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