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