Reprise: new lint: Unintentional return of unit from closures expecting Ord

CHANGELOG.md

@@ -1679,6 +1679,7 @@ Released 2018-09-13
 [`uninit_assumed_init`]: https://rust-lang.github.io/rust-clippy/master/index.html#uninit_assumed_init
 [`unit_arg`]: https://rust-lang.github.io/rust-clippy/master/index.html#unit_arg
 [`unit_cmp`]: https://rust-lang.github.io/rust-clippy/master/index.html#unit_cmp
+[`unit_return_expecting_ord`]: https://rust-lang.github.io/rust-clippy/master/index.html#unit_return_expecting_ord
 [`unknown_clippy_lints`]: https://rust-lang.github.io/rust-clippy/master/index.html#unknown_clippy_lints
 [`unnecessary_cast`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
 [`unnecessary_filter_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_filter_map

clippy_lints/src/lib.rs

@@ -300,6 +300,7 @@ mod trivially_copy_pass_by_ref;
 mod try_err;
 mod types;
 mod unicode;
+mod unit_return_expecting_ord;
 mod unnamed_address;
 mod unnecessary_sort_by;
 mod unnested_or_patterns;
@@ -826,6 +827,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &unicode::NON_ASCII_LITERAL,
         &unicode::UNICODE_NOT_NFC,
         &unicode::ZERO_WIDTH_SPACE,
+        &unit_return_expecting_ord::UNIT_RETURN_EXPECTING_ORD,
         &unnamed_address::FN_ADDRESS_COMPARISONS,
         &unnamed_address::VTABLE_ADDRESS_COMPARISONS,
         &unnecessary_sort_by::UNNECESSARY_SORT_BY,
@@ -891,6 +893,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box attrs::Attributes);
     store.register_late_pass(|| box blocks_in_if_conditions::BlocksInIfConditions);
     store.register_late_pass(|| box unicode::Unicode);
+    store.register_late_pass(|| box unit_return_expecting_ord::UnitReturnExpectingOrd);
     store.register_late_pass(|| box strings::StringAdd);
     store.register_late_pass(|| box implicit_return::ImplicitReturn);
     store.register_late_pass(|| box implicit_saturating_sub::ImplicitSaturatingSub);
@@ -1436,6 +1439,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&types::UNNECESSARY_CAST),
         LintId::of(&types::VEC_BOX),
         LintId::of(&unicode::ZERO_WIDTH_SPACE),
+        LintId::of(&unit_return_expecting_ord::UNIT_RETURN_EXPECTING_ORD),
         LintId::of(&unnamed_address::FN_ADDRESS_COMPARISONS),
         LintId::of(&unnamed_address::VTABLE_ADDRESS_COMPARISONS),
         LintId::of(&unnecessary_sort_by::UNNECESSARY_SORT_BY),
@@ -1692,6 +1696,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&types::CAST_REF_TO_MUT),
         LintId::of(&types::UNIT_CMP),
         LintId::of(&unicode::ZERO_WIDTH_SPACE),
+        LintId::of(&unit_return_expecting_ord::UNIT_RETURN_EXPECTING_ORD),
         LintId::of(&unnamed_address::FN_ADDRESS_COMPARISONS),
         LintId::of(&unnamed_address::VTABLE_ADDRESS_COMPARISONS),
         LintId::of(&unused_io_amount::UNUSED_IO_AMOUNT),

clippy_lints/src/unit_return_expecting_ord.rs

@@ -0,0 +1,177 @@
+use crate::utils::{get_trait_def_id, paths, span_lint, span_lint_and_help};
+use if_chain::if_chain;
+use rustc_hir::def_id::DefId;
+use rustc_hir::{Expr, ExprKind, StmtKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty;
+use rustc_middle::ty::{GenericPredicates, PredicateKind, ProjectionPredicate, TraitPredicate};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::{BytePos, Span};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for functions that expect closures of type
+    /// Fn(...) -> Ord where the implemented closure returns the unit type.
+    /// The lint also suggests to remove the semi-colon at the end of the statement if present.
+    ///
+    /// **Why is this bad?** Likely, returning the unit type is unintentional, and
+    /// could simply be caused by an extra semi-colon. Since () implements Ord
+    /// it doesn't cause a compilation error.
+    /// This is the same reasoning behind the unit_cmp lint.
+    ///
+    /// **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 UNIT_RETURN_EXPECTING_ORD,
+    correctness,
+    "fn arguments of type Fn(...) -> Ord returning the unit type ()."
+}
+
+declare_lint_pass!(UnitReturnExpectingOrd => [UNIT_RETURN_EXPECTING_ORD]);
+
+fn get_trait_predicates_for_trait_id<'tcx>(
+    cx: &LateContext<'tcx>,
+    generics: GenericPredicates<'tcx>,
+    trait_id: Option<DefId>,
+) -> Vec<TraitPredicate<'tcx>> {
+    let mut preds = Vec::new();
+    for (pred, _) in generics.predicates {
+        if_chain! {
+            if let PredicateKind::Trait(poly_trait_pred, _) = pred.kind();
+            let trait_pred = cx.tcx.erase_late_bound_regions(&poly_trait_pred);
+            if let Some(trait_def_id) = trait_id;
+            if trait_def_id == trait_pred.trait_ref.def_id;
+            then {
+                preds.push(trait_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 PredicateKind::Projection(proj_pred) = proj_pred.kind() {
+            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, String)> {
+    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_trait_predicates_for_trait_id(cx, generics, cx.tcx.lang_items().fn_mut_trait());
+        let ord_preds = get_trait_predicates_for_trait_id(cx, generics, get_trait_def_id(cx, &paths::ORD));
+        let partial_ord_preds =
+            get_trait_predicates_for_trait_id(cx, generics, cx.tcx.lang_items().partial_ord_trait());
+        // 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()
+            .rev()
+            .skip(1)
+            .rev()
+            .enumerate()
+            .for_each(|(i, inp)| {
+                for trait_pred in &fn_mut_preds {
+                    if_chain! {
+                        if trait_pred.self_ty() == inp;
+                        if let Some(return_ty_pred) = get_projection_pred(cx, generics, *trait_pred);
+                        then {
+                            if ord_preds.iter().any(|ord| ord.self_ty() == return_ty_pred.ty) {
+                                args_to_check.push((i, "Ord".to_string()));
+                            } else if partial_ord_preds.iter().any(|pord| pord.self_ty() == return_ty_pred.ty) {
+                                args_to_check.push((i, "PartialOrd".to_string()));
+                            }
+                        }
+                    }
+                }
+            });
+    }
+    args_to_check
+}
+
+fn check_arg<'tcx>(cx: &LateContext<'tcx>, arg: &'tcx Expr<'tcx>) -> Option<(Span, Option<Span>)> {
+    if_chain! {
+        if let ExprKind::Closure(_, _fn_decl, body_id, span, _) = arg.kind;
+        if let ty::Closure(_def_id, substs) = &cx.tables().node_type(arg.hir_id).kind;
+        let ret_ty = substs.as_closure().sig().output();
+        let ty = cx.tcx.erase_late_bound_regions(&ret_ty);
+        if ty.is_unit();
+        then {
+            if_chain! {
+                let body = cx.tcx.hir().body(body_id);
+                if let ExprKind::Block(block, _) = body.value.kind;
+                if block.expr.is_none();
+                if let Some(stmt) = block.stmts.last();
+                if let StmtKind::Semi(_) = stmt.kind;
+                then {
+                    let data = stmt.span.data();
+                    // Make a span out of the semicolon for the help message
+                    Some((span, Some(Span::new(data.hi-BytePos(1), data.hi, data.ctxt))))
+                } else {
+                    Some((span, None))
+                }
+            }
+        } else {
+            None
+        }
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for UnitReturnExpectingOrd {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        if let ExprKind::MethodCall(_, _, ref args, _) = expr.kind {
+            let arg_indices = get_args_to_check(cx, expr);
+            for (i, trait_name) in arg_indices {
+                if i < args.len() {
+                    match check_arg(cx, &args[i]) {
+                        Some((span, None)) => {
+                            span_lint(
+                                cx,
+                                UNIT_RETURN_EXPECTING_ORD,
+                                span,
+                                &format!(
+                                    "this closure returns \
+                                   the unit type which also implements {}",
+                                    trait_name
+                                ),
+                            );
+                        },
+                        Some((span, Some(last_semi))) => {
+                            span_lint_and_help(
+                                cx,
+                                UNIT_RETURN_EXPECTING_ORD,
+                                span,
+                                &format!(
+                                    "this closure returns \
+                                   the unit type which also implements {}",
+                                    trait_name
+                                ),
+                                Some(last_semi),
+                                &"probably caused by this trailing semicolon".to_string(),
+                            );
+                        },
+                        None => {},
+                    }
+                }
+            }
+        }
+    }
+}

src/lintlist/mod.rs

@@ -2292,6 +2292,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "types",
     },
+    Lint {
+        name: "unit_return_expecting_ord",
+        group: "correctness",
+        desc: "fn arguments of type Fn(...) -> Ord returning the unit type ().",
+        deprecation: None,
+        module: "unit_return_expecting_ord",
+    },
     Lint {
         name: "unknown_clippy_lints",
         group: "style",

tests/ui/unit_return_expecting_ord.rs

@@ -0,0 +1,36 @@
+#![warn(clippy::unit_return_expecting_ord)]
+#![allow(clippy::needless_return)]
+#![allow(clippy::unused_unit)]
+#![feature(is_sorted)]
+
+struct Struct {
+    field: isize,
+}
+
+fn double(i: isize) -> isize {
+    i * 2
+}
+
+fn unit(_i: isize) {}
+
+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));
+    structs.is_sorted_by_key(|s| {
+        double(s.field);
+    });
+    structs.is_sorted_by_key(|s| {
+        if s.field > 0 {
+            ()
+        } else {
+            return ();
+        }
+    });
+    structs.sort_by_key(|s| {
+        return double(s.field);
+    });
+    structs.sort_by_key(|s| unit(s.field));
+}

tests/ui/unit_return_expecting_ord.stderr

@@ -0,0 +1,39 @@
+error: this closure returns the unit type which also implements Ord
+  --> $DIR/unit_return_expecting_ord.rs:18:25
+   |
+LL |     structs.sort_by_key(|s| {
+   |                         ^^^
+   |
+   = note: `-D clippy::unit-return-expecting-ord` implied by `-D warnings`
+help: probably caused by this trailing semicolon
+  --> $DIR/unit_return_expecting_ord.rs:19:24
+   |
+LL |         double(s.field);
+   |                        ^
+
+error: this closure returns the unit type which also implements PartialOrd
+  --> $DIR/unit_return_expecting_ord.rs:22:30
+   |
+LL |     structs.is_sorted_by_key(|s| {
+   |                              ^^^
+   |
+help: probably caused by this trailing semicolon
+  --> $DIR/unit_return_expecting_ord.rs:23:24
+   |
+LL |         double(s.field);
+   |                        ^
+
+error: this closure returns the unit type which also implements PartialOrd
+  --> $DIR/unit_return_expecting_ord.rs:25:30
+   |
+LL |     structs.is_sorted_by_key(|s| {
+   |                              ^^^
+
+error: this closure returns the unit type which also implements Ord
+  --> $DIR/unit_return_expecting_ord.rs:35:25
+   |
+LL |     structs.sort_by_key(|s| unit(s.field));
+   |                         ^^^
+
+error: aborting due to 4 previous errors
+