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

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