Reserve space in advance

Author: bugadani

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/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 {

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 {