src/tools/clippy/clippy_lints/src/vec_init_then_push.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::source::snippet;
   3 use clippy_utils::ty::is_type_diagnostic_item;
   4 use clippy_utils::{match_def_path, path_to_local, path_to_local_id, paths};
   5 use if_chain::if_chain;
   6 use rustc_ast::ast::LitKind;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::{BindingAnnotation, Block, Expr, ExprKind, HirId, Local, PatKind, QPath, Stmt, StmtKind};
   9 use rustc_lint::{LateContext, LateLintPass, LintContext};
  10 use rustc_middle::lint::in_external_macro;
  11 use rustc_session::{declare_tool_lint, impl_lint_pass};
  12 use rustc_span::{symbol::sym, Span};
  13 use std::convert::TryInto;
  14
  15 declare_clippy_lint! {
  16     /// ### What it does
  17     /// Checks for calls to `push` immediately after creating a new `Vec`.
  18     ///
  19     /// ### Why is this bad?
  20     /// The `vec![]` macro is both more performant and easier to read than
  21     /// multiple `push` calls.
  22     ///
  23     /// ### Example
  24     /// ```rust
  25     /// let mut v = Vec::new();
  26     /// v.push(0);
  27     /// ```
  28     /// Use instead:
  29     /// ```rust
  30     /// let v = vec![0];
  31     /// ```
  32     pub VEC_INIT_THEN_PUSH,
  33     perf,
  34     "`push` immediately after `Vec` creation"
  35 }
  36
  37 impl_lint_pass!(VecInitThenPush => [VEC_INIT_THEN_PUSH]);
  38
  39 #[derive(Default)]
  40 pub struct VecInitThenPush {
  41     searcher: Option<VecPushSearcher>,
  42 }
  43
  44 #[derive(Clone, Copy)]
  45 enum VecInitKind {
  46     New,
  47     WithCapacity(u64),
  48 }
  49 struct VecPushSearcher {
  50     local_id: HirId,
  51     init: VecInitKind,
  52     lhs_is_local: bool,
  53     lhs_span: Span,
  54     err_span: Span,
  55     found: u64,
  56 }
  57 impl VecPushSearcher {
  58     fn display_err(&self, cx: &LateContext<'_>) {
  59         match self.init {
  60             _ if self.found == 0 => return,
  61             VecInitKind::WithCapacity(x) if x > self.found => return,
  62             _ => (),
  63         };
  64
  65         let mut s = if self.lhs_is_local {
  66             String::from("let ")
  67         } else {
  68             String::new()
  69         };
  70         s.push_str(&snippet(cx, self.lhs_span, ".."));
  71         s.push_str(" = vec![..];");
  72
  73         span_lint_and_sugg(
  74             cx,
  75             VEC_INIT_THEN_PUSH,
  76             self.err_span,
  77             "calls to `push` immediately after creation",
  78             "consider using the `vec![]` macro",
  79             s,
  80             Applicability::HasPlaceholders,
  81         );
  82     }
  83 }
  84
  85 impl LateLintPass<'_> for VecInitThenPush {
  86     fn check_block(&mut self, _: &LateContext<'tcx>, _: &'tcx Block<'tcx>) {
  87         self.searcher = None;
  88     }
  89
  90     fn check_local(&mut self, cx: &LateContext<'tcx>, local: &'tcx Local<'tcx>) {
  91         if_chain! {
  92             if !in_external_macro(cx.sess(), local.span);
  93             if let Some(init) = local.init;
  94             if let PatKind::Binding(BindingAnnotation::Mutable, id, _, None) = local.pat.kind;
  95             if let Some(init_kind) = get_vec_init_kind(cx, init);
  96             then {
  97                 self.searcher = Some(VecPushSearcher {
  98                         local_id: id,
  99                         init: init_kind,
 100                         lhs_is_local: true,
 101                         lhs_span: local.ty.map_or(local.pat.span, |t| local.pat.span.to(t.span)),
 102                         err_span: local.span,
 103                         found: 0,
 104                     });
 105             }
 106         }
 107     }
 108
 109     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 110         if_chain! {
 111             if self.searcher.is_none();
 112             if !in_external_macro(cx.sess(), expr.span);
 113             if let ExprKind::Assign(left, right, _) = expr.kind;
 114             if let Some(id) = path_to_local(left);
 115             if let Some(init_kind) = get_vec_init_kind(cx, right);
 116             then {
 117                 self.searcher = Some(VecPushSearcher {
 118                     local_id: id,
 119                     init: init_kind,
 120                     lhs_is_local: false,
 121                     lhs_span: left.span,
 122                     err_span: expr.span,
 123                     found: 0,
 124                 });
 125             }
 126         }
 127     }
 128
 129     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
 130         if let Some(searcher) = self.searcher.take() {
 131             if_chain! {
 132                 if let StmtKind::Expr(expr) | StmtKind::Semi(expr) = stmt.kind;
 133                 if let ExprKind::MethodCall(path, _, [self_arg, _], _) = expr.kind;
 134                 if path_to_local_id(self_arg, searcher.local_id);
 135                 if path.ident.name.as_str() == "push";
 136                 then {
 137                     self.searcher = Some(VecPushSearcher {
 138                         found: searcher.found + 1,
 139                         err_span: searcher.err_span.to(stmt.span),
 140                         .. searcher
 141                     });
 142                 } else {
 143                     searcher.display_err(cx);
 144                 }
 145             }
 146         }
 147     }
 148
 149     fn check_block_post(&mut self, cx: &LateContext<'tcx>, _: &'tcx Block<'tcx>) {
 150         if let Some(searcher) = self.searcher.take() {
 151             searcher.display_err(cx);
 152         }
 153     }
 154 }
 155
 156 fn get_vec_init_kind<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> Option<VecInitKind> {
 157     if let ExprKind::Call(func, args) = expr.kind {
 158         match func.kind {
 159             ExprKind::Path(QPath::TypeRelative(ty, name))
 160                 if is_type_diagnostic_item(cx, cx.typeck_results().node_type(ty.hir_id), sym::Vec) =>
 161             {
 162                 if name.ident.name == sym::new {
 163                     return Some(VecInitKind::New);
 164                 } else if name.ident.name.as_str() == "with_capacity" {
 165                     return args.get(0).and_then(|arg| {
 166                         if_chain! {
 167                             if let ExprKind::Lit(lit) = &arg.kind;
 168                             if let LitKind::Int(num, _) = lit.node;
 169                             then {
 170                                 Some(VecInitKind::WithCapacity(num.try_into().ok()?))
 171                             } else {
 172                                 None
 173                             }
 174                         }
 175                     });
 176                 }
 177             }
 178             ExprKind::Path(QPath::Resolved(_, path))
 179                 if match_def_path(cx, path.res.opt_def_id()?, &paths::DEFAULT_TRAIT_METHOD)
 180                     && is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(expr), sym::Vec) =>
 181             {
 182                 return Some(VecInitKind::New);
 183             }
 184             _ => (),
 185         }
 186     }
 187     None
 188 }