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