src/tools/clippy/clippy_lints/src/unit_return_expecting_ord.rs

   1 use clippy_utils::diagnostics::{span_lint, span_lint_and_help};
   2 use if_chain::if_chain;
   3 use rustc_hir::def_id::DefId;
   4 use rustc_hir::{Closure, Expr, ExprKind, StmtKind};
   5 use rustc_lint::{LateContext, LateLintPass};
   6 use rustc_middle::ty;
   7 use rustc_middle::ty::{GenericPredicates, PredicateKind, ProjectionPredicate, TraitPredicate};
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9 use rustc_span::{sym, BytePos, Span};
  10
  11 declare_clippy_lint! {
  12     /// ### What it does
  13     /// Checks for functions that expect closures of type
  14     /// Fn(...) -> Ord where the implemented closure returns the unit type.
  15     /// The lint also suggests to remove the semi-colon at the end of the statement if present.
  16     ///
  17     /// ### Why is this bad?
  18     /// Likely, returning the unit type is unintentional, and
  19     /// could simply be caused by an extra semi-colon. Since () implements Ord
  20     /// it doesn't cause a compilation error.
  21     /// This is the same reasoning behind the unit_cmp lint.
  22     ///
  23     /// ### Known problems
  24     /// If returning unit is intentional, then there is no
  25     /// way of specifying this without triggering needless_return lint
  26     ///
  27     /// ### Example
  28     /// ```rust
  29     /// let mut twins = vec!((1, 1), (2, 2));
  30     /// twins.sort_by_key(|x| { x.1; });
  31     /// ```
  32     #[clippy::version = "1.47.0"]
  33     pub UNIT_RETURN_EXPECTING_ORD,
  34     correctness,
  35     "fn arguments of type Fn(...) -> Ord returning the unit type ()."
  36 }
  37
  38 declare_lint_pass!(UnitReturnExpectingOrd => [UNIT_RETURN_EXPECTING_ORD]);
  39
  40 fn get_trait_predicates_for_trait_id<'tcx>(
  41     cx: &LateContext<'tcx>,
  42     generics: GenericPredicates<'tcx>,
  43     trait_id: Option<DefId>,
  44 ) -> Vec<TraitPredicate<'tcx>> {
  45     let mut preds = Vec::new();
  46     for (pred, _) in generics.predicates {
  47         if_chain! {
  48             if let PredicateKind::Trait(poly_trait_pred) = pred.kind().skip_binder();
  49             let trait_pred = cx.tcx.erase_late_bound_regions(pred.kind().rebind(poly_trait_pred));
  50             if let Some(trait_def_id) = trait_id;
  51             if trait_def_id == trait_pred.trait_ref.def_id;
  52             then {
  53                 preds.push(trait_pred);
  54             }
  55         }
  56     }
  57     preds
  58 }
  59
  60 fn get_projection_pred<'tcx>(
  61     cx: &LateContext<'tcx>,
  62     generics: GenericPredicates<'tcx>,
  63     trait_pred: TraitPredicate<'tcx>,
  64 ) -> Option<ProjectionPredicate<'tcx>> {
  65     generics.predicates.iter().find_map(|(proj_pred, _)| {
  66         if let ty::PredicateKind::Projection(pred) = proj_pred.kind().skip_binder() {
  67             let projection_pred = cx.tcx.erase_late_bound_regions(proj_pred.kind().rebind(pred));
  68             if projection_pred.projection_ty.substs == trait_pred.trait_ref.substs {
  69                 return Some(projection_pred);
  70             }
  71         }
  72         None
  73     })
  74 }
  75
  76 fn get_args_to_check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> Vec<(usize, String)> {
  77     let mut args_to_check = Vec::new();
  78     if let Some(def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id) {
  79         let fn_sig = cx.tcx.fn_sig(def_id);
  80         let generics = cx.tcx.predicates_of(def_id);
  81         let fn_mut_preds = get_trait_predicates_for_trait_id(cx, generics, cx.tcx.lang_items().fn_mut_trait());
  82         let ord_preds = get_trait_predicates_for_trait_id(cx, generics, cx.tcx.get_diagnostic_item(sym::Ord));
  83         let partial_ord_preds =
  84             get_trait_predicates_for_trait_id(cx, generics, cx.tcx.lang_items().partial_ord_trait());
  85         // Trying to call erase_late_bound_regions on fn_sig.inputs() gives the following error
  86         // The trait `rustc::ty::TypeFoldable<'_>` is not implemented for
  87         // `&[rustc_middle::ty::Ty<'_>]`
  88         let inputs_output = cx.tcx.erase_late_bound_regions(fn_sig.inputs_and_output());
  89         inputs_output
  90             .iter()
  91             .rev()
  92             .skip(1)
  93             .rev()
  94             .enumerate()
  95             .for_each(|(i, inp)| {
  96                 for trait_pred in &fn_mut_preds {
  97                     if_chain! {
  98                         if trait_pred.self_ty() == inp;
  99                         if let Some(return_ty_pred) = get_projection_pred(cx, generics, *trait_pred);
 100                         then {
 101                             if ord_preds
 102                                 .iter()
 103                                 .any(|ord| Some(ord.self_ty()) == return_ty_pred.term.ty())
 104                             {
 105                                 args_to_check.push((i, "Ord".to_string()));
 106                             } else if partial_ord_preds
 107                                 .iter()
 108                                 .any(|pord| pord.self_ty() == return_ty_pred.term.ty().unwrap())
 109                             {
 110                                 args_to_check.push((i, "PartialOrd".to_string()));
 111                             }
 112                         }
 113                     }
 114                 }
 115             });
 116     }
 117     args_to_check
 118 }
 119
 120 fn check_arg<'tcx>(cx: &LateContext<'tcx>, arg: &'tcx Expr<'tcx>) -> Option<(Span, Option<Span>)> {
 121     if_chain! {
 122         if let ExprKind::Closure(&Closure { body, fn_decl_span, .. }) = arg.kind;
 123         if let ty::Closure(_def_id, substs) = &cx.typeck_results().node_type(arg.hir_id).kind();
 124         let ret_ty = substs.as_closure().sig().output();
 125         let ty = cx.tcx.erase_late_bound_regions(ret_ty);
 126         if ty.is_unit();
 127         then {
 128             let body = cx.tcx.hir().body(body);
 129             if_chain! {
 130                 if let ExprKind::Block(block, _) = body.value.kind;
 131                 if block.expr.is_none();
 132                 if let Some(stmt) = block.stmts.last();
 133                 if let StmtKind::Semi(_) = stmt.kind;
 134                 then {
 135                     let data = stmt.span.data();
 136                     // Make a span out of the semicolon for the help message
 137                     Some((fn_decl_span, Some(data.with_lo(data.hi-BytePos(1)))))
 138                 } else {
 139                     Some((fn_decl_span, None))
 140                 }
 141             }
 142         } else {
 143             None
 144         }
 145     }
 146 }
 147
 148 impl<'tcx> LateLintPass<'tcx> for UnitReturnExpectingOrd {
 149     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
 150         if let ExprKind::MethodCall(_, receiver, args, _) = expr.kind {
 151             let arg_indices = get_args_to_check(cx, expr);
 152             let args = std::iter::once(receiver).chain(args.iter()).collect::<Vec<_>>();
 153             for (i, trait_name) in arg_indices {
 154                 if i < args.len() {
 155                     match check_arg(cx, args[i]) {
 156                         Some((span, None)) => {
 157                             span_lint(
 158                                 cx,
 159                                 UNIT_RETURN_EXPECTING_ORD,
 160                                 span,
 161                                 &format!(
 162                                     "this closure returns \
 163                                    the unit type which also implements {trait_name}"
 164                                 ),
 165                             );
 166                         },
 167                         Some((span, Some(last_semi))) => {
 168                             span_lint_and_help(
 169                                 cx,
 170                                 UNIT_RETURN_EXPECTING_ORD,
 171                                 span,
 172                                 &format!(
 173                                     "this closure returns \
 174                                    the unit type which also implements {trait_name}"
 175                                 ),
 176                                 Some(last_semi),
 177                                 "probably caused by this trailing semicolon",
 178                             );
 179                         },
 180                         None => {},
 181                     }
 182                 }
 183             }
 184         }
 185     }
 186 }