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

   1 use if_chain::if_chain;
   2 use rustc_errors::Applicability;
   3 use rustc_hir::{def_id, Expr, ExprKind, Param, PatKind, QPath};
   4 use rustc_lint::{LateContext, LateLintPass, LintContext};
   5 use rustc_middle::lint::in_external_macro;
   6 use rustc_middle::ty::{self, Ty};
   7 use rustc_session::{declare_lint_pass, declare_tool_lint};
   8
   9 use crate::utils::{
  10     implements_trait, is_adjusted, iter_input_pats, snippet_opt, span_lint_and_sugg, span_lint_and_then,
  11     type_is_unsafe_function,
  12 };
  13 use clippy_utils::higher;
  14 use clippy_utils::higher::VecArgs;
  15
  16 declare_clippy_lint! {
  17     /// **What it does:** Checks for closures which just call another function where
  18     /// the function can be called directly. `unsafe` functions or calls where types
  19     /// get adjusted are ignored.
  20     ///
  21     /// **Why is this bad?** Needlessly creating a closure adds code for no benefit
  22     /// and gives the optimizer more work.
  23     ///
  24     /// **Known problems:** If creating the closure inside the closure has a side-
  25     /// effect then moving the closure creation out will change when that side-
  26     /// effect runs.
  27     /// See [#1439](https://github.com/rust-lang/rust-clippy/issues/1439) for more details.
  28     ///
  29     /// **Example:**
  30     /// ```rust,ignore
  31     /// // Bad
  32     /// xs.map(|x| foo(x))
  33     ///
  34     /// // Good
  35     /// xs.map(foo)
  36     /// ```
  37     /// where `foo(_)` is a plain function that takes the exact argument type of
  38     /// `x`.
  39     pub REDUNDANT_CLOSURE,
  40     style,
  41     "redundant closures, i.e., `|a| foo(a)` (which can be written as just `foo`)"
  42 }
  43
  44 declare_clippy_lint! {
  45     /// **What it does:** Checks for closures which only invoke a method on the closure
  46     /// argument and can be replaced by referencing the method directly.
  47     ///
  48     /// **Why is this bad?** It's unnecessary to create the closure.
  49     ///
  50     /// **Known problems:** [#3071](https://github.com/rust-lang/rust-clippy/issues/3071),
  51     /// [#3942](https://github.com/rust-lang/rust-clippy/issues/3942),
  52     /// [#4002](https://github.com/rust-lang/rust-clippy/issues/4002)
  53     ///
  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 in_external_macro(cx.sess(), expr.span) {
  73             return;
  74         }
  75
  76         match expr.kind {
  77             ExprKind::Call(_, args) | ExprKind::MethodCall(_, _, args, _) => {
  78                 for arg in args {
  79                     // skip `foo(macro!())`
  80                     if arg.span.ctxt() == expr.span.ctxt() {
  81                         check_closure(cx, arg)
  82                     }
  83                 }
  84             },
  85             _ => (),
  86         }
  87     }
  88 }
  89
  90 fn check_closure(cx: &LateContext<'_>, expr: &Expr<'_>) {
  91     if let ExprKind::Closure(_, ref decl, eid, _, _) = expr.kind {
  92         let body = cx.tcx.hir().body(eid);
  93         let ex = &body.value;
  94
  95         if ex.span.ctxt() != expr.span.ctxt() {
  96             if let Some(VecArgs::Vec(&[])) = higher::vec_macro(cx, ex) {
  97                 // replace `|| vec![]` with `Vec::new`
  98                 span_lint_and_sugg(
  99                     cx,
 100                     REDUNDANT_CLOSURE,
 101                     expr.span,
 102                     "redundant closure",
 103                     "replace the closure with `Vec::new`",
 104                     "std::vec::Vec::new".into(),
 105                     Applicability::MachineApplicable,
 106                 );
 107             }
 108             // skip `foo(|| macro!())`
 109             return;
 110         }
 111
 112         if_chain!(
 113             if let ExprKind::Call(ref caller, ref args) = ex.kind;
 114
 115             if let ExprKind::Path(_) = caller.kind;
 116
 117             // Not the same number of arguments, there is no way the closure is the same as the function return;
 118             if args.len() == decl.inputs.len();
 119
 120             // Are the expression or the arguments type-adjusted? Then we need the closure
 121             if !(is_adjusted(cx, ex) || args.iter().any(|arg| is_adjusted(cx, arg)));
 122
 123             let fn_ty = cx.typeck_results().expr_ty(caller);
 124
 125             if matches!(fn_ty.kind(), ty::FnDef(_, _) | ty::FnPtr(_) | ty::Closure(_, _));
 126
 127             if !type_is_unsafe_function(cx, fn_ty);
 128
 129             if compare_inputs(&mut iter_input_pats(decl, body), &mut args.iter());
 130
 131             then {
 132                 span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure", |diag| {
 133                     if let Some(snippet) = snippet_opt(cx, caller.span) {
 134                         diag.span_suggestion(
 135                             expr.span,
 136                             "replace the closure with the function itself",
 137                             snippet,
 138                             Applicability::MachineApplicable,
 139                         );
 140                     }
 141                 });
 142             }
 143         );
 144
 145         if_chain!(
 146             if let ExprKind::MethodCall(ref path, _, ref args, _) = ex.kind;
 147
 148             // Not the same number of arguments, there is no way the closure is the same as the function return;
 149             if args.len() == decl.inputs.len();
 150
 151             // Are the expression or the arguments type-adjusted? Then we need the closure
 152             if !(is_adjusted(cx, ex) || args.iter().skip(1).any(|arg| is_adjusted(cx, arg)));
 153
 154             let method_def_id = cx.typeck_results().type_dependent_def_id(ex.hir_id).unwrap();
 155             if !type_is_unsafe_function(cx, cx.tcx.type_of(method_def_id));
 156
 157             if compare_inputs(&mut iter_input_pats(decl, body), &mut args.iter());
 158
 159             if let Some(name) = get_ufcs_type_name(cx, method_def_id, &args[0]);
 160
 161             then {
 162                 span_lint_and_sugg(
 163                     cx,
 164                     REDUNDANT_CLOSURE_FOR_METHOD_CALLS,
 165                     expr.span,
 166                     "redundant closure",
 167                     "replace the closure with the method itself",
 168                     format!("{}::{}", name, path.ident.name),
 169                     Applicability::MachineApplicable,
 170                 );
 171             }
 172         );
 173     }
 174 }
 175
 176 /// Tries to determine the type for universal function call to be used instead of the closure
 177 fn get_ufcs_type_name(cx: &LateContext<'_>, method_def_id: def_id::DefId, self_arg: &Expr<'_>) -> Option<String> {
 178     let expected_type_of_self = &cx.tcx.fn_sig(method_def_id).inputs_and_output().skip_binder()[0];
 179     let actual_type_of_self = &cx.typeck_results().node_type(self_arg.hir_id);
 180
 181     if let Some(trait_id) = cx.tcx.trait_of_item(method_def_id) {
 182         if match_borrow_depth(expected_type_of_self, &actual_type_of_self)
 183             && implements_trait(cx, actual_type_of_self, trait_id, &[])
 184         {
 185             return Some(cx.tcx.def_path_str(trait_id));
 186         }
 187     }
 188
 189     cx.tcx.impl_of_method(method_def_id).and_then(|_| {
 190         //a type may implicitly implement other type's methods (e.g. Deref)
 191         if match_types(expected_type_of_self, &actual_type_of_self) {
 192             return Some(get_type_name(cx, &actual_type_of_self));
 193         }
 194         None
 195     })
 196 }
 197
 198 fn match_borrow_depth(lhs: Ty<'_>, rhs: Ty<'_>) -> bool {
 199     match (&lhs.kind(), &rhs.kind()) {
 200         (ty::Ref(_, t1, mut1), ty::Ref(_, t2, mut2)) => mut1 == mut2 && match_borrow_depth(&t1, &t2),
 201         (l, r) => !matches!((l, r), (ty::Ref(_, _, _), _) | (_, ty::Ref(_, _, _))),
 202     }
 203 }
 204
 205 fn match_types(lhs: Ty<'_>, rhs: Ty<'_>) -> bool {
 206     match (&lhs.kind(), &rhs.kind()) {
 207         (ty::Bool, ty::Bool)
 208         | (ty::Char, ty::Char)
 209         | (ty::Int(_), ty::Int(_))
 210         | (ty::Uint(_), ty::Uint(_))
 211         | (ty::Str, ty::Str) => true,
 212         (ty::Ref(_, t1, mut1), ty::Ref(_, t2, mut2)) => mut1 == mut2 && match_types(t1, t2),
 213         (ty::Array(t1, _), ty::Array(t2, _)) | (ty::Slice(t1), ty::Slice(t2)) => match_types(t1, t2),
 214         (ty::Adt(def1, _), ty::Adt(def2, _)) => def1 == def2,
 215         (_, _) => false,
 216     }
 217 }
 218
 219 fn get_type_name(cx: &LateContext<'_>, ty: Ty<'_>) -> String {
 220     match ty.kind() {
 221         ty::Adt(t, _) => cx.tcx.def_path_str(t.did),
 222         ty::Ref(_, r, _) => get_type_name(cx, &r),
 223         _ => ty.to_string(),
 224     }
 225 }
 226
 227 fn compare_inputs(
 228     closure_inputs: &mut dyn Iterator<Item = &Param<'_>>,
 229     call_args: &mut dyn Iterator<Item = &Expr<'_>>,
 230 ) -> bool {
 231     for (closure_input, function_arg) in closure_inputs.zip(call_args) {
 232         if let PatKind::Binding(_, _, ident, _) = closure_input.pat.kind {
 233             // XXXManishearth Should I be checking the binding mode here?
 234             if let ExprKind::Path(QPath::Resolved(None, ref p)) = function_arg.kind {
 235                 if p.segments.len() != 1 {
 236                     // If it's a proper path, it can't be a local variable
 237                     return false;
 238                 }
 239                 if p.segments[0].ident.name != ident.name {
 240                     // The two idents should be the same
 241                     return false;
 242                 }
 243             } else {
 244                 return false;
 245             }
 246         } else {
 247             return false;
 248         }
 249     }
 250     true
 251 }