Eliminate some temporary vectors

compiler/rustc_ast/src/tokenstream.rs

@@ -221,7 +221,7 @@ impl TokenStream {
             }
         }
         if let Some((pos, comma, sp)) = suggestion {
-            let mut new_stream = vec![];
+            let mut new_stream = Vec::with_capacity(self.0.len() + 1);
             let parts = self.0.split_at(pos + 1);
             new_stream.extend_from_slice(parts.0);
             new_stream.push(comma);

compiler/rustc_ast_lowering/src/expr.rs

@@ -1367,14 +1367,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let mut used_input_regs = FxHashMap::default();
         let mut used_output_regs = FxHashMap::default();
+        let mut required_features: Vec<&str> = vec![];
         for (idx, op) in operands.iter().enumerate() {
             let op_sp = asm.operands[idx].1;
             if let Some(reg) = op.reg() {
+                // Make sure we don't accidentally carry features from the
+                // previous iteration.
+                required_features.clear();
+
                 // Validate register classes against currently enabled target
                 // features. We check that at least one type is available for
                 // the current target.
                 let reg_class = reg.reg_class();
-                let mut required_features: Vec<&str> = vec![];
                 for &(_, feature) in reg_class.supported_types(asm_arch) {
                     if let Some(feature) = feature {
                         if self.sess.target_features.contains(&Symbol::intern(feature)) {

compiler/rustc_ast_lowering/src/lib.rs

@@ -2011,17 +2011,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         //
         // For the "output" lifetime parameters, we just want to
         // generate `'_`.
-        let mut generic_args: Vec<_> = lifetime_params[..input_lifetimes_count]
-            .iter()
-            .map(|&(span, hir_name)| {
+        let mut generic_args = Vec::with_capacity(lifetime_params.len());
+        generic_args.extend(lifetime_params[..input_lifetimes_count].iter().map(
+            |&(span, hir_name)| {
                 // Input lifetime like `'a` or `'1`:
                 GenericArg::Lifetime(hir::Lifetime {
                     hir_id: self.next_id(),
                     span,
                     name: hir::LifetimeName::Param(hir_name),
                 })
-            })
-            .collect();
+            },
+        ));
         generic_args.extend(lifetime_params[input_lifetimes_count..].iter().map(|&(span, _)|
             // Output lifetime like `'_`.
             GenericArg::Lifetime(hir::Lifetime {
@@ -2312,29 +2312,30 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     fn lower_block_noalloc(&mut self, b: &Block, targeted_by_break: bool) -> hir::Block<'hir> {
-        let mut stmts = vec![];
         let mut expr: Option<&'hir _> = None;
 
-        for (index, stmt) in b.stmts.iter().enumerate() {
-            if index == b.stmts.len() - 1 {
-                if let StmtKind::Expr(ref e) = stmt.kind {
-                    expr = Some(self.lower_expr(e));
-                } else {
-                    stmts.extend(self.lower_stmt(stmt));
-                }
-            } else {
-                stmts.extend(self.lower_stmt(stmt));
-            }
-        }
+        let stmts = self.arena.alloc_from_iter(
+            b.stmts
+                .iter()
+                .enumerate()
+                .filter_map(|(index, stmt)| {
+                    if index == b.stmts.len() - 1 {
+                        if let StmtKind::Expr(ref e) = stmt.kind {
+                            expr = Some(self.lower_expr(e));
+                            None
+                        } else {
+                            Some(self.lower_stmt(stmt))
+                        }
+                    } else {
+                        Some(self.lower_stmt(stmt))
+                    }
+                })
+                .flatten(),
+        );
+        let rules = self.lower_block_check_mode(&b.rules);
+        let hir_id = self.lower_node_id(b.id);
 
-        hir::Block {
-            hir_id: self.lower_node_id(b.id),
-            stmts: self.arena.alloc_from_iter(stmts),
-            expr,
-            rules: self.lower_block_check_mode(&b.rules),
-            span: b.span,
-            targeted_by_break,
-        }
+        hir::Block { hir_id, stmts, expr, rules, span: b.span, targeted_by_break }
     }
 
     /// Lowers a block directly to an expression, presuming that it

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -66,7 +66,7 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
 
     let fmt = substr.nonself_args[0].clone();
 
-    let mut stmts = vec![];
+    let mut stmts = Vec::with_capacity(fields.len() + 2);
     match vdata {
         ast::VariantData::Tuple(..) | ast::VariantData::Unit(..) => {
             // tuple struct/"normal" variant

compiler/rustc_lint/src/levels.rs

@@ -30,6 +30,8 @@ fn lint_levels(tcx: TyCtxt<'_>, cnum: CrateNum) -> LintLevelMap {
     let mut builder = LintLevelMapBuilder { levels, tcx, store };
     let krate = tcx.hir().krate();
 
+    builder.levels.id_to_set.reserve(krate.exported_macros.len() + 1);
+
     let push = builder.levels.push(&krate.item.attrs, &store, true);
     builder.levels.register_id(hir::CRATE_HIR_ID);
     for macro_def in krate.exported_macros {

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -784,14 +784,15 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             }
         };
 
+        let attrs: Vec<_> = self.get_item_attrs(id, sess).collect();
         SyntaxExtension::new(
             sess,
             kind,
             self.get_span(id, sess),
             helper_attrs,
             self.root.edition,
             Symbol::intern(name),
-            &self.get_item_attrs(id, sess),
+            &attrs,
         )
     }
 
@@ -1157,7 +1158,8 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                                 // within the crate. We only need this for fictive constructors,
                                 // for other constructors correct visibilities
                                 // were already encoded in metadata.
-                                let attrs = self.get_item_attrs(def_id.index, sess);
+                                let attrs: Vec<_> =
+                                    self.get_item_attrs(def_id.index, sess).collect();
                                 if sess.contains_name(&attrs, sym::non_exhaustive) {
                                     let crate_def_id = self.local_def_id(CRATE_DEF_INDEX);
                                     vis = ty::Visibility::Restricted(crate_def_id);
@@ -1283,8 +1285,8 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         }
     }
 
-    fn get_item_variances(&self, id: DefIndex) -> Vec<ty::Variance> {
-        self.root.tables.variances.get(self, id).unwrap_or_else(Lazy::empty).decode(self).collect()
+    fn get_item_variances(&'a self, id: DefIndex) -> impl Iterator<Item = ty::Variance> + 'a {
+        self.root.tables.variances.get(self, id).unwrap_or_else(Lazy::empty).decode(self)
     }
 
     fn get_ctor_kind(&self, node_id: DefIndex) -> CtorKind {
@@ -1308,7 +1310,11 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         }
     }
 
-    fn get_item_attrs(&self, node_id: DefIndex, sess: &Session) -> Vec<ast::Attribute> {
+    fn get_item_attrs(
+        &'a self,
+        node_id: DefIndex,
+        sess: &'a Session,
+    ) -> impl Iterator<Item = ast::Attribute> + 'a {
         // The attributes for a tuple struct/variant are attached to the definition, not the ctor;
         // we assume that someone passing in a tuple struct ctor is actually wanting to
         // look at the definition
@@ -1325,7 +1331,6 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .get(self, item_id)
             .unwrap_or_else(Lazy::empty)
             .decode((self, sess))
-            .collect::<Vec<_>>()
     }
 
     fn get_struct_field_names(&self, id: DefIndex, sess: &Session) -> Vec<Spanned<Symbol>> {

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -138,7 +138,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
         cdata.get_deprecation(def_id.index).map(DeprecationEntry::external)
     }
     item_attrs => { tcx.arena.alloc_from_iter(
-        cdata.get_item_attrs(def_id.index, tcx.sess).into_iter()
+        cdata.get_item_attrs(def_id.index, tcx.sess)
     ) }
     fn_arg_names => { cdata.get_fn_param_names(tcx, def_id.index) }
     rendered_const => { cdata.get_rendered_const(def_id.index) }
@@ -415,11 +415,7 @@ impl CStore {
 
         let span = data.get_span(id.index, sess);
 
-        // Mark the attrs as used
-        let attrs = data.get_item_attrs(id.index, sess);
-        for attr in attrs.iter() {
-            sess.mark_attr_used(attr);
-        }
+        let attrs = data.get_item_attrs(id.index, sess).collect();
 
         let ident = data.item_ident(id.index, sess);
 
@@ -428,7 +424,7 @@ impl CStore {
                 ident,
                 id: ast::DUMMY_NODE_ID,
                 span,
-                attrs: attrs.to_vec(),
+                attrs,
                 kind: ast::ItemKind::MacroDef(data.get_macro(id.index, sess)),
                 vis: ast::Visibility {
                     span: span.shrink_to_lo(),

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -353,16 +353,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
 
         // Keep only those bounds which may apply, and propagate overflow if it occurs.
-        let mut param_candidates = vec![];
         for bound in matching_bounds {
             let wc = self.evaluate_where_clause(stack, bound)?;
             if wc.may_apply() {
-                param_candidates.push(ParamCandidate(bound));
+                candidates.vec.push(ParamCandidate(bound));
             }
         }
 
-        candidates.vec.extend(param_candidates);
-
         Ok(())
     }