clippy_lints/src/vec_init_then_push.rs

   1 use crate::utils::{match_def_path, path_to_local, path_to_local_id, paths, snippet, span_lint_and_sugg};
   2 use clippy_utils::ty::is_type_diagnostic_item;
   3 use if_chain::if_chain;
   4 use rustc_ast::ast::LitKind;
   5 use rustc_errors::Applicability;
   6 use rustc_hir::{BindingAnnotation, Block, Expr, ExprKind, HirId, Local, PatKind, QPath, Stmt, StmtKind};
   7 use rustc_lint::{LateContext, LateLintPass, LintContext};
   8 use rustc_middle::lint::in_external_macro;
   9 use rustc_session::{declare_tool_lint, impl_lint_pass};
  10 use rustc_span::{symbol::sym, Span};
  11 use std::convert::TryInto;
  12
  13 declare_clippy_lint! {
  14     /// **What it does:** Checks for calls to `push` immediately after creating a new `Vec`.
  15     ///
  16     /// **Why is this bad?** The `vec![]` macro is both more performant and easier to read than
  17     /// multiple `push` calls.
  18     ///
  19     /// **Known problems:** None.
  20     ///
  21     /// **Example:**
  22     ///
  23     /// ```rust
  24     /// let mut v = Vec::new();
  25     /// v.push(0);
  26     /// ```
  27     /// Use instead:
  28     /// ```rust
  29     /// let v = vec![0];
  30     /// ```
  31     pub VEC_INIT_THEN_PUSH,
  32     perf,
  33     "`push` immediately after `Vec` creation"
  34 }
  35
  36 impl_lint_pass!(VecInitThenPush => [VEC_INIT_THEN_PUSH]);
  37
  38 #[derive(Default)]
  39 pub struct VecInitThenPush {
  40     searcher: Option<VecPushSearcher>,
  41 }
  42
  43 #[derive(Clone, Copy)]
  44 enum VecInitKind {
  45     New,
  46     WithCapacity(u64),
  47 }
  48 struct VecPushSearcher {
  49     local_id: HirId,
  50     init: VecInitKind,
  51     lhs_is_local: bool,
  52     lhs_span: Span,
  53     err_span: Span,
  54     found: u64,
  55 }
  56 impl VecPushSearcher {
  57     fn display_err(&self, cx: &LateContext<'_>) {
  58         match self.init {
  59             _ if self.found == 0 => return,
  60             VecInitKind::WithCapacity(x) if x > self.found => return,
  61             _ => (),
  62         };
  63
  64         let mut s = if self.lhs_is_local {
  65             String::from("let ")
  66         } else {
  67             String::new()
  68         };
  69         s.push_str(&snippet(cx, self.lhs_span, ".."));
  70         s.push_str(" = vec![..];");
  71
  72         span_lint_and_sugg(
  73             cx,
  74             VEC_INIT_THEN_PUSH,
  75             self.err_span,
  76             "calls to `push` immediately after creation",
  77             "consider using the `vec![]` macro",
  78             s,
  79             Applicability::HasPlaceholders,
  80         );
  81     }
  82 }
  83
  84 impl LateLintPass<'_> for VecInitThenPush {
  85     fn check_block(&mut self, _: &LateContext<'tcx>, _: &'tcx Block<'tcx>) {
  86         self.searcher = None;
  87     }
  88
  89     fn check_local(&mut self, cx: &LateContext<'tcx>, local: &'tcx Local<'tcx>) {
  90         if_chain! {
  91             if !in_external_macro(cx.sess(), local.span);
  92             if let Some(init) = local.init;
  93             if let PatKind::Binding(BindingAnnotation::Mutable, id, _, None) = local.pat.kind;
  94             if let Some(init_kind) = get_vec_init_kind(cx, init);
  95             then {
  96                 self.searcher = Some(VecPushSearcher {
  97                         local_id: id,
  98                         init: init_kind,
  99                         lhs_is_local: true,
 100                         lhs_span: local.ty.map_or(local.pat.span, |t| local.pat.span.to(t.span)),
 101                         err_span: local.span,
 102                         found: 0,
 103                     });
 104             }
 105         }
 106     }
 107
 108     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 109         if self.searcher.is_none() {
 110             if_chain! {
 111                 if !in_external_macro(cx.sess(), expr.span);
 112                 if let ExprKind::Assign(left, right, _) = expr.kind;
 113                 if let Some(id) = path_to_local(left);
 114                 if let Some(init_kind) = get_vec_init_kind(cx, right);
 115                 then {
 116                     self.searcher = Some(VecPushSearcher {
 117                         local_id: id,
 118                         init: init_kind,
 119                         lhs_is_local: false,
 120                         lhs_span: left.span,
 121                         err_span: expr.span,
 122                         found: 0,
 123                     });
 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_type) =>
 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_type) =>
 181             {
 182                 return Some(VecInitKind::New);
 183             }
 184             _ => (),
 185         }
 186     }
 187     None
 188 }