Use Vec::with_capacity() as possible

Author: JohnTitor

clippy_lints/src/booleans.rs

@@ -359,7 +359,7 @@ impl<'a, 'tcx> NonminimalBoolVisitor<'a, 'tcx> {
                 }
                 simplified.push(simple_negated);
             }
-            let mut improvements = Vec::new();
+            let mut improvements = Vec::with_capacity(simplified.len());
             'simplified: for suggestion in &simplified {
                 let simplified_stats = terminal_stats(suggestion);
                 let mut improvement = false;

clippy_lints/src/matches.rs

@@ -724,7 +724,7 @@ fn is_panic_block(block: &Block<'_>) -> bool {
 
 fn check_match_ref_pats(cx: &LateContext<'_, '_>, ex: &Expr<'_>, arms: &[Arm<'_>], expr: &Expr<'_>) {
     if has_only_ref_pats(arms) {
-        let mut suggs = Vec::new();
+        let mut suggs = Vec::with_capacity(arms.len() + 1);
         let (title, msg) = if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, ref inner) = ex.kind {
             let span = ex.span.source_callsite();
             suggs.push((span, Sugg::hir_with_macro_callsite(cx, inner, "..").to_string()));

clippy_lints/src/shadow.rs

@@ -101,7 +101,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Shadow {
 }
 
 fn check_fn<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, decl: &'tcx FnDecl<'_>, body: &'tcx Body<'_>) {
-    let mut bindings = Vec::new();
+    let mut bindings = Vec::with_capacity(decl.inputs.len());
     for arg in iter_input_pats(decl, body) {
         if let PatKind::Binding(.., ident, _) = arg.pat.kind {
             bindings.push((ident.name, ident.span))