src/tools/clippy/clippy_lints/src/eta_reduction.rs

   1 use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
   2 use clippy_utils::higher::VecArgs;
   3 use clippy_utils::source::snippet_opt;
   4 use clippy_utils::ty::{implements_trait, is_type_diagnostic_item};
   5 use clippy_utils::usage::local_used_after_expr;
   6 use clippy_utils::{higher, is_adjusted, path_to_local, path_to_local_id};
   7 use if_chain::if_chain;
   8 use rustc_errors::Applicability;
   9 use rustc_hir::def_id::DefId;
  10 use rustc_hir::{Closure, Expr, ExprKind, Param, PatKind, Unsafety};
  11 use rustc_lint::{LateContext, LateLintPass};
  12 use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow};
  13 use rustc_middle::ty::binding::BindingMode;
  14 use rustc_middle::ty::{self, Ty, TypeVisitable};
  15 use rustc_session::{declare_lint_pass, declare_tool_lint};
  16 use rustc_span::symbol::sym;
  17
  18 declare_clippy_lint! {
  19     /// ### What it does
  20     /// Checks for closures which just call another function where
  21     /// the function can be called directly. `unsafe` functions or calls where types
  22     /// get adjusted are ignored.
  23     ///
  24     /// ### Why is this bad?
  25     /// Needlessly creating a closure adds code for no benefit
  26     /// and gives the optimizer more work.
  27     ///
  28     /// ### Known problems
  29     /// If creating the closure inside the closure has a side-
  30     /// effect then moving the closure creation out will change when that side-
  31     /// effect runs.
  32     /// See [#1439](https://github.com/rust-lang/rust-clippy/issues/1439) for more details.
  33     ///
  34     /// ### Example
  35     /// ```rust,ignore
  36     /// xs.map(|x| foo(x))
  37     /// ```
  38     ///
  39     /// Use instead:
  40     /// ```rust,ignore
  41     /// // where `foo(_)` is a plain function that takes the exact argument type of `x`.
  42     /// xs.map(foo)
  43     /// ```
  44     #[clippy::version = "pre 1.29.0"]
  45     pub REDUNDANT_CLOSURE,
  46     style,
  47     "redundant closures, i.e., `|a| foo(a)` (which can be written as just `foo`)"
  48 }
  49
  50 declare_clippy_lint! {
  51     /// ### What it does
  52     /// Checks for closures which only invoke a method on the closure
  53     /// argument and can be replaced by referencing the method directly.
  54     ///
  55     /// ### Why is this bad?
  56     /// It's unnecessary to create the closure.
  57     ///
  58     /// ### Example
  59     /// ```rust,ignore
  60     /// Some('a').map(|s| s.to_uppercase());
  61     /// ```
  62     /// may be rewritten as
  63     /// ```rust,ignore
  64     /// Some('a').map(char::to_uppercase);
  65     /// ```
  66     #[clippy::version = "1.35.0"]
  67     pub REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
  68     pedantic,
  69     "redundant closures for method calls"
  70 }
  71
  72 declare_lint_pass!(EtaReduction => [REDUNDANT_CLOSURE, REDUNDANT_CLOSURE_FOR_METHOD_CALLS]);
  73
  74 impl<'tcx> LateLintPass<'tcx> for EtaReduction {
  75     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  76         if expr.span.from_expansion() {
  77             return;
  78         }
  79         let body = match expr.kind {
  80             ExprKind::Closure(&Closure { body, .. }) => cx.tcx.hir().body(body),
  81             _ => return,
  82         };
  83         if body.value.span.from_expansion() {
  84             if body.params.is_empty() {
  85                 if let Some(VecArgs::Vec(&[])) = higher::VecArgs::hir(cx, body.value) {
  86                     // replace `|| vec![]` with `Vec::new`
  87                     span_lint_and_sugg(
  88                         cx,
  89                         REDUNDANT_CLOSURE,
  90                         expr.span,
  91                         "redundant closure",
  92                         "replace the closure with `Vec::new`",
  93                         "std::vec::Vec::new".into(),
  94                         Applicability::MachineApplicable,
  95                     );
  96                 }
  97             }
  98             // skip `foo(|| macro!())`
  99             return;
 100         }
 101
 102         let closure_ty = cx.typeck_results().expr_ty(expr);
 103
 104         if_chain!(
 105             if !is_adjusted(cx, body.value);
 106             if let ExprKind::Call(callee, args) = body.value.kind;
 107             if let ExprKind::Path(_) = callee.kind;
 108             if check_inputs(cx, body.params, None, args);
 109             let callee_ty = cx.typeck_results().expr_ty_adjusted(callee);
 110             let call_ty = cx.typeck_results().type_dependent_def_id(body.value.hir_id)
 111                 .map_or(callee_ty, |id| cx.tcx.type_of(id));
 112             if check_sig(cx, closure_ty, call_ty);
 113             let substs = cx.typeck_results().node_substs(callee.hir_id);
 114             // This fixes some false positives that I don't entirely understand
 115             if substs.is_empty() || !cx.typeck_results().expr_ty(expr).has_late_bound_regions();
 116             // A type param function ref like `T::f` is not 'static, however
 117             // it is if cast like `T::f as fn()`. This seems like a rustc bug.
 118             if !substs.types().any(|t| matches!(t.kind(), ty::Param(_)));
 119             let callee_ty_unadjusted = cx.typeck_results().expr_ty(callee).peel_refs();
 120             if !is_type_diagnostic_item(cx, callee_ty_unadjusted, sym::Arc);
 121             if !is_type_diagnostic_item(cx, callee_ty_unadjusted, sym::Rc);
 122             then {
 123                 span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure", |diag| {
 124                     if let Some(mut snippet) = snippet_opt(cx, callee.span) {
 125                         if let Some(fn_mut_id) = cx.tcx.lang_items().fn_mut_trait()
 126                             && implements_trait(cx, callee_ty.peel_refs(), fn_mut_id, &[])
 127                             && path_to_local(callee).map_or(false, |l| local_used_after_expr(cx, l, expr))
 128                         {
 129                                 // Mutable closure is used after current expr; we cannot consume it.
 130                                 snippet = format!("&mut {snippet}");
 131                         }
 132                         diag.span_suggestion(
 133                             expr.span,
 134                             "replace the closure with the function itself",
 135                             snippet,
 136                             Applicability::MachineApplicable,
 137                         );
 138                     }
 139                 });
 140             }
 141         );
 142
 143         if_chain!(
 144             if !is_adjusted(cx, body.value);
 145             if let ExprKind::MethodCall(path, receiver, args, _) = body.value.kind;
 146             if check_inputs(cx, body.params, Some(receiver), args);
 147             let method_def_id = cx.typeck_results().type_dependent_def_id(body.value.hir_id).unwrap();
 148             let substs = cx.typeck_results().node_substs(body.value.hir_id);
 149             let call_ty = cx.tcx.bound_type_of(method_def_id).subst(cx.tcx, substs);
 150             if check_sig(cx, closure_ty, call_ty);
 151             then {
 152                 span_lint_and_then(cx, REDUNDANT_CLOSURE_FOR_METHOD_CALLS, expr.span, "redundant closure", |diag| {
 153                     let name = get_ufcs_type_name(cx, method_def_id);
 154                     diag.span_suggestion(
 155                         expr.span,
 156                         "replace the closure with the method itself",
 157                         format!("{name}::{}", path.ident.name),
 158                         Applicability::MachineApplicable,
 159                     );
 160                 })
 161             }
 162         );
 163     }
 164 }
 165
 166 fn check_inputs(
 167     cx: &LateContext<'_>,
 168     params: &[Param<'_>],
 169     receiver: Option<&Expr<'_>>,
 170     call_args: &[Expr<'_>],
 171 ) -> bool {
 172     if receiver.map_or(params.len() != call_args.len(), |_| params.len() != call_args.len() + 1) {
 173         return false;
 174     }
 175     let binding_modes = cx.typeck_results().pat_binding_modes();
 176     let check_inputs = |param: &Param<'_>, arg| {
 177         match param.pat.kind {
 178             PatKind::Binding(_, id, ..) if path_to_local_id(arg, id) => {},
 179             _ => return false,
 180         }
 181         // checks that parameters are not bound as `ref` or `ref mut`
 182         if let Some(BindingMode::BindByReference(_)) = binding_modes.get(param.pat.hir_id) {
 183             return false;
 184         }
 185
 186         match *cx.typeck_results().expr_adjustments(arg) {
 187             [] => true,
 188             [
 189                 Adjustment {
 190                     kind: Adjust::Deref(None),
 191                     ..
 192                 },
 193                 Adjustment {
 194                     kind: Adjust::Borrow(AutoBorrow::Ref(_, mu2)),
 195                     ..
 196                 },
 197             ] => {
 198                 // re-borrow with the same mutability is allowed
 199                 let ty = cx.typeck_results().expr_ty(arg);
 200                 matches!(*ty.kind(), ty::Ref(.., mu1) if mu1 == mu2.into())
 201             },
 202             _ => false,
 203         }
 204     };
 205     std::iter::zip(params, receiver.into_iter().chain(call_args.iter())).all(|(param, arg)| check_inputs(param, arg))
 206 }
 207
 208 fn check_sig<'tcx>(cx: &LateContext<'tcx>, closure_ty: Ty<'tcx>, call_ty: Ty<'tcx>) -> bool {
 209     let call_sig = call_ty.fn_sig(cx.tcx);
 210     if call_sig.unsafety() == Unsafety::Unsafe {
 211         return false;
 212     }
 213     if !closure_ty.has_late_bound_regions() {
 214         return true;
 215     }
 216     let ty::Closure(_, substs) = closure_ty.kind() else {
 217         return false;
 218     };
 219     let closure_sig = cx.tcx.signature_unclosure(substs.as_closure().sig(), Unsafety::Normal);
 220     cx.tcx.erase_late_bound_regions(closure_sig) == cx.tcx.erase_late_bound_regions(call_sig)
 221 }
 222
 223 fn get_ufcs_type_name(cx: &LateContext<'_>, method_def_id: DefId) -> String {
 224     let assoc_item = cx.tcx.associated_item(method_def_id);
 225     let def_id = assoc_item.container_id(cx.tcx);
 226     match assoc_item.container {
 227         ty::TraitContainer => cx.tcx.def_path_str(def_id),
 228         ty::ImplContainer => {
 229             let ty = cx.tcx.type_of(def_id);
 230             match ty.kind() {
 231                 ty::Adt(adt, _) => cx.tcx.def_path_str(adt.did()),
 232                 _ => ty.to_string(),
 233             }
 234         },
 235     }
 236 }