src/tools/clippy/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 `&[&rustc::ty::TyS<'_>]`
  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.iter().any(|ord| Some(ord.self_ty()) == return_ty_pred.term.ty()) {
 102                                 args_to_check.push((i, "Ord".to_string()));
 103                             } else if partial_ord_preds.iter().any(|pord| {
 104                                 pord.self_ty() == return_ty_pred.term.ty().unwrap()
 105                             }) {
 106                                 args_to_check.push((i, "PartialOrd".to_string()));
 107                             }
 108                         }
 109                     }
 110                 }
 111             });
 112     }
 113     args_to_check
 114 }
 115
 116 fn check_arg<'tcx>(cx: &LateContext<'tcx>, arg: &'tcx Expr<'tcx>) -> Option<(Span, Option<Span>)> {
 117     if_chain! {
 118         if let ExprKind::Closure(_, _fn_decl, body_id, span, _) = arg.kind;
 119         if let ty::Closure(_def_id, substs) = &cx.typeck_results().node_type(arg.hir_id).kind();
 120         let ret_ty = substs.as_closure().sig().output();
 121         let ty = cx.tcx.erase_late_bound_regions(ret_ty);
 122         if ty.is_unit();
 123         then {
 124             let body = cx.tcx.hir().body(body_id);
 125             if_chain! {
 126                 if let ExprKind::Block(block, _) = body.value.kind;
 127                 if block.expr.is_none();
 128                 if let Some(stmt) = block.stmts.last();
 129                 if let StmtKind::Semi(_) = stmt.kind;
 130                 then {
 131                     let data = stmt.span.data();
 132                     // Make a span out of the semicolon for the help message
 133                     Some((span, Some(data.with_lo(data.hi-BytePos(1)))))
 134                 } else {
 135                     Some((span, None))
 136                 }
 137             }
 138         } else {
 139             None
 140         }
 141     }
 142 }
 143
 144 impl<'tcx> LateLintPass<'tcx> for UnitReturnExpectingOrd {
 145     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
 146         if let ExprKind::MethodCall(_, args, _) = expr.kind {
 147             let arg_indices = get_args_to_check(cx, expr);
 148             for (i, trait_name) in arg_indices {
 149                 if i < args.len() {
 150                     match check_arg(cx, &args[i]) {
 151                         Some((span, None)) => {
 152                             span_lint(
 153                                 cx,
 154                                 UNIT_RETURN_EXPECTING_ORD,
 155                                 span,
 156                                 &format!(
 157                                     "this closure returns \
 158                                    the unit type which also implements {}",
 159                                     trait_name
 160                                 ),
 161                             );
 162                         },
 163                         Some((span, Some(last_semi))) => {
 164                             span_lint_and_help(
 165                                 cx,
 166                                 UNIT_RETURN_EXPECTING_ORD,
 167                                 span,
 168                                 &format!(
 169                                     "this closure returns \
 170                                    the unit type which also implements {}",
 171                                     trait_name
 172                                 ),
 173                                 Some(last_semi),
 174                                 "probably caused by this trailing semicolon",
 175                             );
 176                         },
 177                         None => {},
 178                     }
 179                 }
 180             }
 181         }
 182     }
 183 }