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::is_type_diagnostic_item;
   5 use clippy_utils::usage::local_used_after_expr;
   6 use clippy_utils::{get_enclosing_loop_or_closure, 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::subst::Subst;
  14 use rustc_middle::ty::{self, ClosureKind, Ty, TypeFoldable};
  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     /// // Bad
  37     /// xs.map(|x| foo(x))
  38     ///
  39     /// // Good
  40     /// xs.map(foo)
  41     /// ```
  42     /// where `foo(_)` is a plain function that takes the exact argument type of
  43     /// `x`.
  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(_, _, id, _, _) => cx.tcx.hir().body(id),
  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 let ExprKind::Call(callee, args) = body.value.kind;
 106             if let ExprKind::Path(_) = callee.kind;
 107             if check_inputs(cx, body.params, args);
 108             let callee_ty = cx.typeck_results().expr_ty_adjusted(callee);
 109             let call_ty = cx.typeck_results().type_dependent_def_id(body.value.hir_id)
 110                 .map_or(callee_ty, |id| cx.tcx.type_of(id));
 111             if check_sig(cx, closure_ty, call_ty);
 112             let substs = cx.typeck_results().node_substs(callee.hir_id);
 113             // This fixes some false positives that I don't entirely understand
 114             if substs.is_empty() || !cx.typeck_results().expr_ty(expr).has_late_bound_regions();
 115             // A type param function ref like `T::f` is not 'static, however
 116             // it is if cast like `T::f as fn()`. This seems like a rustc bug.
 117             if !substs.types().any(|t| matches!(t.kind(), ty::Param(_)));
 118             let callee_ty_unadjusted = cx.typeck_results().expr_ty(callee).peel_refs();
 119             if !is_type_diagnostic_item(cx, callee_ty_unadjusted, sym::Arc);
 120             if !is_type_diagnostic_item(cx, callee_ty_unadjusted, sym::Rc);
 121             then {
 122                 span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure", |diag| {
 123                     if let Some(mut snippet) = snippet_opt(cx, callee.span) {
 124                         if_chain! {
 125                             if let ty::Closure(_, substs) = callee_ty.peel_refs().kind();
 126                             if substs.as_closure().kind() == ClosureKind::FnMut;
 127                             if get_enclosing_loop_or_closure(cx.tcx, expr).is_some()
 128                                 || path_to_local(callee).map_or(false, |l| local_used_after_expr(cx, l, callee));
 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     std::iter::zip(params, call_args).all(|(param, arg)| {
 173         match param.pat.kind {
 174             PatKind::Binding(_, id, ..) if path_to_local_id(arg, id) => {},
 175             _ => return false,
 176         }
 177         match *cx.typeck_results().expr_adjustments(arg) {
 178             [] => true,
 179             [
 180                 Adjustment {
 181                     kind: Adjust::Deref(None),
 182                     ..
 183                 },
 184                 Adjustment {
 185                     kind: Adjust::Borrow(AutoBorrow::Ref(_, mu2)),
 186                     ..
 187                 },
 188             ] => {
 189                 // re-borrow with the same mutability is allowed
 190                 let ty = cx.typeck_results().expr_ty(arg);
 191                 matches!(*ty.kind(), ty::Ref(.., mu1) if mu1 == mu2.into())
 192             },
 193             _ => false,
 194         }
 195     })
 196 }
 197
 198 fn check_sig<'tcx>(cx: &LateContext<'tcx>, closure_ty: Ty<'tcx>, call_ty: Ty<'tcx>) -> bool {
 199     let call_sig = call_ty.fn_sig(cx.tcx);
 200     if call_sig.unsafety() == Unsafety::Unsafe {
 201         return false;
 202     }
 203     if !closure_ty.has_late_bound_regions() {
 204         return true;
 205     }
 206     let substs = match closure_ty.kind() {
 207         ty::Closure(_, substs) => substs,
 208         _ => return false,
 209     };
 210     let closure_sig = cx.tcx.signature_unclosure(substs.as_closure().sig(), Unsafety::Normal);
 211     cx.tcx.erase_late_bound_regions(closure_sig) == cx.tcx.erase_late_bound_regions(call_sig)
 212 }
 213
 214 fn get_ufcs_type_name(cx: &LateContext<'_>, method_def_id: DefId) -> String {
 215     match cx.tcx.associated_item(method_def_id).container {
 216         ty::TraitContainer(def_id) => cx.tcx.def_path_str(def_id),
 217         ty::ImplContainer(def_id) => {
 218             let ty = cx.tcx.type_of(def_id);
 219             match ty.kind() {
 220                 ty::Adt(adt, _) => cx.tcx.def_path_str(adt.did),
 221                 _ => ty.to_string(),
 222             }
 223         },
 224     }
 225 }