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

   1 use rustc_errors::Applicability;
   2 use rustc_hir::{
   3     intravisit::{walk_expr, NestedVisitorMap, Visitor},
   4     Expr, ExprKind, Stmt, StmtKind,
   5 };
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_middle::hir::map::Map;
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9 use rustc_span::{source_map::Span, sym, Symbol};
  10
  11 use if_chain::if_chain;
  12
  13 use clippy_utils::diagnostics::span_lint_and_then;
  14 use clippy_utils::is_trait_method;
  15 use clippy_utils::source::snippet_with_applicability;
  16 use clippy_utils::ty::has_iter_method;
  17
  18 declare_clippy_lint! {
  19     /// ### What it does
  20     /// Checks for usage of `for_each` that would be more simply written as a
  21     /// `for` loop.
  22     ///
  23     /// ### Why is this bad?
  24     /// `for_each` may be used after applying iterator transformers like
  25     /// `filter` for better readability and performance. It may also be used to fit a simple
  26     /// operation on one line.
  27     /// But when none of these apply, a simple `for` loop is more idiomatic.
  28     ///
  29     /// ### Example
  30     /// ```rust
  31     /// let v = vec![0, 1, 2];
  32     /// v.iter().for_each(|elem| {
  33     ///     println!("{}", elem);
  34     /// })
  35     /// ```
  36     /// Use instead:
  37     /// ```rust
  38     /// let v = vec![0, 1, 2];
  39     /// for elem in v.iter() {
  40     ///     println!("{}", elem);
  41     /// }
  42     /// ```
  43     #[clippy::version = "1.53.0"]
  44     pub NEEDLESS_FOR_EACH,
  45     pedantic,
  46     "using `for_each` where a `for` loop would be simpler"
  47 }
  48
  49 declare_lint_pass!(NeedlessForEach => [NEEDLESS_FOR_EACH]);
  50
  51 impl LateLintPass<'_> for NeedlessForEach {
  52     fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
  53         let expr = match stmt.kind {
  54             StmtKind::Expr(expr) | StmtKind::Semi(expr) => expr,
  55             _ => return,
  56         };
  57
  58         if_chain! {
  59             // Check the method name is `for_each`.
  60             if let ExprKind::MethodCall(method_name, _, [for_each_recv, for_each_arg], _) = expr.kind;
  61             if method_name.ident.name == Symbol::intern("for_each");
  62             // Check `for_each` is an associated function of `Iterator`.
  63             if is_trait_method(cx, expr, sym::Iterator);
  64             // Checks the receiver of `for_each` is also a method call.
  65             if let ExprKind::MethodCall(_, _, [iter_recv], _) = for_each_recv.kind;
  66             // Skip the lint if the call chain is too long. e.g. `v.field.iter().for_each()` or
  67             // `v.foo().iter().for_each()` must be skipped.
  68             if matches!(
  69                 iter_recv.kind,
  70                 ExprKind::Array(..) | ExprKind::Call(..) | ExprKind::Path(..)
  71             );
  72             // Checks the type of the `iter` method receiver is NOT a user defined type.
  73             if has_iter_method(cx, cx.typeck_results().expr_ty(iter_recv)).is_some();
  74             // Skip the lint if the body is not block because this is simpler than `for` loop.
  75             // e.g. `v.iter().for_each(f)` is simpler and clearer than using `for` loop.
  76             if let ExprKind::Closure(_, _, body_id, ..) = for_each_arg.kind;
  77             let body = cx.tcx.hir().body(body_id);
  78             if let ExprKind::Block(..) = body.value.kind;
  79             then {
  80                 let mut ret_collector = RetCollector::default();
  81                 ret_collector.visit_expr(&body.value);
  82
  83                 // Skip the lint if `return` is used in `Loop` in order not to suggest using `'label`.
  84                 if ret_collector.ret_in_loop {
  85                     return;
  86                 }
  87
  88                 let (mut applicability, ret_suggs) = if ret_collector.spans.is_empty() {
  89                     (Applicability::MachineApplicable, None)
  90                 } else {
  91                     (
  92                         Applicability::MaybeIncorrect,
  93                         Some(
  94                             ret_collector
  95                                 .spans
  96                                 .into_iter()
  97                                 .map(|span| (span, "continue".to_string()))
  98                                 .collect(),
  99                         ),
 100                     )
 101                 };
 102
 103                 let sugg = format!(
 104                     "for {} in {} {}",
 105                     snippet_with_applicability(cx, body.params[0].pat.span, "..", &mut applicability),
 106                     snippet_with_applicability(cx, for_each_recv.span, "..", &mut applicability),
 107                     snippet_with_applicability(cx, body.value.span, "..", &mut applicability),
 108                 );
 109
 110                 span_lint_and_then(cx, NEEDLESS_FOR_EACH, stmt.span, "needless use of `for_each`", |diag| {
 111                     diag.span_suggestion(stmt.span, "try", sugg, applicability);
 112                     if let Some(ret_suggs) = ret_suggs {
 113                         diag.multipart_suggestion("...and replace `return` with `continue`", ret_suggs, applicability);
 114                     }
 115                 })
 116             }
 117         }
 118     }
 119 }
 120
 121 /// This type plays two roles.
 122 /// 1. Collect spans of `return` in the closure body.
 123 /// 2. Detect use of `return` in `Loop` in the closure body.
 124 ///
 125 /// NOTE: The functionality of this type is similar to
 126 /// [`clippy_utils::visitors::find_all_ret_expressions`], but we can't use
 127 /// `find_all_ret_expressions` instead of this type. The reasons are:
 128 /// 1. `find_all_ret_expressions` passes the argument of `ExprKind::Ret` to a callback, but what we
 129 ///    need here is `ExprKind::Ret` itself.
 130 /// 2. We can't trace current loop depth with `find_all_ret_expressions`.
 131 #[derive(Default)]
 132 struct RetCollector {
 133     spans: Vec<Span>,
 134     ret_in_loop: bool,
 135     loop_depth: u16,
 136 }
 137
 138 impl<'tcx> Visitor<'tcx> for RetCollector {
 139     type Map = Map<'tcx>;
 140
 141     fn visit_expr(&mut self, expr: &Expr<'_>) {
 142         match expr.kind {
 143             ExprKind::Ret(..) => {
 144                 if self.loop_depth > 0 && !self.ret_in_loop {
 145                     self.ret_in_loop = true;
 146                 }
 147
 148                 self.spans.push(expr.span);
 149             },
 150
 151             ExprKind::Loop(..) => {
 152                 self.loop_depth += 1;
 153                 walk_expr(self, expr);
 154                 self.loop_depth -= 1;
 155                 return;
 156             },
 157
 158             _ => {},
 159         }
 160
 161         walk_expr(self, expr);
 162     }
 163
 164     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 165         NestedVisitorMap::None
 166     }
 167 }