clippy_lints/src/unit_return_expecting_ord.rs

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