())
-        && !document_hidden
-        && tcx.is_doc_hidden(did)
-    {
-        return;
-    }
-
-    if let Some(generics) = trait_.as_ref().and_then(|t| t.generics()) {
-        stack.extend(generics);
-    }
-
-    while let Some(ty) = stack.pop() {
-        if let Some(did) = ty.def_id(&cx.cache)
-            && !document_hidden
+        if let Some(did) = trait_.as_ref().map(|t| t.def_id())
             && tcx.is_doc_hidden(did)
         {
             return;
         }
-        if let Some(generics) = ty.generics() {
+
+        if let Some(generics) = trait_.as_ref().and_then(|t| t.generics()) {
             stack.extend(generics);
         }
+
+        while let Some(ty) = stack.pop() {
+            if let Some(did) = ty.def_id(&cx.cache)
+                && tcx.is_doc_hidden(did)
+            {
+                return;
+            }
+            if let Some(generics) = ty.generics() {
+                stack.extend(generics);
+            }
+        }
     }
 
     if let Some(did) = trait_.as_ref().map(|t| t.def_id()) {