src/tools/clippy/clippy_lints/src/needless_for_each.rs

   1 use rustc_errors::Applicability;
   2 use rustc_hir::{
   3     intravisit::{walk_expr, Visitor},
   4     Closure, Expr, ExprKind, Stmt, StmtKind,
   5 };
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_session::{declare_lint_pass, declare_tool_lint};
   8 use rustc_span::{source_map::Span, sym, Symbol};
   9
  10 use if_chain::if_chain;
  11
  12 use clippy_utils::diagnostics::span_lint_and_then;
  13 use clippy_utils::is_trait_method;
  14 use clippy_utils::source::snippet_with_applicability;
  15 use clippy_utils::ty::has_iter_method;
  16
  17 declare_clippy_lint! {
  18     /// ### What it does
  19     /// Checks for usage of `for_each` that would be more simply written as a
  20     /// `for` loop.
  21     ///
  22     /// ### Why is this bad?
  23     /// `for_each` may be used after applying iterator transformers like
  24     /// `filter` for better readability and performance. It may also be used to fit a simple
  25     /// operation on one line.
  26     /// But when none of these apply, a simple `for` loop is more idiomatic.
  27     ///
  28     /// ### Example
  29     /// ```rust
  30     /// let v = vec![0, 1, 2];
  31     /// v.iter().for_each(|elem| {
  32     ///     println!("{}", elem);
  33     /// })
  34     /// ```
  35     /// Use instead:
  36     /// ```rust
  37     /// let v = vec![0, 1, 2];
  38     /// for elem in v.iter() {
  39     ///     println!("{}", elem);
  40     /// }
  41     /// ```
  42     #[clippy::version = "1.53.0"]
  43     pub NEEDLESS_FOR_EACH,
  44     pedantic,
  45     "using `for_each` where a `for` loop would be simpler"
  46 }
  47
  48 declare_lint_pass!(NeedlessForEach => [NEEDLESS_FOR_EACH]);
  49
  50 impl<'tcx> LateLintPass<'tcx> for NeedlessForEach {
  51     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
  52         let expr = match stmt.kind {
  53             StmtKind::Expr(expr) | StmtKind::Semi(expr) => expr,
  54             _ => return,
  55         };
  56
  57         if_chain! {
  58             // Check the method name is `for_each`.
  59             if let ExprKind::MethodCall(method_name, for_each_recv, [for_each_arg], _) = expr.kind;
  60             if method_name.ident.name == Symbol::intern("for_each");
  61             // Check `for_each` is an associated function of `Iterator`.
  62             if is_trait_method(cx, expr, sym::Iterator);
  63             // Checks the receiver of `for_each` is also a method call.
  64             if let ExprKind::MethodCall(_, iter_recv, [], _) = for_each_recv.kind;
  65             // Skip the lint if the call chain is too long. e.g. `v.field.iter().for_each()` or
  66             // `v.foo().iter().for_each()` must be skipped.
  67             if matches!(
  68                 iter_recv.kind,
  69                 ExprKind::Array(..) | ExprKind::Call(..) | ExprKind::Path(..)
  70             );
  71             // Checks the type of the `iter` method receiver is NOT a user defined type.
  72             if has_iter_method(cx, cx.typeck_results().expr_ty(iter_recv)).is_some();
  73             // Skip the lint if the body is not block because this is simpler than `for` loop.
  74             // e.g. `v.iter().for_each(f)` is simpler and clearer than using `for` loop.
  75             if let ExprKind::Closure(&Closure { body, .. }) = for_each_arg.kind;
  76             let body = cx.tcx.hir().body(body);
  77             if let ExprKind::Block(..) = body.value.kind;
  78             then {
  79                 let mut ret_collector = RetCollector::default();
  80                 ret_collector.visit_expr(body.value);
  81
  82                 // Skip the lint if `return` is used in `Loop` in order not to suggest using `'label`.
  83                 if ret_collector.ret_in_loop {
  84                     return;
  85                 }
  86
  87                 let (mut applicability, ret_suggs) = if ret_collector.spans.is_empty() {
  88                     (Applicability::MachineApplicable, None)
  89                 } else {
  90                     (
  91                         Applicability::MaybeIncorrect,
  92                         Some(
  93                             ret_collector
  94                                 .spans
  95                                 .into_iter()
  96                                 .map(|span| (span, "continue".to_string()))
  97                                 .collect(),
  98                         ),
  99                     )
 100                 };
 101
 102                 let sugg = format!(
 103                     "for {} in {} {}",
 104                     snippet_with_applicability(cx, body.params[0].pat.span, "..", &mut applicability),
 105                     snippet_with_applicability(cx, for_each_recv.span, "..", &mut applicability),
 106                     snippet_with_applicability(cx, body.value.span, "..", &mut applicability),
 107                 );
 108
 109                 span_lint_and_then(cx, NEEDLESS_FOR_EACH, stmt.span, "needless use of `for_each`", |diag| {
 110                     diag.span_suggestion(stmt.span, "try", sugg, applicability);
 111                     if let Some(ret_suggs) = ret_suggs {
 112                         diag.multipart_suggestion("...and replace `return` with `continue`", ret_suggs, applicability);
 113                     }
 114                 })
 115             }
 116         }
 117     }
 118 }
 119
 120 /// This type plays two roles.
 121 /// 1. Collect spans of `return` in the closure body.
 122 /// 2. Detect use of `return` in `Loop` in the closure body.
 123 ///
 124 /// NOTE: The functionality of this type is similar to
 125 /// [`clippy_utils::visitors::find_all_ret_expressions`], but we can't use
 126 /// `find_all_ret_expressions` instead of this type. The reasons are:
 127 /// 1. `find_all_ret_expressions` passes the argument of `ExprKind::Ret` to a callback, but what we
 128 ///    need here is `ExprKind::Ret` itself.
 129 /// 2. We can't trace current loop depth with `find_all_ret_expressions`.
 130 #[derive(Default)]
 131 struct RetCollector {
 132     spans: Vec<Span>,
 133     ret_in_loop: bool,
 134     loop_depth: u16,
 135 }
 136
 137 impl<'tcx> Visitor<'tcx> for RetCollector {
 138     fn visit_expr(&mut self, expr: &Expr<'_>) {
 139         match expr.kind {
 140             ExprKind::Ret(..) => {
 141                 if self.loop_depth > 0 && !self.ret_in_loop {
 142                     self.ret_in_loop = true;
 143                 }
 144
 145                 self.spans.push(expr.span);
 146             },
 147
 148             ExprKind::Loop(..) => {
 149                 self.loop_depth += 1;
 150                 walk_expr(self, expr);
 151                 self.loop_depth -= 1;
 152                 return;
 153             },
 154
 155             _ => {},
 156         }
 157
 158         walk_expr(self, expr);
 159     }
 160 }