Auto merge of #53824 - ljedrz:begone_onevector, r=michaelwoerister

Remove OneVector, increase related SmallVec capacities

Removes the `OneVector` type alias (equivalent to `SmallVec<[T; 1]>`); it is used in scenarios where the capacity of 1 is often exceeded, which might be nullifying the performance wins (due to spilling to the heap) expected when using `SmallVec` instead of `Vec`.

The numbers I used in this PR are very rough estimates - it would probably be a good idea to adjust some/all of them, which is what this proposal is all about.

It might be a good idea to additionally create some local type aliases for the `SmallVec`s in the `Folder` trait, as they are repeated in quite a few spots; I'd be happy to apply this sort of adjustments.

1 files changed, 7 insertions, 6 deletions

diff --git a/src/libsyntax/config.rs b/src/libsyntax/config.rs
index 5d978b6b9e6..3d9f4a92f81 100644
--- a/src/libsyntax/config.rs
+++ b/src/libsyntax/config.rs
@@ -15,7 +15,7 @@ use ast;
 use source_map::Spanned;
 use edition::Edition;
 use parse::{token, ParseSess};
-use OneVector;
+use smallvec::SmallVec;
 use errors::Applicability;
 
 use ptr::P;
@@ -338,22 +338,23 @@ impl<'a> fold::Folder for StripUnconfigured<'a> {
         Some(P(fold::noop_fold_expr(expr, self)))
     }
 
-    fn fold_stmt(&mut self, stmt: ast::Stmt) -> OneVector<ast::Stmt> {
+    fn fold_stmt(&mut self, stmt: ast::Stmt) -> SmallVec<[ast::Stmt; 1]> {
         match self.configure_stmt(stmt) {
             Some(stmt) => fold::noop_fold_stmt(stmt, self),
-            None => return OneVector::new(),
+            None => return SmallVec::new(),
         }
     }
 
-    fn fold_item(&mut self, item: P<ast::Item>) -> OneVector<P<ast::Item>> {
+    fn fold_item(&mut self, item: P<ast::Item>) -> SmallVec<[P<ast::Item>; 1]> {
         fold::noop_fold_item(configure!(self, item), self)
     }
 
-    fn fold_impl_item(&mut self, item: ast::ImplItem) -> OneVector<ast::ImplItem> {
+    fn fold_impl_item(&mut self, item: ast::ImplItem) -> SmallVec<[ast::ImplItem; 1]>
+    {
         fold::noop_fold_impl_item(configure!(self, item), self)
     }
 
-    fn fold_trait_item(&mut self, item: ast::TraitItem) -> OneVector<ast::TraitItem> {
+    fn fold_trait_item(&mut self, item: ast::TraitItem) -> SmallVec<[ast::TraitItem; 1]> {
         fold::noop_fold_trait_item(configure!(self, item), self)
     }