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::UsedAfterExprVisitor;
   5 use clippy_utils::{get_enclosing_loop_or_closure, higher, 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     pub REDUNDANT_CLOSURE,
  43     style,
  44     "redundant closures, i.e., `|a| foo(a)` (which can be written as just `foo`)"
  45 }
  46
  47 declare_clippy_lint! {
  48     /// ### What it does
  49     /// Checks for closures which only invoke a method on the closure
  50     /// argument and can be replaced by referencing the method directly.
  51     ///
  52     /// ### Why is this bad?
  53     /// It's unnecessary to create the closure.
  54     ///
  55     /// ### Example
  56     /// ```rust,ignore
  57     /// Some('a').map(|s| s.to_uppercase());
  58     /// ```
  59     /// may be rewritten as
  60     /// ```rust,ignore
  61     /// Some('a').map(char::to_uppercase);
  62     /// ```
  63     pub REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
  64     pedantic,
  65     "redundant closures for method calls"
  66 }
  67
  68 declare_lint_pass!(EtaReduction => [REDUNDANT_CLOSURE, REDUNDANT_CLOSURE_FOR_METHOD_CALLS]);
  69
  70 impl<'tcx> LateLintPass<'tcx> for EtaReduction {
  71     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  72         if expr.span.from_expansion() {
  73             return;
  74         }
  75         let body = match expr.kind {
  76             ExprKind::Closure(_, _, id, _, _) => cx.tcx.hir().body(id),
  77             _ => return,
  78         };
  79         if body.value.span.from_expansion() {
  80             if body.params.is_empty() {
  81                 if let Some(VecArgs::Vec(&[])) = higher::VecArgs::hir(cx, &body.value) {
  82                     // replace `|| vec![]` with `Vec::new`
  83                     span_lint_and_sugg(
  84                         cx,
  85                         REDUNDANT_CLOSURE,
  86                         expr.span,
  87                         "redundant closure",
  88                         "replace the closure with `Vec::new`",
  89                         "std::vec::Vec::new".into(),
  90                         Applicability::MachineApplicable,
  91                     );
  92                 }
  93             }
  94             // skip `foo(|| macro!())`
  95             return;
  96         }
  97
  98         let closure_ty = cx.typeck_results().expr_ty(expr);
  99
 100         if_chain!(
 101             if let ExprKind::Call(callee, args) = body.value.kind;
 102             if let ExprKind::Path(_) = callee.kind;
 103             if check_inputs(cx, body.params, args);
 104             let callee_ty = cx.typeck_results().expr_ty_adjusted(callee);
 105             let call_ty = cx.typeck_results().type_dependent_def_id(body.value.hir_id)
 106                 .map_or(callee_ty, |id| cx.tcx.type_of(id));
 107             if check_sig(cx, closure_ty, call_ty);
 108             let substs = cx.typeck_results().node_substs(callee.hir_id);
 109             // This fixes some false positives that I don't entirely understand
 110             if substs.is_empty() || !cx.typeck_results().expr_ty(expr).has_late_bound_regions();
 111             // A type param function ref like `T::f` is not 'static, however
 112             // it is if cast like `T::f as fn()`. This seems like a rustc bug.
 113             if !substs.types().any(|t| matches!(t.kind(), ty::Param(_)));
 114             then {
 115                 span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure", |diag| {
 116                     if let Some(mut snippet) = snippet_opt(cx, callee.span) {
 117                         if_chain! {
 118                             if let ty::Closure(_, substs) = callee_ty.peel_refs().kind();
 119                             if substs.as_closure().kind() == ClosureKind::FnMut;
 120                             if get_enclosing_loop_or_closure(cx.tcx, expr).is_some()
 121                                 || UsedAfterExprVisitor::is_found(cx, callee);
 122
 123                             then {
 124                                 // Mutable closure is used after current expr; we cannot consume it.
 125                                 snippet = format!("&mut {}", snippet);
 126                             }
 127                         }
 128                         diag.span_suggestion(
 129                             expr.span,
 130                             "replace the closure with the function itself",
 131                             snippet,
 132                             Applicability::MachineApplicable,
 133                         );
 134                     }
 135                 });
 136             }
 137         );
 138
 139         if_chain!(
 140             if let ExprKind::MethodCall(path, _, args, _) = body.value.kind;
 141             if check_inputs(cx, body.params, args);
 142             let method_def_id = cx.typeck_results().type_dependent_def_id(body.value.hir_id).unwrap();
 143             let substs = cx.typeck_results().node_substs(body.value.hir_id);
 144             let call_ty = cx.tcx.type_of(method_def_id).subst(cx.tcx, substs);
 145             if check_sig(cx, closure_ty, call_ty);
 146             then {
 147                 span_lint_and_then(cx, REDUNDANT_CLOSURE_FOR_METHOD_CALLS, expr.span, "redundant closure", |diag| {
 148                     let name = get_ufcs_type_name(cx, method_def_id);
 149                     diag.span_suggestion(
 150                         expr.span,
 151                         "replace the closure with the method itself",
 152                         format!("{}::{}", name, path.ident.name),
 153                         Applicability::MachineApplicable,
 154                     );
 155                 })
 156             }
 157         );
 158     }
 159 }
 160
 161 fn check_inputs(cx: &LateContext<'_>, params: &[Param<'_>], call_args: &[Expr<'_>]) -> bool {
 162     if params.len() != call_args.len() {
 163         return false;
 164     }
 165     std::iter::zip(params, call_args).all(|(param, arg)| {
 166         match param.pat.kind {
 167             PatKind::Binding(_, id, ..) if path_to_local_id(arg, id) => {},
 168             _ => return false,
 169         }
 170         match *cx.typeck_results().expr_adjustments(arg) {
 171             [] => true,
 172             [Adjustment {
 173                 kind: Adjust::Deref(None),
 174                 ..
 175             }, Adjustment {
 176                 kind: Adjust::Borrow(AutoBorrow::Ref(_, mu2)),
 177                 ..
 178             }] => {
 179                 // re-borrow with the same mutability is allowed
 180                 let ty = cx.typeck_results().expr_ty(arg);
 181                 matches!(*ty.kind(), ty::Ref(.., mu1) if mu1 == mu2.into())
 182             },
 183             _ => false,
 184         }
 185     })
 186 }
 187
 188 fn check_sig<'tcx>(cx: &LateContext<'tcx>, closure_ty: Ty<'tcx>, call_ty: Ty<'tcx>) -> bool {
 189     let call_sig = call_ty.fn_sig(cx.tcx);
 190     if call_sig.unsafety() == Unsafety::Unsafe {
 191         return false;
 192     }
 193     if !closure_ty.has_late_bound_regions() {
 194         return true;
 195     }
 196     let substs = match closure_ty.kind() {
 197         ty::Closure(_, substs) => substs,
 198         _ => return false,
 199     };
 200     let closure_sig = cx.tcx.signature_unclosure(substs.as_closure().sig(), Unsafety::Normal);
 201     cx.tcx.erase_late_bound_regions(closure_sig) == cx.tcx.erase_late_bound_regions(call_sig)
 202 }
 203
 204 fn get_ufcs_type_name(cx: &LateContext<'_>, method_def_id: DefId) -> String {
 205     match cx.tcx.associated_item(method_def_id).container {
 206         ty::TraitContainer(def_id) => cx.tcx.def_path_str(def_id),
 207         ty::ImplContainer(def_id) => {
 208             let ty = cx.tcx.type_of(def_id);
 209             match ty.kind() {
 210                 ty::Adt(adt, _) => cx.tcx.def_path_str(adt.did),
 211                 _ => ty.to_string(),
 212             }
 213         },
 214     }
 215 }