resolve: improve/remove allocations

src/librustc_resolve/lib.rs

@@ -3021,10 +3021,7 @@ fn smart_resolve_path_fragment(&mut self,
             let mut err = this.session.struct_span_err_with_code(base_span, &base_msg, code);
 
             // Emit help message for fake-self from other languages like `this`(javascript)
-            let fake_self: Vec<Ident> = ["this", "my"].iter().map(
-                |s| Ident::from_str(*s)
-            ).collect();
-            if fake_self.contains(&item_str)
+            if ["this", "my"].contains(&&*item_str.as_str())
                 && this.self_value_is_available(path[0].span, span) {
                 err.span_suggestion_with_applicability(
                     span,
@@ -4377,10 +4374,9 @@ fn lookup_import_candidates_from_module<FilterFn>(&mut self,
         where FilterFn: Fn(Def) -> bool
     {
         let mut candidates = Vec::new();
-        let mut worklist = Vec::new();
         let mut seen_modules = FxHashSet();
         let not_local_module = crate_name != keywords::Crate.ident();
-        worklist.push((start_module, Vec::<ast::PathSegment>::new(), not_local_module));
+        let mut worklist = vec![(start_module, Vec::<ast::PathSegment>::new(), not_local_module)];
 
         while let Some((in_module,
                         path_segments,
@@ -4467,13 +4463,8 @@ fn lookup_import_candidates<FilterFn>(&mut self,
                                           -> Vec<ImportSuggestion>
         where FilterFn: Fn(Def) -> bool
     {
-        let mut suggestions = vec![];
-
-        suggestions.extend(
-            self.lookup_import_candidates_from_module(
-                lookup_name, namespace, self.graph_root, keywords::Crate.ident(), &filter_fn
-            )
-        );
+        let mut suggestions = self.lookup_import_candidates_from_module(
+            lookup_name, namespace, self.graph_root, keywords::Crate.ident(), &filter_fn);
 
         if self.session.rust_2018() {
             let extern_prelude_names = self.extern_prelude.clone();
@@ -4502,9 +4493,8 @@ fn find_module(&mut self,
                    -> Option<(Module<'a>, ImportSuggestion)>
     {
         let mut result = None;
-        let mut worklist = Vec::new();
         let mut seen_modules = FxHashSet();
-        worklist.push((self.graph_root, Vec::new()));
+        let mut worklist = vec![(self.graph_root, Vec::new())];
 
         while let Some((in_module, path_segments)) = worklist.pop() {
             // abort if the module is already found

src/librustc_resolve/macros.rs

@@ -203,9 +203,7 @@ fn visit_ast_fragment_with_placeholders(&mut self, mark: Mark, fragment: &AstFra
         self.current_module = invocation.module.get();
         self.current_module.unresolved_invocations.borrow_mut().remove(&mark);
         self.current_module.unresolved_invocations.borrow_mut().extend(derives);
-        for &derive in derives {
-            self.invocations.insert(derive, invocation);
-        }
+        self.invocations.extend(derives.iter().map(|&derive| (derive, invocation)));
         let mut visitor = BuildReducedGraphVisitor {
             resolver: self,
             current_legacy_scope: invocation.parent_legacy_scope.get(),
@@ -277,11 +275,12 @@ fn find_legacy_attr_invoc(&mut self, attrs: &mut Vec<ast::Attribute>, allow_deri
                     if traits.is_empty() {
                         attrs.remove(i);
                     } else {
-                        let mut tokens = Vec::new();
+                        let mut tokens = Vec::with_capacity(traits.len() - 1);
                         for (j, path) in traits.iter().enumerate() {
                             if j > 0 {
                                 tokens.push(TokenTree::Token(attrs[i].span, Token::Comma).into());
                             }
+                            tokens.reserve((path.segments.len() * 2).saturating_sub(1));
                             for (k, segment) in path.segments.iter().enumerate() {
                                 if k > 0 {
                                     tokens.push(TokenTree::Token(path.span, Token::ModSep).into());