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

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::higher::{get_vec_init_kind, VecInitKind};
   3 use clippy_utils::source::snippet;
   4 use clippy_utils::{path_to_local, path_to_local_id};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{BindingAnnotation, Block, Expr, ExprKind, HirId, Local, PatKind, Stmt, StmtKind};
   8 use rustc_lint::{LateContext, LateLintPass, LintContext};
   9 use rustc_middle::lint::in_external_macro;
  10 use rustc_session::{declare_tool_lint, impl_lint_pass};
  11 use rustc_span::Span;
  12
  13 declare_clippy_lint! {
  14     /// ### What it does
  15     /// Checks for calls to `push` immediately after creating a new `Vec`.
  16     ///
  17     /// ### Why is this bad?
  18     /// The `vec![]` macro is both more performant and easier to read than
  19     /// multiple `push` calls.
  20     ///
  21     /// ### Example
  22     /// ```rust
  23     /// let mut v = Vec::new();
  24     /// v.push(0);
  25     /// ```
  26     /// Use instead:
  27     /// ```rust
  28     /// let v = vec![0];
  29     /// ```
  30     pub VEC_INIT_THEN_PUSH,
  31     perf,
  32     "`push` immediately after `Vec` creation"
  33 }
  34
  35 impl_lint_pass!(VecInitThenPush => [VEC_INIT_THEN_PUSH]);
  36
  37 #[derive(Default)]
  38 pub struct VecInitThenPush {
  39     searcher: Option<VecPushSearcher>,
  40 }
  41
  42 struct VecPushSearcher {
  43     local_id: HirId,
  44     init: VecInitKind,
  45     lhs_is_local: bool,
  46     lhs_span: Span,
  47     err_span: Span,
  48     found: u64,
  49 }
  50 impl VecPushSearcher {
  51     fn display_err(&self, cx: &LateContext<'_>) {
  52         match self.init {
  53             _ if self.found == 0 => return,
  54             VecInitKind::WithLiteralCapacity(x) if x > self.found => return,
  55             VecInitKind::WithExprCapacity(_) => return,
  56             _ => (),
  57         };
  58
  59         let mut s = if self.lhs_is_local {
  60             String::from("let ")
  61         } else {
  62             String::new()
  63         };
  64         s.push_str(&snippet(cx, self.lhs_span, ".."));
  65         s.push_str(" = vec![..];");
  66
  67         span_lint_and_sugg(
  68             cx,
  69             VEC_INIT_THEN_PUSH,
  70             self.err_span,
  71             "calls to `push` immediately after creation",
  72             "consider using the `vec![]` macro",
  73             s,
  74             Applicability::HasPlaceholders,
  75         );
  76     }
  77 }
  78
  79 impl LateLintPass<'_> for VecInitThenPush {
  80     fn check_block(&mut self, _: &LateContext<'tcx>, _: &'tcx Block<'tcx>) {
  81         self.searcher = None;
  82     }
  83
  84     fn check_local(&mut self, cx: &LateContext<'tcx>, local: &'tcx Local<'tcx>) {
  85         if_chain! {
  86             if !in_external_macro(cx.sess(), local.span);
  87             if let Some(init) = local.init;
  88             if let PatKind::Binding(BindingAnnotation::Mutable, id, _, None) = local.pat.kind;
  89             if let Some(init_kind) = get_vec_init_kind(cx, init);
  90             then {
  91                 self.searcher = Some(VecPushSearcher {
  92                         local_id: id,
  93                         init: init_kind,
  94                         lhs_is_local: true,
  95                         lhs_span: local.ty.map_or(local.pat.span, |t| local.pat.span.to(t.span)),
  96                         err_span: local.span,
  97                         found: 0,
  98                     });
  99             }
 100         }
 101     }
 102
 103     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 104         if_chain! {
 105             if self.searcher.is_none();
 106             if !in_external_macro(cx.sess(), expr.span);
 107             if let ExprKind::Assign(left, right, _) = expr.kind;
 108             if let Some(id) = path_to_local(left);
 109             if let Some(init_kind) = get_vec_init_kind(cx, right);
 110             then {
 111                 self.searcher = Some(VecPushSearcher {
 112                     local_id: id,
 113                     init: init_kind,
 114                     lhs_is_local: false,
 115                     lhs_span: left.span,
 116                     err_span: expr.span,
 117                     found: 0,
 118                 });
 119             }
 120         }
 121     }
 122
 123     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
 124         if let Some(searcher) = self.searcher.take() {
 125             if_chain! {
 126                 if let StmtKind::Expr(expr) | StmtKind::Semi(expr) = stmt.kind;
 127                 if let ExprKind::MethodCall(path, _, [self_arg, _], _) = expr.kind;
 128                 if path_to_local_id(self_arg, searcher.local_id);
 129                 if path.ident.name.as_str() == "push";
 130                 then {
 131                     self.searcher = Some(VecPushSearcher {
 132                         found: searcher.found + 1,
 133                         err_span: searcher.err_span.to(stmt.span),
 134                         .. searcher
 135                     });
 136                 } else {
 137                     searcher.display_err(cx);
 138                 }
 139             }
 140         }
 141     }
 142
 143     fn check_block_post(&mut self, cx: &LateContext<'tcx>, _: &'tcx Block<'tcx>) {
 144         if let Some(searcher) = self.searcher.take() {
 145             searcher.display_err(cx);
 146         }
 147     }
 148 }