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new lint: Unintentional return of unit from closures expecting Ord
This lint catches cases where the last statement of a closure expecting an instance of Ord has a trailing semi-colon. It compiles since the closure ends up return () which also implements Ord but causes unexpected results in cases such as sort_by_key. Fixes #5080
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,136 @@ | ||
| use crate::utils::{get_trait_def_id, paths, span_lint}; | ||
| use if_chain::if_chain; | ||
| use rustc::ty::{GenericPredicates, Predicate, ProjectionPredicate, TraitPredicate}; | ||
| use rustc_hir::{Expr, ExprKind, StmtKind}; | ||
| use rustc_lint::{LateContext, LateLintPass}; | ||
| use rustc_session::{declare_lint_pass, declare_tool_lint}; | ||
|
|
||
| declare_clippy_lint! { | ||
| /// **What it does:** Checks for functions that expect closures of type | ||
| /// Fn(...) -> Ord where the implemented closure has a semi-colon | ||
| /// at the end of the last statement. | ||
| /// | ||
| /// **Why is this bad?** Likely the semi-colon is unintentional which | ||
| /// returns () instead of the result of last statement. Since () implements Ord | ||
| /// it doesn't cause a compilation error | ||
| /// | ||
| /// **Known problems:** If returning unit is intentional, then there is no | ||
| /// way of specifying this without triggering needless_return lint | ||
| /// | ||
| /// **Example:** | ||
| /// | ||
| /// ```rust | ||
| /// let mut twins = vec!((1,1), (2,2)); | ||
| /// twins.sort_by_key(|x| { x.1; }); | ||
| /// ``` | ||
| pub UNINTENTIONAL_UNIT_RETURN, | ||
| nursery, | ||
| "fn arguments of type Fn(...) -> Once having last statements with a semi-colon, suggesting to remove the semi-colon if it is unintentional." | ||
| } | ||
|
|
||
| declare_lint_pass!(UnintentionalUnitReturn => [UNINTENTIONAL_UNIT_RETURN]); | ||
|
|
||
| fn unwrap_trait_pred<'tcx>(cx: &LateContext<'_, 'tcx>, pred: &Predicate<'tcx>) -> Option<TraitPredicate<'tcx>> { | ||
| if let Predicate::Trait(poly_trait_pred, _) = pred { | ||
| let trait_pred = cx.tcx.erase_late_bound_regions(&poly_trait_pred); | ||
| Some(trait_pred) | ||
| } else { | ||
| None | ||
| } | ||
| } | ||
|
|
||
| fn get_predicates_for_trait_path<'tcx>( | ||
| cx: &LateContext<'_, 'tcx>, | ||
| generics: GenericPredicates<'tcx>, | ||
| trait_path: &[&str], | ||
| ) -> Vec<&'tcx Predicate<'tcx>> { | ||
| let mut preds = Vec::new(); | ||
| generics.predicates.iter().for_each(|(pred, _)| { | ||
| if let Some(trait_pred) = unwrap_trait_pred(cx, pred) { | ||
| if let Some(trait_def_id) = get_trait_def_id(cx, trait_path) { | ||
| if trait_def_id == trait_pred.trait_ref.def_id { | ||
| preds.push(pred); | ||
| } | ||
| } | ||
| } | ||
| }); | ||
| preds | ||
| } | ||
|
|
||
| fn get_projection_pred<'tcx>( | ||
| cx: &LateContext<'_, 'tcx>, | ||
| generics: GenericPredicates<'tcx>, | ||
| pred: TraitPredicate<'tcx>, | ||
| ) -> Option<ProjectionPredicate<'tcx>> { | ||
| generics.predicates.iter().find_map(|(proj_pred, _)| { | ||
| if let Predicate::Projection(proj_pred) = proj_pred { | ||
| let projection_pred = cx.tcx.erase_late_bound_regions(proj_pred); | ||
| if projection_pred.projection_ty.substs == pred.trait_ref.substs { | ||
| return Some(projection_pred); | ||
| } | ||
| } | ||
| None | ||
| }) | ||
| } | ||
|
|
||
| fn get_args_to_check<'tcx>(cx: &LateContext<'_, 'tcx>, expr: &'tcx Expr<'tcx>) -> Vec<usize> { | ||
| let mut args_to_check = Vec::new(); | ||
| if let Some(def_id) = cx.tables.type_dependent_def_id(expr.hir_id) { | ||
| let fn_sig = cx.tcx.fn_sig(def_id); | ||
| let generics = cx.tcx.predicates_of(def_id); | ||
| let fn_mut_preds = get_predicates_for_trait_path(cx, generics, &paths::FN_MUT); | ||
| let ord_preds = get_predicates_for_trait_path(cx, generics, &paths::ORD); | ||
| // Trying to call erase_late_bound_regions on fn_sig.inputs() gives the following error | ||
| // The trait `rustc::ty::TypeFoldable<'_>` is not implemented for `&[&rustc::ty::TyS<'_>]` | ||
| let inputs_output = cx.tcx.erase_late_bound_regions(&fn_sig.inputs_and_output()); | ||
| inputs_output.iter().enumerate().for_each(|(i, inp)| { | ||
| // Ignore output param | ||
| if i == inputs_output.len() - 1 { | ||
| return; | ||
| } | ||
| fn_mut_preds.iter().for_each(|pred| { | ||
| let trait_pred = unwrap_trait_pred(cx, pred).unwrap(); | ||
| if trait_pred.self_ty() == *inp { | ||
| if let Some(projection_pred) = get_projection_pred(cx, generics, trait_pred) { | ||
| let ret_is_ord = ord_preds | ||
| .iter() | ||
| .any(|ord_pred| unwrap_trait_pred(cx, ord_pred).unwrap().self_ty() == projection_pred.ty); | ||
| if ret_is_ord { | ||
| args_to_check.push(i); | ||
| } | ||
| } | ||
| } | ||
| }); | ||
| }); | ||
| } | ||
| args_to_check | ||
| } | ||
|
|
||
| impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnintentionalUnitReturn { | ||
| fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'tcx>) { | ||
| let arg_indices = get_args_to_check(cx, expr); | ||
| if_chain! { | ||
| if let ExprKind::MethodCall(_, _, ref args) = expr.kind; | ||
| then { | ||
| for i in arg_indices { | ||
| if_chain! { | ||
| if i < args.len(); | ||
| if let ExprKind::Closure(_, _fn_decl, body_id, _span, _) = args[i].kind; | ||
| let body = cx.tcx.hir().body(body_id); | ||
| if let ExprKind::Block(block, _) = body.value.kind; | ||
| if let Some(stmt) = block.stmts.last(); | ||
| if let StmtKind::Semi(_) = stmt.kind; | ||
| then { | ||
| //TODO : Maybe only filter the closures where the last statement return type also is an unit | ||
| span_lint(cx, | ||
| UNINTENTIONAL_UNIT_RETURN, | ||
| stmt.span, | ||
| "Semi-colon on the last line of this closure returns \ | ||
| the unit type which also implements Ord."); | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,17 @@ | ||
| #![warn(clippy::unintentional_unit_return)] | ||
|
|
||
| struct Struct { | ||
| field: isize, | ||
| } | ||
|
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||
| fn double(i: isize) -> isize { | ||
| i * 2 | ||
| } | ||
|
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||
| fn main() { | ||
| let mut structs = vec![Struct { field: 2 }, Struct { field: 1 }]; | ||
| structs.sort_by_key(|s| { | ||
| double(s.field); | ||
| }); | ||
| structs.sort_by_key(|s| double(s.field)); | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,10 @@ | ||
| error: Semi-colon on the last line of this closure returns the unit type which also implements Ord. | ||
| --> $DIR/unintentional_unit_return.rs:14:9 | ||
| | | ||
| LL | double(s.field); | ||
| | ^^^^^^^^^^^^^^^^ | ||
| | | ||
| = note: `-D clippy::unintentional-unit-return` implied by `-D warnings` | ||
|
|
||
| error: aborting due to previous error | ||
|
|