src/tools/clippy/clippy_lints/src/slow_vector_initialization.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::sugg::Sugg;
   3 use clippy_utils::ty::is_type_diagnostic_item;
   4 use clippy_utils::{get_enclosing_block, is_expr_path_def_path, path_to_local, path_to_local_id, paths, SpanlessEq};
   5 use if_chain::if_chain;
   6 use rustc_ast::ast::LitKind;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::intravisit::{walk_block, walk_expr, walk_stmt, NestedVisitorMap, Visitor};
   9 use rustc_hir::{BindingAnnotation, Block, Expr, ExprKind, HirId, PatKind, QPath, Stmt, StmtKind};
  10 use rustc_lint::{LateContext, LateLintPass};
  11 use rustc_middle::hir::map::Map;
  12 use rustc_session::{declare_lint_pass, declare_tool_lint};
  13 use rustc_span::symbol::sym;
  14
  15 declare_clippy_lint! {
  16     /// ### What it does
  17     /// Checks slow zero-filled vector initialization
  18     ///
  19     /// ### Why is this bad?
  20     /// These structures are non-idiomatic and less efficient than simply using
  21     /// `vec![0; len]`.
  22     ///
  23     /// ### Example
  24     /// ```rust
  25     /// # use core::iter::repeat;
  26     /// # let len = 4;
  27     ///
  28     /// // Bad
  29     /// let mut vec1 = Vec::with_capacity(len);
  30     /// vec1.resize(len, 0);
  31     ///
  32     /// let mut vec2 = Vec::with_capacity(len);
  33     /// vec2.extend(repeat(0).take(len));
  34     ///
  35     /// // Good
  36     /// let mut vec1 = vec![0; len];
  37     /// let mut vec2 = vec![0; len];
  38     /// ```
  39     #[clippy::version = "1.32.0"]
  40     pub SLOW_VECTOR_INITIALIZATION,
  41     perf,
  42     "slow vector initialization"
  43 }
  44
  45 declare_lint_pass!(SlowVectorInit => [SLOW_VECTOR_INITIALIZATION]);
  46
  47 /// `VecAllocation` contains data regarding a vector allocated with `with_capacity` and then
  48 /// assigned to a variable. For example, `let mut vec = Vec::with_capacity(0)` or
  49 /// `vec = Vec::with_capacity(0)`
  50 struct VecAllocation<'tcx> {
  51     /// HirId of the variable
  52     local_id: HirId,
  53
  54     /// Reference to the expression which allocates the vector
  55     allocation_expr: &'tcx Expr<'tcx>,
  56
  57     /// Reference to the expression used as argument on `with_capacity` call. This is used
  58     /// to only match slow zero-filling idioms of the same length than vector initialization.
  59     len_expr: &'tcx Expr<'tcx>,
  60 }
  61
  62 /// Type of slow initialization
  63 enum InitializationType<'tcx> {
  64     /// Extend is a slow initialization with the form `vec.extend(repeat(0).take(..))`
  65     Extend(&'tcx Expr<'tcx>),
  66
  67     /// Resize is a slow initialization with the form `vec.resize(.., 0)`
  68     Resize(&'tcx Expr<'tcx>),
  69 }
  70
  71 impl<'tcx> LateLintPass<'tcx> for SlowVectorInit {
  72     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  73         // Matches initialization on reassignements. For example: `vec = Vec::with_capacity(100)`
  74         if_chain! {
  75             if let ExprKind::Assign(left, right, _) = expr.kind;
  76
  77             // Extract variable
  78             if let Some(local_id) = path_to_local(left);
  79
  80             // Extract len argument
  81             if let Some(len_arg) = Self::is_vec_with_capacity(cx, right);
  82
  83             then {
  84                 let vi = VecAllocation {
  85                     local_id,
  86                     allocation_expr: right,
  87                     len_expr: len_arg,
  88                 };
  89
  90                 Self::search_initialization(cx, vi, expr.hir_id);
  91             }
  92         }
  93     }
  94
  95     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
  96         // Matches statements which initializes vectors. For example: `let mut vec = Vec::with_capacity(10)`
  97         if_chain! {
  98             if let StmtKind::Local(local) = stmt.kind;
  99             if let PatKind::Binding(BindingAnnotation::Mutable, local_id, _, None) = local.pat.kind;
 100             if let Some(init) = local.init;
 101             if let Some(len_arg) = Self::is_vec_with_capacity(cx, init);
 102
 103             then {
 104                 let vi = VecAllocation {
 105                     local_id,
 106                     allocation_expr: init,
 107                     len_expr: len_arg,
 108                 };
 109
 110                 Self::search_initialization(cx, vi, stmt.hir_id);
 111             }
 112         }
 113     }
 114 }
 115
 116 impl SlowVectorInit {
 117     /// Checks if the given expression is `Vec::with_capacity(..)`. It will return the expression
 118     /// of the first argument of `with_capacity` call if it matches or `None` if it does not.
 119     fn is_vec_with_capacity<'tcx>(cx: &LateContext<'_>, expr: &Expr<'tcx>) -> Option<&'tcx Expr<'tcx>> {
 120         if_chain! {
 121             if let ExprKind::Call(func, [arg]) = expr.kind;
 122             if let ExprKind::Path(QPath::TypeRelative(ty, name)) = func.kind;
 123             if name.ident.as_str() == "with_capacity";
 124             if is_type_diagnostic_item(cx, cx.typeck_results().node_type(ty.hir_id), sym::Vec);
 125             then {
 126                 Some(arg)
 127             } else {
 128                 None
 129             }
 130         }
 131     }
 132
 133     /// Search initialization for the given vector
 134     fn search_initialization<'tcx>(cx: &LateContext<'tcx>, vec_alloc: VecAllocation<'tcx>, parent_node: HirId) {
 135         let enclosing_body = get_enclosing_block(cx, parent_node);
 136
 137         if enclosing_body.is_none() {
 138             return;
 139         }
 140
 141         let mut v = VectorInitializationVisitor {
 142             cx,
 143             vec_alloc,
 144             slow_expression: None,
 145             initialization_found: false,
 146         };
 147
 148         v.visit_block(enclosing_body.unwrap());
 149
 150         if let Some(ref allocation_expr) = v.slow_expression {
 151             Self::lint_initialization(cx, allocation_expr, &v.vec_alloc);
 152         }
 153     }
 154
 155     fn lint_initialization<'tcx>(
 156         cx: &LateContext<'tcx>,
 157         initialization: &InitializationType<'tcx>,
 158         vec_alloc: &VecAllocation<'_>,
 159     ) {
 160         match initialization {
 161             InitializationType::Extend(e) | InitializationType::Resize(e) => {
 162                 Self::emit_lint(cx, e, vec_alloc, "slow zero-filling initialization");
 163             },
 164         };
 165     }
 166
 167     fn emit_lint<'tcx>(cx: &LateContext<'tcx>, slow_fill: &Expr<'_>, vec_alloc: &VecAllocation<'_>, msg: &str) {
 168         let len_expr = Sugg::hir(cx, vec_alloc.len_expr, "len");
 169
 170         span_lint_and_then(cx, SLOW_VECTOR_INITIALIZATION, slow_fill.span, msg, |diag| {
 171             diag.span_suggestion(
 172                 vec_alloc.allocation_expr.span,
 173                 "consider replace allocation with",
 174                 format!("vec![0; {}]", len_expr),
 175                 Applicability::Unspecified,
 176             );
 177         });
 178     }
 179 }
 180
 181 /// `VectorInitializationVisitor` searches for unsafe or slow vector initializations for the given
 182 /// vector.
 183 struct VectorInitializationVisitor<'a, 'tcx> {
 184     cx: &'a LateContext<'tcx>,
 185
 186     /// Contains the information.
 187     vec_alloc: VecAllocation<'tcx>,
 188
 189     /// Contains the slow initialization expression, if one was found.
 190     slow_expression: Option<InitializationType<'tcx>>,
 191
 192     /// `true` if the initialization of the vector has been found on the visited block.
 193     initialization_found: bool,
 194 }
 195
 196 impl<'a, 'tcx> VectorInitializationVisitor<'a, 'tcx> {
 197     /// Checks if the given expression is extending a vector with `repeat(0).take(..)`
 198     fn search_slow_extend_filling(&mut self, expr: &'tcx Expr<'_>) {
 199         if_chain! {
 200             if self.initialization_found;
 201             if let ExprKind::MethodCall(path, _, [self_arg, extend_arg], _) = expr.kind;
 202             if path_to_local_id(self_arg, self.vec_alloc.local_id);
 203             if path.ident.name == sym!(extend);
 204             if self.is_repeat_take(extend_arg);
 205
 206             then {
 207                 self.slow_expression = Some(InitializationType::Extend(expr));
 208             }
 209         }
 210     }
 211
 212     /// Checks if the given expression is resizing a vector with 0
 213     fn search_slow_resize_filling(&mut self, expr: &'tcx Expr<'_>) {
 214         if_chain! {
 215             if self.initialization_found;
 216             if let ExprKind::MethodCall(path, _, [self_arg, len_arg, fill_arg], _) = expr.kind;
 217             if path_to_local_id(self_arg, self.vec_alloc.local_id);
 218             if path.ident.name == sym!(resize);
 219
 220             // Check that is filled with 0
 221             if let ExprKind::Lit(ref lit) = fill_arg.kind;
 222             if let LitKind::Int(0, _) = lit.node;
 223
 224             // Check that len expression is equals to `with_capacity` expression
 225             if SpanlessEq::new(self.cx).eq_expr(len_arg, self.vec_alloc.len_expr);
 226
 227             then {
 228                 self.slow_expression = Some(InitializationType::Resize(expr));
 229             }
 230         }
 231     }
 232
 233     /// Returns `true` if give expression is `repeat(0).take(...)`
 234     fn is_repeat_take(&self, expr: &Expr<'_>) -> bool {
 235         if_chain! {
 236             if let ExprKind::MethodCall(take_path, _, take_args, _) = expr.kind;
 237             if take_path.ident.name == sym!(take);
 238
 239             // Check that take is applied to `repeat(0)`
 240             if let Some(repeat_expr) = take_args.get(0);
 241             if self.is_repeat_zero(repeat_expr);
 242
 243             // Check that len expression is equals to `with_capacity` expression
 244             if let Some(len_arg) = take_args.get(1);
 245             if SpanlessEq::new(self.cx).eq_expr(len_arg, self.vec_alloc.len_expr);
 246
 247             then {
 248                 return true;
 249             }
 250         }
 251
 252         false
 253     }
 254
 255     /// Returns `true` if given expression is `repeat(0)`
 256     fn is_repeat_zero(&self, expr: &Expr<'_>) -> bool {
 257         if_chain! {
 258             if let ExprKind::Call(fn_expr, [repeat_arg]) = expr.kind;
 259             if is_expr_path_def_path(self.cx, fn_expr, &paths::ITER_REPEAT);
 260             if let ExprKind::Lit(ref lit) = repeat_arg.kind;
 261             if let LitKind::Int(0, _) = lit.node;
 262
 263             then {
 264                 true
 265             } else {
 266                 false
 267             }
 268         }
 269     }
 270 }
 271
 272 impl<'a, 'tcx> Visitor<'tcx> for VectorInitializationVisitor<'a, 'tcx> {
 273     type Map = Map<'tcx>;
 274
 275     fn visit_stmt(&mut self, stmt: &'tcx Stmt<'_>) {
 276         if self.initialization_found {
 277             match stmt.kind {
 278                 StmtKind::Expr(expr) | StmtKind::Semi(expr) => {
 279                     self.search_slow_extend_filling(expr);
 280                     self.search_slow_resize_filling(expr);
 281                 },
 282                 _ => (),
 283             }
 284
 285             self.initialization_found = false;
 286         } else {
 287             walk_stmt(self, stmt);
 288         }
 289     }
 290
 291     fn visit_block(&mut self, block: &'tcx Block<'_>) {
 292         if self.initialization_found {
 293             if let Some(s) = block.stmts.get(0) {
 294                 self.visit_stmt(s);
 295             }
 296
 297             self.initialization_found = false;
 298         } else {
 299             walk_block(self, block);
 300         }
 301     }
 302
 303     fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
 304         // Skip all the expressions previous to the vector initialization
 305         if self.vec_alloc.allocation_expr.hir_id == expr.hir_id {
 306             self.initialization_found = true;
 307         }
 308
 309         walk_expr(self, expr);
 310     }
 311
 312     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 313         NestedVisitorMap::None
 314     }
 315 }